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M. Antonijevic1∗, Z. Simic2, and Z. Petrovic2 1Technical Faculty of Bor, University of Belgrade, P.O. Box 50, 19210 Bor, Serbia 2Faculty of Science, Department of Chemistry, University of Kragujevac, Serbia (Received: 3 February 2009. Accepted: 29 April 2009) For the purpose of this paper the following natural minerals: pyrite (FeS2, chalcopyrite (CuFeS2, galena (PbS), covellite (CuS) and chalcocite (Cu2S), were used as the indicator electrode for coulometric–potentiometric and potentiometric titrations of complex natural acid–base systems. These results were compared with those obtained by using of glass indicator electrode. The acidity of humic acid macromolecules was determined by combined coulometric–potentiometric method in alkaline solution which contained the sample of the substance that was being tested. Hydrogen ions generated by anode oxidation of hydrogen dissolved in palladium, while for the purpose of determining acidity of fulvic acids either in water and non-water environment (methanol, ethanol, acetone) tetraethylammonium hydroxide solution in methanol was used. Keywords: Sulphide Mineral, Sensor, Humic Acids, Fulvic Acids, Coulometry, Potentiometry. 1. INTRODUCTION Humic substances are complex polymers which have tree- dimensional network structure of changeable composition which can differ from one site to another. Humic sub- stances can be divided into 3 main groups: fulvic acids, humic acids and humin. Fulvic acids have the smallest molecular weight—approximately 1000 Daltons, molecu- lar weight of humic acids is approximately 3000 Daltons and finally the humin alone contains the polymers of molecular weight which goes up to several hundred thou- sand Daltons. The ratio of humin, humic and fulvic acids quantities depends on the soil type, climate, plant cover etc. Physical and physic-chemical properties of humic materials depend on its age and they are prone to change over time. High content of this complex supstances in soil is desired, because of their responsibility for soil fertility, soil protection from degradation and contamination. According to Felbeck1 the change in the plant material structure causes the surface bondage between those parts of the plant tissues that are resistant to microbiological degradation with soil. During the first phase of humifi- cation, the molecules of the humic acids that have high ∗Corresponding author; E-mail: mantonijevic@tf.bor.ac.rs molecular weights are formed, which, again, gradually degrade to fulvic acids and eventually to carbon dioxide and water. By performing decomposition of the plant mate- rial, microorganisms synthesize phenols and amino acids and excrete them into the surrounding environment where due to the reaction with oxygen from the air the oxida- tion of these substances occur and this is followed by fur- ther polymerization to humic acids and humin. Humic acid molecule contains carbon, ether, ester, carbonyl groups and also quinone and methoxy group are likely to be found. Thus, we are talking about complex macromolecules with great number of different oxygen functional groups. The presence of the units of furan, alkylbenzene, methoxy- benzene, alkylphenol, methoxyphenol, benzoic acid and various condensed aromatic systems was confirmed. Anionic character of humic acids is responsible for their solubility, buffer capacity, metal-binding and other chemi- cal reactions. Hydrogen bonds give significant stability to these molecules. Fulvic acids are soluble in both base and acid under all pH conditions, and they are especially soluble in polar organic solvents. They have smaller molecular weights and are also characterized by hydrophilic functional groups. They are stronger acids than humic acids. They are Sensor Lett. 2009, Vol. 7, No. 4 1546-198X/2009/7/001/007 doi:10.1166/sl.2009.1103 1 R E S E A R C H A R T IC L E Natural Sulphide Minerals as Sensors for Determination of Total Acidity of Humic and Fulvic Acids Antonijevic et al. hydrophilic colloids and they facilitate the process of rinis- ing alumminium and iron from the soil. Alumminium and iron are present in the soil due to decomposition of miner- als. Their salts, fulvates are water-soluble, they are rinised from the ground; fulvates facilitate dissolution of minerals that are in the soil. Different amounts of these acids occure in the humic substances of different soil types (15–70% out of the total humic substances). They also contain more alcohol and carboxyl groups, while humic acids contain more phenol and cheto groups. To extract fulvic acids one should extract humin in non- acid environment, and then by using aciditation precipi- tate humic acids. Fulvic acids which are yellow-brown in colour remain in the solution. Although glass electrode is the one most frequently used for the purpose of potentiometric acid–base titra- tion, the practice of utilization of other pH elec- trodes such as oxide and sulphide electrodes with solid membrane,2–8 monocrystals sensor electrodes,910 electro- conductive organic polymer sensor electrodes, etc. In this paper natural sulfide minerals from pyrite, chalcopyrite, galena, covellite and chalcocite were used as sensors for potentiometric and coulometric–potentiometric determina- tion of acidity of the humic acids extracted from the soil according to the established procedure. 2. EXPERIMENTAL DETAILS 2.1. Extraction and Characterization of Humic and Fulvic Acids Extraction and purification of humic and fulvic acids from the soil was performed according to the established procedure (IHSS 1983).11 Humic acid were extracted from the soil near Kragujevac (Gruža) in Serbia. Extrac- tion was conducted in Socklets apparatus with the mixture of the dichloromethane and water (azeotropic mix- ture) at 37.8 C. Characterization of the isolated humic acid was done using IR spectrophotometer Perkin-Elmer “Spectrum-one 197.” Humic acids extracted from the soil are characterized by the vibrational frequencies of the main functional groups. 2.2. Electrodes Natural pyrite (FeS2, chalcopyrite (CuFeS2, galena (PbS), covellite (CuS) and chalcocite (Cu2S) crystals in a form of cube were used as potentiometric sensors. Copper wire was pasted on one side of this cube by electroconductible glue based on silver. Contact of min- erals and copper conductor was realized by this. Such prepared electrode was immersed with autopolymerized methyl methacrylate to the desired height. Upon solidifi- cation, working surface of electrode was polished to the high glow on felt with impregnated diamond and then Al2O3 paste (0.3 m). After that, electrode was rinsed with distilled water and alcohol, and upon drying on air immediately inserted into electrochemical cell. Fresh min- eral surface were formed by polishing on clay before each experiment. Glass and SCE were commercial electrodes, whereas H2/Pd generator electrode was laboratory-made. 12 2.3. Methods On the other hand, HA are less soluble than FA and so back titration method (coulometric–potentiometric titration of HA solution which contain exacty quantite of hydrox- ide) was used in the case of HA. It was difficult to deter- mine the precise and accurate carboxyl contents of HA by direct potentiometric titration. Inversely, total acidity of fulvic acids can be determined by direct potentiometric titration of the acids with hydroxide solution. 2.3.1. Coulometric–Potentiometric Titration Suspension of humic acid was prepared by dispersing the acid in 0.1 M NaCl solution. Concentration humic acid was 0.6 g/dm3. 10 cm3 of the suspension was potentimet- rically titrated by standard solution of NaOH (0.0965 M) using glass electrode (Radiometer). Based on amount of consumed NaOH during the titration it was calculated of volume of NaOH which need to add in humic acid sus- pension for total neutralisation. Fresh soulution of humic acid was prepared and neutralised by the amount of NaOH. Aliquot (10.00 cm3 of the suspension was trans- fered to coulometric cell. In order to determine electric- ity consumption for coulometric–potentiometric titration of the humic acids we used apparatus described in the paper.12 In order to determine end point by potentiomet- ric detection we used the following electrode pairs: glass electrode-SCE, FeS2-SCE, CuFeS2-SCE, PbS-SCE, CuS- SCE i Cu2S-SCE. Catholite was separated from analyte by sinter glass G-4. For generation of H+ ions H2/Pd genera- tor electrode was used as an anode and platinum electrode as a cathode. Thence, the titration agent is coulometrically generated on H2/Pd electrode as a generator electrode, as given by the following reaction: Pd(H2−2e= 2H++Pd. The chosen pair of electrodes was immerged into a dish in which the titration was to be conducted. After each addition of H+ ions, we read the potential from the volt- meter. In order to monitor the potential we used one of the above mentioned electrode pairs. Right before and after the end point of the titration we performed the reading after 30 seconds of electrolysis at the constant 2 mA current in order to avoid secondary reaction of electrooxidation of the substrate itself. The quantity of electric current used till arriving to the end point of titration was calculated by using the second derivative. 2 Sensor Letters 7, 1–7, 2009 R E S E A R C H A R T IC L E Antonijevic et al. Natural Sulphide Minerals as Sensors for Determination of Total Acidity of Humic and Fulvic Acids 2.3.2. Potentiometric Titration Potentiometric titration of fulvic acids were carried out in water and non-aqua media (methanole, ethanole and ace- tone). The aqueous solutions of fulvic acids were prepared by dissolving of 0.5000 g of fulvic acid in disstiled water in flask of 50 cm3. A portion of 2.00 cm3 of the solution was transfered in potentiometric cell, diluted up to 20 cm3 with desired solvents and titrated by applying above men- tioned pairs of electrodes with 0.1 M tetraethylammo- nium hydroxide solution in methanol (0.1 M TEAH) (“Fluka AG”). 3. RESULTS AND DISCUSSION 3.1. Characterization of Humic and Fulvic Acids In present work, Figures 1 and 2 show IR spectra for Humic and Fulvic acids. Based on the spectral data the following can be concluded: the substance is mainly of aromatic character. Widened and very intensive range in region 3200–3500 cm−1 occurred due to the presence of the polymerized amino or hydroxide groups. The occur- rence of the sharp peak at 3697.03 is due to the partial breaking of intermolecular hydrogen bond, i.e., the exis- tence of a free –OH group. The range of the medium intensity at 1637.09 cm−1 comes from -diketones which are in enolic form, but it can also be caused by –C C– bond which is conjugated with carboxyl or carbonyl group. The presence of ether bonds, alcohol group and glycosides can be also seen from the very intensive within the range of 1038.78 cm−1. The presence of substituted aromatic compounds is validated and recorded within the range of 4000,0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 450,0 6,0 10 15 20 25 30 35 40 45 50 55 60 65 67,0 cm–1 % T 3697,03 3619,98 3434,07 1637,09 1384,18 1031,78 914,62 798,11 778,97 695,25 647,52 534,91 470,95 Fig. 1. IR spectra of Humic acids. 534.91 and 470.95 cm−1. Palladino et al.,13 Fukushima et al.14 and Massini et al.15 found similar IR spectra for humic acid. 3.2. Coulometric–Potentiometric Determination of Total Acidity of Humic Acids In coulometric cell (−) AE IE H2/Pd (+) (where are AE—auxiliary electrode, IE—indicator electrode) at the current of 2 mA in order to avoid oxidation of the humic acids, electro generated ions H+ were added to the solu- tion. The change of the potential within the indicator cir- cuit AE IE In depends upon the change of concentration of H+ ions according to Nernst equation E = E0ind+5916 log [H++ Ej where Ej stands for the potential which depends on interaction between the reference electrode surface and the solution. The change of the potential dur- ing titration of the indicator electrode points to neutraliza- tion of OH− ions by generated H+ ions within in the humic acids solution. From the quantity of OH− ions added in the solution, added quantities of humic acid and the quantity of the coulometrically generated H+ ions on the end points of the titration gives the total acidity of humic acids. As far as the indicator ion selective electrodes with solid membrane that are without inner solution are concerned, there is a significant application of the sensors based on natural minerals and they are becoming more frequently used in both water and non-water solvents. Sulphide min- erals of pyrite, chalcopyrite and galena can be used in acid–base determinations.16 Thus they were used in this paper for the purpose of determination of the overall acid- ity of the humic acids. The change of potential during the titration shows that there is sufficient rise of potential Sensor Letters 7, 1–7, 2009 3 R E S E A R C H A R T IC L E Natural Sulphide Minerals as Sensors for Determination of Total Acidity of Humic and Fulvic Acids Antonijevic et al. 4000,0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 450,0 40,0 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 83,0 cm–1 % T 3697,19 3435,16 2920,42 2850,95 2345,95 1629,72 1031,28 797,78 778,34 694,97 526,84 470,92 Fig. 2. IR spectra of Fulvic acids. (Fig. 3, Table II) during titration as well as on the point of equivalency. The rise of potential on EP caused by application of the above mentioned electrodes is slightly smaller from the one obtained by the application of glass electrode, however it is sufficient for the purpose of determining the equivalence point with solid precision and reproductivity. Potential dur- ing the titration of humic and fulvic acids, when applying the above mentioned indicator electrodes, is stable and it is rapidly established. The oxidation of the chalcopyrite min- erals can be described by the following equations: CuFeS2+8H2O = Cu2++Fe3++2SO2−4 +16H++17e− (1) CuFeS2+4H++O2 = Cu2++Fe2++2So+2H2O (2) CuFeS2+4O2 = Cu2++Fe2++2SO2−4 (3) –400 –300 –200 –100 0 100 200 300 m V µ(H+) (I division = 100 µ eqv H+) 1 2 3 4 5 6 Fig. 3. Coulometric–potentiometric curves of Humic acids with poten- tiometric end-point detection: (1) CuFeS2-SCE, (2) FeS2-SCE, (3) CuS- SCE, (4) PbS-SCE, (5) Cu2S-SCE, (6) Glass-SCE. As it can be seen from the above given equations, the end products of oxidation of chalcopyrite are sulfur and sulphate. Regardless the nature of the products that are formed as a result of oxidation of sulphide, a certain num- ber of iron and copper ions will always be present in prox- imity to the very surface of minerals, thus making a double electric layer. The oxidation states of these cations depend on the nature of the solvent and on the conditions present in the solution itself. Iron and copper ions hydrolyze, and the hydrolysis of the iron ions can be represented by the following equation: Fen++2kH2O= FeOHn−k+k + kH3O+ (4) Since the mineral chalcopyrite is known for its semicon- ducting features, hydroxide formed on the surface of the mineral Fe(OH)n−k+k /CuFeS2 behaves as hydroxide/metal electrode whose potential is given by the equation: E = Eoox+RT /nF lnaoxakH3O+ (5) Table I. Atribution of the infrared absorption frequencies. Frequency interval (cm−1) Atribution 3436 3435 –O–H stretching 1631 1629 Aromatic O–C–O bond vibrations in COO−, aromatic C–C bond, C–O bond in aromatic aldehydes and ketones 2920 2922 –C–H stretching in –CH2 or –CH3 groups 2850 –C–H stretching in –CH2 or –CH3 groups 1384 Deformation models of aliphatic –CH2 or –CH3 groups in phenols 1031 –C–O–C– aliphatic stretching 798, 778, 695, 694 Aromatic –C–H vibrations or –C–H deformation –C(O)OH groups 4 Sensor Letters 7, 1–7, 2009 R E S E A R C H A R T IC L E Antonijevic et al. Natural Sulphide Minerals as Sensors for Determination of Total Acidity of Humic and Fulvic Acids Table II. Results of coulometric–potentiometric determination of humic acids by use of sulphide minerals as indicator electrodes. Electrode Total potential Total potential Total acidity oh couple change (mV) change at EP (mV) HA (mmol/g) Glass-SCE 572 270 9.70 FeS2-SCE 626 235 9.60 CuFeS2-SCE 346 145 9.75 PbS-SCE 370 160 9.60 Cu2S-SCE 284 100 9.70 CuS-SCE 337 122 9.65 where: OX= Fe(OH)n−k+k /CuFeS2. If during electrolyses the solid layer is being formed near the very surface of the electrode, then the equation for the potential equation can be simplified: E = Eoox+RT /nF lnakH3O+ (6) From this follows that the potential of the electrode depends on the activity of hydronium ions. The same equa- tion can also be applied to non-water solutions of weak acids. Since in diluted water solutions of weak acids the water activity is constant, this water activity will not have any influence the on potential of solution; however in non- water solution the change of potential during titration will be influenced by presence of hydrogen ions from acids as well as by the presence of water itself that is being cre- ated during neutralization process. Equation (7) describes dependence of the potential on the existent hydrogen ions and water in non-water solvent. E = Eoox+RT /nF lnakH3O+/aH2O2k (7) For galena (PbS) electrode, similarly to previous dis- cussion, it can be demonstrated that the potential of both water and non-water solutions can be described by Eq. (7). By applying galena electrode, mineral PbS can be par- tially dissolved in water solution thus producing different reaction products compared to the mentioned chalcopyrite. The surface layer of PbS mineral can be partially sheathed with PbCO3, PbS2O3, Pb(OH)2 produced according to the following reaction: 2PbS+5H2O= PbS2O3+PbOH2+8H++8e− (8) 2PbS+7H2O= 2PbOH2+S2O2−3 +10H++8e− (9) 2PbS+2CO2−3 +3H2O = 2PbCO3+S2O2−3 +6H++8e (10) Similarly to above mentioned indicator electrodes, the sensor-based electrodes made from the following minerals: chalcopyrite (FeS2, chalcocite (Cu2S) and covellite (CuS) can also be applied. In paper,13 the experimental results confirm that con- stant current coulometry is a very powerful method to vary stepwise the analytical acidity of Humic Acid solutions with the highest accuracy. Authors found that total acidity of Humic acids was 9.5 mmol g−1 what is similar with results presented in this work. 3.3. Potentiometric Determination of Total Acidity of Fulvic Acids The adequate choice of solvents used for determining acids and bases enables us to perform titration under optimal conditions. Choosing the solvents is usually performed according to two criteria. First criterion is based on using relative (empirical) scale acidity, and the other is based on using the absolute scale acidity, thermodynamic constant of solvent autoprotolysis and thermodynamic constants of titrated acids and bases. The selection of the solvent that is to be used in acid–base titration is performed according to the value and the position on the relative scale acidity. The higher the relative acidity scale, the greater the differenti- ating effect of the solvent on the strength of the dissolved electrolytes as well as the possibility of differential titra- tion of the acid mixture, i.e., the base mixture. In base solvents or in solvents whose acidity scale is for its greater part found in alkaline area, weak acids can also be titrated. Thus in acid solvents, or in solvents whose acidity scale is for its greater part found in acid area, weak bases can be titrated. Both size and position of the relative acidity scale depend on the experimental conditions used for determi- nation (nature of the titers, concentration of electrolytes, present admixtures, formation of complexes and insoluble compounds, etc.) and it equals the deduction of the val- ues of potential of the semineutralization of a strong acid (HClO4 or HCl) and strong base (R4NOH or C2H5OK) which is given by equation: Es = E1/2HA−E1–2B. Poten- tiometric titration in lower alcohols (methanol, ethanol) due to expended scale of acidity both in acid and base area compared to water (methanol: 690 mV, ethanol: 790 mV, acetone: 1360 mV, water: 540 mV)17 as a consequence enables better conditions for titration of humic and fulvic acids in these solvents (Tables III and IV). Tables III and IV show that the sharpest increase of potentials during titration of fulvic acids, both during the process of titration itself as well as on the equivalency point are obtained when the glass electrode is used as the indicator electrode (285 mV/0.3 cm3 of 0.1 M TEAH in water, while at the same conditions in methanol, ethanol and acetone these increases of potential are even greater and are 302 mV/0.3 cm3 of 0.1 M TEAH for methanol, Table III. Results of potentiometric titrations of Fulvic acids in water with 0.1 M tetraethylammonium hydroxide solution in methanol using of sulphide minerals as indicator electrodes. Electrode Total potential Total potential Total acidity of couple change (mV) change at EP (mV) FA (mmol/g) Glass-SCE 598 285 5.7 FeS2-SCE 664 248 5.95 CuFeS2-SCE 378 158 5.75 PbS-SCE 391 176 5.6 Cu2S-SCE 299 115 5.7 CuS-SCE 291 109 5.6 Sensor Letters 7, 1–7, 2009 5 R E S E A R C H A R T IC L E Natural Sulphide Minerals as Sensors for Determination of Total Acidity of Humic and Fulvic Acids Antonijevic et al. Table IV. Results of potentiometric determination of Fulvic acids in non-aqua media with 0.1 M tetraethylammonium hydroxide solution in methanol use of sulphide minerals as indicator electrodes. Total Potential Total Electrode potential change at end acidity of FA Solvent couple change (mV) point (mV) (mmol/g) Methanol Glass-SCE 630 302 5.7 FeS2-SCE 681 268 5.85 CuFeS2-SCE 395 172 5.9 PbS-SCE 420 192 5.7 Cu2S-SCE 338 131 5.6 CuS-SCE 322 122 5.8 Ethanol Glass-SCE 654 318 5.4 FeS2-SCE 695 260 5.35 CuFeS2-SCE 391 179 5.6 PbS-SCE 431 198 5.4 Cu2S-SCE 332 138 5.5 CuS-SCE 290 122 5.35 Acetone Glass-SCE 705 342 5.4 FeS2-SCE 722 282 5.2 CuFeS2-SCE 405 192 5.25 PbS-SCE 445 210 5.3 Cu2S-SCE 358 144 5.2 CuS-SCE 325 138 5.25 318 mV/0.3 cm3 for ethanol and 342 mV/0.3 cm3 for ace- tone, respectively. By applying pyrite electrode the total change of potential during the titration of fulvic acids is slightly bigger than when the glass electrode is used for the same purpose, while on EP it is slightly smaller and it is 248 mV/0.3 cm3 for water, 268 mV/0.3 cm3 for methanol, 260 mV/0.3 cm3 for ethanol and 282 mV/0.3 cm3 for ace- tone, respectively. The change of potential during the titra- tion as well as on the equivalency point among all above mentioned pairs of electrodes is the greatest for the glass electrode for all solvents: either water or all other above mentioned solvents. This change is the greatest in acetone (the total change is 705 mV TEAH, while for EP it is 342 mV/0.3 cm3 which is explained by the high acidity scale of acetone (Fig. 5). Also, the improved conditions for the titration compared to water, are achieved in alco- hols because by applying all of the above mentioned elec- trodes, greater changes of potential occur on EP compared to water solutions (Figs. 3 and 4). From applied sensors based on sulphide minerals, the greatest changes are recorded for the pyrite electrode (in acetone total change during titration is 702 mV, and for EP 342 mV/0.3 cm3, respectively), while the changes are the smallest in water. The smallest changes of potential dur- ing titration are produced by applying the chalcocite and covellite as mineral for indicator electrodes. However they are still sufficient for determination of EP with satisfactory precision and reproductivity. The obtained results are in good conformity with the results obtained by application of glass electrode as indicator electrode. Fernandez et al.18 found that total acidity (carboxilyc- type group content+ phenolic type group content) of ful- vic acids isolated from composted sewage sludge (CS), –400 –300 –200 –100 0 100 200 300 400 m V V(I division = 0.5 ml) 1 2 3 4 5 6 Fig. 4. Potentiometric curves of Fulvic acid with 0.1 M tetraethylam- monium hydroxide solution in methanol using sulphide mineral as indi- cator electrodes in water: (1) Glass-SCE, (2) FeS2-SCE, (3) CuFeS2-SCE, (4) PbS-SCE, (5) CuS-SCE, (6) Cu2S-SCE. thermally-dried sewage sludge (TS) and soils amended with CS or TS are between 6.10 and 8.51 mmol g−1. These data obtained by potentiometric titration of fulvic acids (the pH was adjusted to a value of 10.7 by addition of 0.1 M KOH) with 0.1 M nitric acid. Total acidity of fulvic acids of a different origin (inshore soils, peat, marine sediments, and soil (lysimetric) waters) were evaluated by means of two alternative methods— colloid titration and potentiometric titration.19 In the paper,20 amphiphilic properties of Aso fulvic acid (AFA) and Aso humic acid (AHA) are evaluated through the study on the binding of N -alkylpyridinium bromide (CnPy +Br−, n= 12, 14 and 16), using a potentio- metric titration method with surfactant-ion-selective mem- brane electrodes in aqueous solution of pH 9.18 and ionic strength of 0.03 M at 25 C. The papers18–20 show that potentiometric titrations can be used for charachterization and determination of total acidity of fulvic acids which is in agreement with the results obtained in this study. –500 –400 –300 –200 –100 1 2 3 4 5 6 0 100 200 300 400 V(I division = 0.5 ml)m V Fig. 5. Potentiometric curves of Fulvic acid with 0.1 M tetraethylam- monium hydroxide solution in methanol using sulphide mineral as indica- tor electrodes in acetone: (1) Glass-SCE, (2) CuFeS2-SCE, (3) PbS-SCE, (4) Cu2S-SCE, (5) CuS-SCE, (6) FeS2-SCE. 6 Sensor Letters 7, 1–7, 2009 R E S E A R C H A R T IC L E Antonijevic et al. Natural Sulphide Minerals as Sensors for Determination of Total Acidity of Humic and Fulvic Acids 4. CONCLUSION Sensors based on natural minerals of pyrite, chalcopyrite, galena, chalcocite and covellite can be applied in water for coulometric–potentiometric determination of the total acidity of humic acids, as well as in water, methanol, ethanol and acetone for the potentiometric determination of the total acidity of fulvic acids. The application of non- water solvents: methanol, ethanol and acetone led to the improvement of the conditions for determination of fulvic acids. The mentioned coulometric–potentiometric method, presented in this paper can also be applied to other acid– base systems either from natural environment or those applied in technics, such as synthetic polymers, biological membranes etc. Acknowledgment: The authors wish to thank the Min- istry of Science and Environmental Protection of Serbia for financial support (Project No. 142 012). References and Notes 1. G. T. Felbek, Soil Biochemistry 2, 55 (1973). 2. F. Cuta and E. Havelka, Coll. Czech. Chem. Comm. 28, 3005 (1963). 3. J. M. Kolthoff and B. D. Hartong, Rev. Trav. Chim. 44, 113 (1925). 4. G. Edvall, Med. Biol. Eng. 16, 661 (1978). 5. S. Glab, G. Edwall, P. A. Jongren, and F. Ingman, Talanta 28, 301 (1981). 6. M. S. Jovanovic´ and R. B. Babic´, Glasnik Hem Društva, Beograd 29, 11 (1964). 7. W. T. Grubb and L. H. King, Anal. Chem. 52, 270 (1980). 8. G. A. Perley and J. B. Goldshalk, U.S. Patent, 2, 416, 949 (1974). 9. M. M. Antonijevic´, R. P. Mihajlovic´, B. V. Vukanovic´, and S. Jovanovic´, Analysis 25, 152 (1997). 10. R. P. Mihajlovic´ and Z. D. Stanic´, J. Solid State Electrochem. 9, 558 (2005). 11. D. Ðurka, P. Pfendt, D. Jovancˇic´evic´, O. Cvetkovic´, and H. Wehner, Environ. Chem. Lett. 39, 39 (2005). 12. R. Mihajlovic, Z. Simic, Lj. Mihajlovic, A. Jokic, M. Vukašinovic, and N. Rakicevic, Anal. Chim. Acta 318, 287 (1996). 13. G. Palladino, D. Ferri, C. Manfredi, and E. Vasca, Anal. Chim. Acta 582, 164 (2007). 14. M. Fukushima, S. Tanaka, K. Hasebe, M. Taga, and H. Nakamura, Anal. Chim Acta 302, 365 (1995). 15. J. C. Massini, G. A. Abate, E. C. Lima, L. C. Hahn, M. S. Nakamura, J. Lichtig, and H. R. Nagatomy, Anal. Chim Acta 364, 223 (1998). 16. Z. Stanic, Ph.D. Thesis, University of Kragujevac (2006). 17. A. P. Kreshkov, Analiticeskaya khimiya v Nevodnykh rastvorakh, Khimiya, Moscow (1982). 18. J. M. Fernandez, C. Plaza, N. Senesi, and A. Polo, Chemosphere 69, 630 (2007). 19. S. Bratskaya, A. Golikov, T. Lutsenko, O. Nesterova, and V. Dudarchik, Chemosphere 73, 557 (2008). 20. M. M. Yee, T. Miyajima, and T. Takisawa, Colloids and Surfaces A: Physicochem. Eng. Aspects 272, 182 (2006). Sensor Letters 7, 1–7, 2009 7 R E S E A R C H A R T IC L E Copyright © 2010 American Scientific Publishers All rights reserved Printed in the United States of America SENSOR LETTERS Vol. 8, 1–8, 2010 Use of Sulphide Minerals as Electrode Sensors for Acid–Base Potentiometric Titrations in Non-Aqueous Solvents and Their Application for the Determination of Certain Biologically Active Substances Zoran Simic´1, Zorka Stanic´1∗, and Milan Antonijevic´2 1Department of Chemistry, Faculty of Sciences, University of Kragujevac, R. Domanovic´ 12, P.O. Box 60, 34000 Kragujevac, Serbia 2Technical Faculty Bor, University of Belgrade, 19210 BOR, Belgrade, Serbia (Received: 16 February 2010. Accepted: 14 May 2010) The use of certain sulphide minerals as electrode sensors, for acid–base potentiometric titrations in propionitrile/ethylene carbonate and nitromethane/ethylene carbonate as solvents, was investi- gated. The coulometrically generated base was obtained by the cathodic reduction of m-cresol and 3-methoxy phenol in the above mentioned solvents and it was used for the potentiometric titra- tion of p-toluenesulphonic, trichloroacetic, 5-sulphosalicylic and oxalic acids. For the potentiometric determination of the equivalence point, the use of the following electrode couples was investi- gated: glass—SCE, FeS2–SCE, CuFeS2–SCE, PbS–SCE, CuS–SCE and Cu2S–SCE. The natural minerals pyrite, chalcopyrite, galena, covellite and chalcocite were used as the indicator electrode sensors for the potentiometric determinations of some organic acids and bases, which represent biologically active substances, and also some substances that are toxic for the environment. The electrodes used for this purpose showed a linear dependence of the potential on the logarithm of the concentration of p-toluenesulphonic acid in the concentration range 0.5 to 100 mmoldm−3 with a sub-Nernstian slope. The standard deviation for such determination is less than 0.9%. Keywords: Sulphide Mineral Sensors, Non-Aqueous Solvents, Coulometry, Potentiometry. 1. INTRODUCTION In the appropriate non-aqueous solvents weak organic bases and acids are soluble and behave as strong bases and acids, allowing their accurate potentiometric titrations, thus, significantly contributing to the fundamental and applied research in the field of analytical chemistry. For the titration of bases in a non-aqueous environ- ment, perchloric acid in an appropriate solvent is most frequently used as the titrant. In order to avoid the pres- ence of water from the externally added HClO4, H + ions can be coulometrically generated by oxidation of certain organic compounds1 and hydrogen or deuterium adsorbed on palladium.2 The oxidation of hydrogen adsorbed on a palladium electrode proceeded with 100% current effi- ciency in a number of non-aqueous solvents. Vajgand, Mihajlovic´ and associates34 applied a palladium electrode ∗Corresponding author; E-mail: zorkas@kg.ac.rs saturated with hydrogen for the generation of H+ ions in acetic acid, acetic anhydride, some nitriles, ketones, alco- hols, cyclic esters, nitromethane, sulfolane, etc. By using generated H+ ions, many organic bases have been quan- titatively determined and some physical–chemical proper- ties of the solvents and dissolved substances (pKs , relative acidity scale, pKa) were also determined. 56 Some of the non-aqueous solvents that are suitable for the titration of acids are reduced on the cathode and form bases with quantitative contribution. The extensive use of coulometric determinations in alcohols and their mix- tures with ketones,78 benzene,9 and dimethylformamide8 is based on the reduction of alcohols on platinum elec- trodes used as cathodes. By reducing alcohol, an equiv- alent quantity of lyate ions is formed, which further reacts during titration with the investigated acid, follow- ing the equations: 2ROH+ 2e− ⇒ H2 + 2RO−, that is RO− +HA⇒ ROH+A−. If the solvent is not quantita- tively reduced on the cathode, it is possible to achieve Sensor Lett. 2010, Vol. 8, No. 6 1546-198X/2010/8/001/008 doi:10.1166/sl.2010.1346 1 R E S E A R C H A R T IC L E Use of Sulphide Minerals as Electrode Sensors for Acid–Base Potentiometric Titrations in Non-Aqueous Solvents Simic´ et al. quantitative reduction of the depolarizer by addition of suitable electro-active compounds, producing equivalent amounts of lyate ions. Bos and Dahmen10 added m-cresol (0.1 M) as the depolarizer to DMSO containing 0.2 M TEAP as supporting electrolyte. By the generation of m-cresolate ions, the quantitative determinations of picric, 2,4-dinitrobenzenesulfonic and salicylic acid, as well as of 2,6-dinitrophenol were performed. Streuli11 used water as a cathodic depolarizer in acetone—tetrabutylammonium perchlorate and acetone—tetrabutylammonium bromide. In this way, p-toluenesulphonic acid, benzoic acid and 2,4-dinitrophenol were quantitatively determined. In their work, Champion and Bush12 generated tetrabutylammo- nium hydroxide in tetrahydrofuran containing tetrabuty- lammonium perchlorate and 0.2% water. These authors under these particular conditions generated the base with 100% current efficiency and successfully determined a number of weak organic acids. The coulometric determi- nations of acids, apart from theoretical discussions, have found wide practical application. For example, a method for the determination of carbon in different substances by titration of the carbon dioxide released during the combus- tion of such substances has been elaborated.13 In addition, a method for the determination of the carbon contained in steel by titration in 2-propanol and dimethylformamide as solvents was developed.1415 Iwamoto16 titrated the hydro- gen ions originating from the reaction of aluminium with 8-oxychinoline and successfully determined the quantity of aluminium in the solution. Proceeding from the fact that propionitrile, nitromethane and ethylene carbonate are suit- able solvents for electrochemical acid–base investigations, in this work, the mixtures of these solvents were applied for the coulometric determination of acids through genera- tion of lyate ions by the reduction of cathodic depolarizers (m-cresol and 3-methoxy phenol). The glass electrode is the electrode most frequently employed in both aqueous and non-aqueous environments. However, despite its simlicity the glass membrane has a number of disadvantages, such as high electrical resis- tance, instability in a fluoride environment, problems with miniaturization, contamination of the glass membrane, non-applicability in extremely high or low pH values, etc. In a non-aqueous environment, there is a number of prob- lems, such as weak response, non-Nernstian dependence, low sensitivity, etc. On application of a glass electrode in non-aqueous environments, dehydration of the mem- brane frequently occurs, which reduces its useful life. In view of the aforementioned limitations, other types of pH electrodes have been developed based on a liquid membrane,17 PVC membrane in which tri-n-dodecylamine was imprinted,18 poly(ethylene) glycol hydrogel prepared by photolithography as pH-sensitive surface,19 metal and metalloid electrodes,2021 metal oxide electrodes,22–24 elec- trodes based on silica gel,25 talc,26 silicon nitride (Si4 N3), 27 chinhydron,28 vitreous carbon,29 H2/Pd and D2//Pd electrodes,1 monocrystalline sulphide electrodes,30–33 and wireless sensor for remote pH monitoring.34 Antonijevic´ et al.3536 applied natural sulphide minerals for the poten- tiometric determination of acids and bases as well as redox systems in both aqueous and non-aqueous environments. The investigations333536 showed that these electrodes can be successfully applied as indicator electrodes for the titra- tions of weak acids and bases. This paper describes investigations of the possibilities to use sulphide minerals as electrode sensors (FeS2, CuFeS2, PbS, CuS and Cu2S) for acid–base titrations in the above mentioned non-aqueous solvents and also the possibilities for the determination of some biological active compouns with acid–base properties, like barbituric acid, lysine, cys- teine, arginine. Sulpholane (tetramethylenesulphone) has been used as the medium for non-aqueous potentiomet- ric titration of barbituric acid, phenobarbital, amobarbital, barbital, secobarbital, sulphapyridine, sulphadiazine and sulphamerazine with tetrabutylammonium hydroxide.37 In coulometric titration using the biamperometric end-point detection, 0.1–20 mols of 2-thiobarbituric acid were suc- cessfully determined.38 A new potentiometric method was proposed to determine lysine in pharmaceutical samples.39 Results obtained by the potentiometric method are consis- tent with those taken by the standard method for amino- acid analysis. Elsewhere, a method is described for the determination of lysine, based on a flow injection dif- ferential potentiometric system.40 A chemically modified electrode was used as a sensitive electrochemical sensor for the detection and for the amperometric and differen- tial pulse voltammetric determination of cysteine.41 The results were compared with those taken by the potentio- metric method. Additionally, the potentiometric method as well as the biosensor4243 were used for the determina- tion of arginine. Taking into consideration the advantages of indicator electrodes with a solid membrane, natu- ral minerals of pyrite, chalcopyrite, galena, covellite and chalcocite were used as sensors for the potentiometric determination of acids and bases in propionitrile/ethylene carbonate and nitromethane/ethylene carbonate, as well as of some biologically active compounds (arginine, cysteine, lysine, barbituric acid), in propionitrile/ethylene carbonate as solvents. 2. EXPERIMENTAL DETAILS 2.1. Reagents Propionitrile and nitromethane (Fluka AG) were purified before application according to the procedure described in the literature.44 Ethylene carbonate was used without further purifica- tion. Propionitrile/ethylene carbonate and nitromethane/ ethylene carbonate were used as 1:1 mixtures. The acids and bases in this study were supplied by “Fluka AG” and 2 Sensor Letters 8, 1–8, 2010 R E S E A R C H A R T IC L E Simic´ et al. Use of Sulphide Minerals as Electrode Sensors for Acid–Base Potentiometric Titrations in Non-Aqueous Solvents “Carlo Erba” and were additionally purified prior to their application.1 The solutions of acids and bases were prepared by dis- solving a weighed amount of the required substance in the appropriate solvent. In cases the solutions were not pri- mary standards, their titles were determined by titration with hydrogen cations, coulometrically generated by oxi- dation of hydrogen adsorbed on palladium electrode, while the concentrations of the employed acid solutions were controlled by titration with a standard solution of 0.1 M tetrabutylammonium hydroxide (TBAH) in methanol. The title of the TBAH standard solution was coulomet- rically determined by oxidation of hydrogen adsorbed on the palladium electrode. 2.2. Apparatus and Electrodes The apparaturs employed for the coulometric titration of the acids was previously described.1 The polarization curves were recorded on a polarograph “Polarographic analyzer PA2” using “Elektra” software. A hydrogen palladium electrode was made according to the procedure described in the literature.2 The indicator electrodes based on the sulphide minerals were made by polishing the mineral and using the polished side as the working surface. The mineral was inserted into a glass tube and fixed with a methacrylate-based resin. Metallic mer- cury was added into the tube in order to make contact with the disk of the sulphide mineral and the inserted copper wirer. The working surface of the electrode was polished with Al2O3 powder, washed with water and ethanol, dried in air and it was ready for use. The potential was measured during the titration by means of an “Iskra” pH meter MA 5740. The variation of the potential of the covellite and chalcocite electrodes with time was followed in the respec- tive solvents. The response of CuS and Cu2S indicator electrodes in the respective solvents was investigated by measuring the potential in solutions of p-toluensulphonic acid (within the concentration range 0.5 mM–100 mM) in TBAP media. A conventional glass electrode (G 200 B, Radiome- ter) was used as the indicator electrode and the electrode was conditioned, in the appropriate solvent, 48 h before use. A modified saturated calomel electrode, SCE (401 Radiometer, Copenhagen) was applied as the reference electrode. The modification was realised by substituting the aqueous solution of KCl with a saturated solution of KCl in methanol. This modification was done in order to reduce the diffusion potential between the inner solution and the investigated solution. 2.3. Procedures 2.3.1. Coulometric Titration The procedure for the coulometric titration of the acids was previously described.45 2.3.2. Potentiometric Titration A certain volume (8.00 cm3 of the required solvent, that had previously been titrated, was placed in the titration vessel, followed by the addition of certain volume of the investigated acid or base (1.00 cm3 and thymol blue as indicator. The electrode couples FeS2–SCE, CuFeS2–SCE, PbS–SCE, CuS–SCE and Cu2S–SCE were immersed in the solution. The potentiometric titration was then carried out by the addition of 0.1 M standard TBAH or 0.1 M standard HClO4 in aliquots of 0.05 cm 3 and the potential was recorded after each addition of the titrant. The poten- tial measurements were taken at 2–4 min intervals during the course of the titration. The equivalence point (EP) was determined by the second derivative. 3. RESULTS AND DISCUSSION 3.1. Coulometric Determination of Acids Classical acid–base determinations in non-aqueous sol- vents are associated with experimental difficulties, such as the preparation of standard solvents, the relatively large volume required for the determination, the long time nec- essary for the overall determination, etc. A coulometrically generated titrant (strong acid or base) directly reacts with the substance to be analysed. The application of several substances, that are eas- ily reduced on the cathode producing lyate (base) ions as titrants in propionitrile/ethylene carbonate and nitromethane/ethylene carbonate as solvents, was investi- gated. The reduction potential of a compound, to be used for the cathodic generation of lyate (base) ions, must be more positive than the reduction potential of the solvent and the other components in the solution (indicator, titrated Fig. 1. Change in cathodic potential with current density in pro- pionitrile/ethylene carbonate containing TEAP as supporting elec- trolyte: 1. m-cresol; 2. oxalic acid; 3. p-toluensulphonic acid; 4. 5- sulphosalicylic; 5. solvent. Sensor Letters 8, 1–8, 2010 3 R E S E A R C H A R T IC L E Use of Sulphide Minerals as Electrode Sensors for Acid–Base Potentiometric Titrations in Non-Aqueous Solvents Simic´ et al. Fig. 2. Change in cathodic potential with current density in nitromethane/ethylene carbonate containing TEAP as supporting elec- trolyte: 1. 3-methoxy phenol; 2. oxalic acid; 3. 5-sulphosalicylic acid; 4. p-toluenesulphonic; 5. solvent. acid, conductive electrolyte). Since the reduction poten- tials of m-cresol in propionitrile/ethylene carbonate and 3- methoxyphenol in nitromethane/ethylene carbonate could not be found in the literature, IE curves were recorded under the same conditions as those applied for the titration of acids (Figs. 1 and 2). The figures show that the reduc- tion potentials of the previously mentioned compounds are 0.3 V–0.6 V more positive than the reduction potentials of the other compounds present in the solution. The following electrode reactions are taking place: 2C6H4 CH3OH+2e− ⇒ H2+2C6H4 CH3O− (1) 2C6H4 OCH3OH+2e− ⇒ H2+2C6H4 OCH3O− (2) Table I. Results of coulometric titrations of acids in solvent mixtures with lyate ions obtained by the reduction of m-cresol and 3-methoxy phenol with potentiometric end-point detection; I= 5 mA. Solvent Depolarizer Acid Taken (mg) Recovery (%) Potential jumps (mV/0.3 cm3 Propionitrile/ m-Cresol p-Toluenesulphonic acid 8.11 100.6±0.9 145 a ethylene carbonate 8.11 101.5±0.7 120 b 5-Sulphosalicylic acid 10.32 100.3±0.8 135 a 10.32 98.5±0.7 105 b Trichloracetic acid 7.34 99.4±0.9 148 a 7.34 99.7±0.9 128 b Oxalic acid 8.66 100.8±0.7 104 a 8.66 100.8±0.7 90 b Nitromethane/ 3-Methoxy phenol p-Toluenesulphonic acid 6.84 100.8±0.8 152 a ethylene carbonate 6.84 101.6±0.7 118 b 5-Sulphosalicylic acid 9.25 99.7±0.9 142 a 9.25 98.9±0.7 116 b Trichloracetic acid 7.34 100.7±0.8 158 a 7.34 101.4±0.8 125 b Oxalic acid 8.66 101.7±0.6 112 a 8.66 100.7±0.6 98 b a—FeS2–SCE; b—CuFeS2-SCE The determination of p-toluenesulphonic, 5- sulfosalicylic, trichloroacetic and oxalic acids is perfomed by potentiometric titration through coulometric generation of lyate ions using FeS2 and CuFeS2 as indicator elec- trodes (Table I). The rise of the potential at the equivalence point caused by the application of the above mentioned electrodes is slightly smaller than that obtained by the application of the glass electrode, however, it is sufficient for the precise determination of the equivalence point. The potential during the titration of acids, when applying FeS2 and CuFeS2 as indicator electrodes, is stable and it is rapidly established. 3.2. Potentiometric Titrations of Different Compounds Using Sulphide Minerals as Indicator Electrodes Pyrite, chalcopyrite, galena, covellite and chalcocite are natural minerals, which have been the subjects of stud- ies for their applications in metallurgy and technology. The electrochemical corrosion creates a large number of species by clearly defined mechanisms. In addition, the mechanism of the oxidation processes, taking place on these sulphide minerals, is still under investigation.3346–52 PbS+2H2O+2h+ ⇒ Pb OH2+S+2H+ (3) PbS+2H2O+2xh+ ⇒ Pb1−xS+xPb OH2 (4) 2PbS+7H2O⇒ 2Pb OH2+S2O2−3 +10H++8e− (5) CuFeS2+4H++O2 ⇒ Cu2++Fe2++2S0+2H2O (6) 2CuFeS2+6H++2e− ⇒ Cu2S+2Fe2++3H2S (7) CuFeS2 ⇒ Cu2++Fe3++2S0+5e− (8) FeS2+8H2O⇒ Fe2++2SO2−4 +16H++14e− (9) 4 Sensor Letters 8, 1–8, 2010 R E S E A R C H A R T IC L E Simic´ et al. Use of Sulphide Minerals as Electrode Sensors for Acid–Base Potentiometric Titrations in Non-Aqueous Solvents Fig. 3. Plots of the CuS electrode potential versus logc (concentrations) p-toluenesulphonic acid in: (1) nitromethane/ethylene carbonate and (2) propionitrile/ethylene carbonate. FeS2+7/2O2+H2O⇒ Fe2++2SO2−4 +2H+ (10) 2FeS2+15/2O2+4H2O⇒ Fe2O3+4SO2−4 +8H+ (11) CuS+2H++2e− ⇒ Cu2S+H2S (12) Cu2S⇒ CuS+Cu2++2e− (13) The products that are formed during oxidation (Cu2+, Cu+, Fe3+, Fe2+, So, SO2−4 ) depend on numerous factors, such as: the pH value, the nature of the oxidant, tempera- ture, the nature and concentration of the cations and anions present, chelating agents, etc. The Cu, Fe and Pb ions, sul- phur and sulphates, that are obtained as the end products of the oxidation of the sulphide minerals, are susceptible to hydrolysis depending on the pH of the environment. Cun++2kH2O⇒ Cu OH n−k+k +kH3O+ (14) Copper, iron and lead hydroxides, formed by hydrolysis on the surface of the mineral, build a precipitated surface layer on the sulphide mineral, that acts as a sensor for the corresponding H3O + ions. The formula for the potential of Fig. 4. Plots of the CuS electrode potential versus time in: (1) propi- onitrile/ethylene carbonate and (2) nitromethane/ethylene carbonate. Fig. 5. Plots of the Cu2S electrode potential versus logc (concentra- tions) p-toluenesulphonic acid in: (1) nitromethane/ethylene carbonate and (2) propionitrile/ethylene carbonate. such a sensor can be defined as: E = Eoox+ RT nF ln akH3O+ (15) where ox=Me OH n−k+k /MekSn. Equation (15) is the basis for the employment of sulphide minerals of iron, Fig. 6. Plots of the Cu2S electrode potential versus time in: (1) propi- onitrile/ethylene carbonate and (2) nitromethane/ethylene carbonate. Fig. 7. Potentiometric titration curves in propionitrile/ethylene carbon- ate: (a) lysine; (b) barbituric acid; 1. CuFeS2–SCE; 2. PbS–SCE; 3. CuS– SCE; 4. Cu2S–SCE; 5. FeS2–SCE. Sensor Letters 8, 1–8, 2010 5 R E S E A R C H A R T IC L E Use of Sulphide Minerals as Electrode Sensors for Acid–Base Potentiometric Titrations in Non-Aqueous Solvents Simic´ et al. copper and lead in both aqueous and non-aqueous environments. The dependence of the CuS electrode potential on the –logc was studied in solutions of coulometrically generated HClO4 in concentrations ranging from 0.5– 100.0 mmoldm−3. It was found that the CuS electrode showed a linear dependence of the potential versus –logc, with a slope of 46 mV/logc in propionitrile/ethylene car- bonate and 49 mV/logc in nitromethane/ethylene carbonate Table II. Results of potentiometric titrations of acids and bases in propionitrile/ethylene carbonate with TBAH or perchloric acid. Electrode couple Titrated substances (Taken mg) Found (%) Potential jumps (mV/0.3 cm3 Glass–SCE Benzoic acid 24.7 98.8±0.6 150 Arginine 21.8 100.2±0.7 120 Cysteine (hydrochloride) 20.1 99.3±0.8 165 Lizine 29.4 101.4±0.5 75 NN ′ -Diphenylguanidine 17.5 98.6±0.5 145 Antranylic acid 26.4 100.7±0.9 125 Barbituric acid 14.2 101.2±0.3 165 −Nitroso--naphtol 32.6 99.0±0.8 75 FeS2–SCE Benzoic acid 24.7 101.6±0.8 108 Arginine 21.8 98.5±0.6 124 Cysteine (hydrochloride) 20.1 100.7±0.7 125 Lizine 29.4 101.6±0.7 110 NN ′ -Diphenylguanidine 17.5 99.4±0.7 105 Antranylic acid 26.4 99.8±0.7 84 Barbituric acid 14.2 99.5±0.7 114 -Nitroso--naphtol 32.6 99.0±0.8 60 CuS–SCE Benzoic acid 24.7 100.8±0.6 99 Arginine 21.8 101.4±0.6 118 Cysteine (hydrochloride) 20.1 99.0±0.8 112 Lizine 29.4 100.8±0.7 100 NN ′ -Diphenylguanidine 17.5 99.5±0.3 94 Antranylic acid 26.4 100.9±0.9 75 Barbituric acid 14.2 99.5±0.7 107 −Nitroso--naphtol 32.6 101.2±0.8 56 Cu2S–SCE Benzoic acid 24.7 99.1±0.9 110 Arginine 21.8 100.9±0.8 130 Cysteine (hydrochloride) 20.1 101.4±0.6 128 Lizine 29.4 99.5±0.8 115 NN ′ -Diphenylguanidine 17.5 100.4±0.6 108 Antranylic acid 26.4 98.6±0.8 82 Barbituric acid 14.2 101.4±0.8 115 -Nitroso--naphtol 32.6 99.1±0.6 58 PbS–SCE Benzoic acid 24.7 98.4±0.5 98 Arginine 21.8 99.5±0.7 115 Cysteine (hydrochloride) 20.1 100.3±0.5 115 Lizine 29.4 101.2±0.9 102 NN ′ -Diphenylguanidine 17.5 99.1±0.4 95 Antranylic acid 26.4 100.6±0.4 80 Barbituric acid 14.2 101.3±0.5 106 -Nitroso--naphtol 32.6 100.8±0.3 55 CuFeS2–SCE Benzoic acid 24.7 98.2±0.8 105 Arginine 21.8 98.8±0.4 121 Cysteine (hydrochloride) 20.1 99.3±0.7 122 Lizine 29.4 98.9±0.6 108 NN ′ -Diphenylguanidine 17.5 101.7±0.4 102 Antranylic acid 26.4 99.3±0.9 79 Barbituric acid 14.2 98.7±0.2 111 -Nitroso--naphtol 32.6 100.7±0.5 58 (Fig. 3). In order to examine the rate of the potential shift, the variation of the potential with time in these solvents was investigated (Fig. 4). The figure shows that the poten- tial of the electrode is shifted in both solvents by 3–5 mV per hour. If the potentiometric titration lasts approximately 30 minutes, then the potential shift of the CuS electrode, within this time interval, is approximately 2 mV. Since for the determination of the EP, only the values of the potential around the EP are important, and this procedure lasts for 6 Sensor Letters 8, 1–8, 2010 R E S E A R C H A R T IC L E Simic´ et al. Use of Sulphide Minerals as Electrode Sensors for Acid–Base Potentiometric Titrations in Non-Aqueous Solvents few minutes, the shift of the electrode potential during this time can be practically ignored. The CuS electrode in these solvents showed a sub-Nerstian dependence. The dependence of the potential on the concentration of HClO4 for a Cu2S electrode showed that the value of the slope was 51 mV/logc in nitromethane/ethylene car- bonate and 55 mV/logc in propionitrile/ethylene carbonate (Fig. 5), which indicate a sub-Nerstian dependence, as in the case of the CuS electrode. After addition of acids or bases to the solution, the potential is rapidly established. In order to investigate the possibilities of the Cu2S as an indicator electrode, the electrode was coupled with a SCE and the change of the potential with time was monitored (Fig. 6). The potential slightly changes over time. The behaviour of sulphide minerals as indicator elec- trodes was examined by titration of organic acids and bases of different strengths and structures, such as: ben- zoic acid (pKa H2O = 4.19), anthranilic acid (pKa H2O = 4.95), cysteine hydrochloride (pKa H2O cysteine = 8.33), barbituric acid (pKa H2O = 3.9), NN ′-diphenylguanidine (pKa H2O = 10.12), arginine (pKa H2O = 12.48) and lysine (pKa H2O = 10.79). For the titration of acids and bases, 0.1 M TBAH in methanol and 0.1 M solution HClO4 were respectively used. Titration curves of the determined sub- stances are given in Figure 7, while the results of the determinations are given in Table II. When for the titra- tion of the investigated acids the electrode couple FeS2– SCE was applied, the increase of the potential at the EP was: for benzoic acid, 108 mV/0.3 cm3; anthracitic acid, 84 mV/0.3 cm3; barbituric acid, 114 mV/0.3 cm3 and cysteine hydrochloride, 125 mV/0.3 cm3, while for the titrated bases on application of the same electrode couple, the increase of the potential at the EP was: for NN ′-diphenylguanidine, 105 mV/0.3 cm3; arginine, 124 mV/0.3 cm3 and lysine, 110 mV/0.3 cm3. Similar increases of the potential at the EP were obtained, when the other electrode couples (except Cu2S–SCE and CuS– SCE) were employed. When the Cu2S and CuS elec- trodes were used as indicator electrodes, the increases were slightly smaller but still sufficient for determining the EP with good precision and reproducibility (Table II). In addi- tion, a sufficient increase of the potential at the EP was obtained, when the weakest acid -nitroso--naphthol was titrated in these solvents, applying a Cu2S–SCE or CuS– SCE electrode couple. In literature3953–58 different methods were used for inves- tigation of barbituric acid, lysine, cysteine, and arginine in various solvents and solvent mixtures, using a direct poten- tiometry and coulometry. The hydrogen electrode, some metal electrodes, sulphides and various enzyme electrodes were used for the determination of the EP. The modified carbon-paste electrode was applied as a sensitive sensor for potentiometric, amperometric, and DPV determination of cysteine.41 A linear response in the concentration range from 2 M to 0.01 M was obtained with a detection limit of 1 M for the potentiometric determination of cysteine. The performance characteristics of the modified electrode in conjunction with the simplicity of its preparation and the renewability of its surface by simple polishing, demon- strates its analytical utility as a sensor for the determi- nation of cysteine. A potentiometric biosensor based on the monitoring of the ammonium, enzymatically released, was used in determining lysine in pharmaceutical samples (the detection limit was 6.0×10−6 M).39 A new coulomet- ric titration method with potentiometric end-point detec- tion for the determination of barbituric acid was presented elsewhere.57 In the presented method, 1–200 mols of bar- bituric acid were successfully determined. Monomolecular and bimolecular bienzymatic layers immobilized directly on the ammonium sensitive membrane were used for the detection of arginine.58 Therefore, the presented biosensors were stable, responded rapidly, the linear ranges of both biosensors were 0.l–30 mM and the detection limits were below 10−5 M. Comparing the proposed determination of these bio- logically active substances with the methods previously reported,3953–58 one may conclude that it is characterized by simple electrolytic cell and electrodes, short analysis time, simple procedure, and commonly available reagents. It was showed that the applied sulphide electrodes gave results in good agreement with the results presented in previous papers. The applicability of the electrodes was demonstrated by the determination of the chosen com- pounds in pharmaceutical dosage forms. 4. CONCLUSION By the coulometric generation of lyate ions, through the cathodic reduction of m-cresol and 3-methoxyphenol in propionitrile/ethylene carbonate and nitromethane/ethylene carbonate as solvents, compounds with weak acidic prop- erties could be successfully titrated. Indicator electrodes based on FeS2, CuFeS2, PbS, CuS and Cu2S minerals could be successfully applied for the potentiometric determination of weak organic acids and bases as well as biologically active compounds and some substances toxic for the living environment presenting weak acidic and basic properties. Their simplicity of use, chemical inertness and the stability of the potential dur- ing titrations enable the application of these electrodes for titrations in non-aqueous solvents and their mixtures. Acknowledgment: This work is supported by the Min- istry of Science and Technological Development of the Republic of Serbia (Project No. 142060). References and Notes 1. R. Mihajlovic´, V. Vajgand, and Z. Simiæ, Anal. Chim. Acta 265, 35 (1992). 2. R. Mihajlovic´, Z. Simic´, L. Mihajlovic´, A. Jokic´, M. Vukašinovic´, and N. Rakiæevic´, Anal. Chim. Acta 318, 287 (1996). 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Printed in Brazil - ©2011 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 A *e-mail: zorkas@kg.ac.rs Application of Pyrite and Chalcopyrite Electrodes for the Acid-Base Determinations in Nitriles Zoran Simić,a Zorka D. Stanić*,a and Milan Antonijević b aDepartment of Chemistry, Faculty of Science, University of Kragujevac, R. Domanović 12, P.O. Box 60, 34000 Kragujevac, Serbia bTechnical Faculty Bor, University of Belgrade, 19210 BOR, Belgrade, Serbia Os minerais naturais pirita e chalcopirita tem sido usados como sensores em eletrodos indicadores usando acetronitrila e propionitrila como solventes. Os resultados mostram vantagens excepcionais da pirita e chacopirita como sensores eletroquímicos e também a possibilidade de aplicação desses eletrodos para determinações em solventes não aquosos onde eletrodos de vidro mostram dificuldades. O comportamento dos eletrodos indicadores de pirita e chalcopirita em nitrila foi avaliado pela titulação de diversas bases e ácidos de forças diferentes usando prótons e íons liato gerados pela oxidação de alguns ésteres de ácido gálico e fenóis diidróxi, bem como m-cresol e 3-metóxifenol. Os resultados obtidos com esses eletrodos foram comparados com os de eletrodos de vidro nas mesmas condições, obtendo-se boa concordância, reprodutividade e precisão. Os desvios padrão na determinação dos ácidos e bases investigados foram menores do que 0,9%. Em titulações potenciométricas com pirita e chalcopirita como eletrodos indicadores, o potencial é instantaneamente estabelecido. Ambos os eletrodos podem ser usados sem qualquer limitação temporal ou divergências potenciais consideráveis. The natural minerals pyrite and chalcopyrite have been used as the indicator electrode sensors using acetonitrile and propionitrile as solvents. The results show the exceptional advantage of pyrite and chalcopyrite as electrochemical sensors and also the possibility of the application of these electrodes for the determination in nonaqueous solutions where glass electrode shows many defaults. The behaviour of the pyrite and chalcopyrite indicator electrodes in nitriles were checked by titrating several bases and acids of different strengths using protons and lyate ions generated by the oxidation of some gallic acid esters and dihydroxy phenols, as well as m-cresol and 3-metoxy phenol. The results obtained by using these electrodes were compared with those obtained by the application of a glass electrode under the same conditions, and good agreement, reproducibility and accuracy were obtained. The standard deviation of the determination of the investigated acids and bases was less than 0.9%. In the potentiometric titrations with pyrite and chalcopyrite indicator electrodes, the potential is instantaneously established throughout. Both electrodes can be used without any time limit or without considerable divergence in potentials. Keywords: pyrite, chalcopyrite, hydrogen and lyate ions, nonaqueous solutions Introduction Low molecular weight nitrides, as solvents from the dipolar aprotic solvents (DAS) group, have been very successfully applied in electrochemical investigations as they are chemically inert, and can be oxidized and reduced only with difficulty. From this group of solvents, acetonitrile was found the widest application, whereas other nitriles, such as propionitrile, butionitrile and benzonitrile, have been used more rarely. Although the above-mentioned solvents possess all properties of a good solvent for electrochemical investigations, only a few data (with the exception of acetonitrile) on their use in acid-base determinations have been reported.1 Using coulometric methods for the determination of acids and bases and owing to the electrochemically obtained titrant with a high current efficiency, it is possible to analyze solutions of low concentrations and to determine accurately and reproducibly small amounts of substances. In the course of the coulometric determinations of bases in Application of Pyrite and Chalcopyrite Electrodes for the Acid-Base Determinations in Nitriles J. Braz. Chem. Soc.710 non-aqueous solvents, the electrochemical generation of hydrogen ions is realized by the oxidation of an appropriate electroreactive compounds (anodic depolarizers). Previously, the generation of hydrogen ions was performed quantitatively by the oxidation of hydroquinone and thus a new field of investigation,1 with the application of some other compounds of low oxidation potentials, was opened. Some authors,2-5 have applied a whole range of organic compounds, mercury and hydrogen dissolved in palladium as the medium for the generation of H+ ions in the coulometric titrations of bases with visual, photometric and potentiometric end point determination. For the titration of bases in some aprotic solvents, instead of a standard solution of perchloric acid that is unstable in these solvents, Mihajlovic et al.6 used hydrogen ions generated by the electro-oxidation of some organic depolarizers at a platinum anode. It was emphasized that the application hydrogen or deuterium dissolved in palladium have advantages over many classical depolarizers because no foreign substances are introduced into the solution being analyzed.7 As the medium for the catholic generation of lyate ions in the coulometric determinations of acids, some alcohols, m-cresol and dimethylsulfoxide were used.8-10 Under the certain conditions, a reduction of the above-mentioned compounds is performed, whereby a hydrogen is separated at a platinum cathode and lyate ions are generated with a high percentage current efficiency. With the use of this method, many weak organic acids were determined. In organic acids coulometric determination, bases were generated in a solution of some organic salts or tetraethyl- ammonium halogenide. Streuli et al.11 coulometrically titrated benzoic acid in acetone using tetraethyl-ammonium bromide and tetraethyl-ammonium perchlorate as the supporting electrolyte and generated hydroxide ions by the reduction of water, which was added to the solution up to a concentration of 1% (m/v). Fritz and Gainer12 titrated acids by generating the base in t-butanol, while Johansson13 generated the base in 2-propanol, and a mixture of 2-propanol and methyl acetone as solvents. For more than 60 years, pH-glass electrodes have been widely used, and pH measurements in various samples are still made using these electrodes. They are very popular due to their high selectivity and dynamic pH range. However, in spite of the distinctive potential characteristic of pH-glass electrodes and their use in routine pH measurements for so many years, they have certain limitations, such as high resistance, brittleness, instability in hydrofluoric acid and in fluoride-containing media as well as in nonaqueous solutions. They are also challenging in the construction of microelectrodes for biological applications and in vivo measurements. That is why studies related to the construction of nonglass pH electrodes have been gaining momentum. The construction of pH electrodes based on neutral carriers will solve most of these problems. While developing alternative possibilities for the H+ ions determination during acid-base titrations, many authors investigated applications of a metal and metalloid electrodes,14,15 metal oxide electrodes,16-18 monocrystalline sulphide electrodes.19-22 Sulphide minerals (pyrite, chalcopyrite and galena) have been investigated as indicator electrodes for acid-base23,24 and redox titration in some non-aqueous solvents. The anodic dissolution process of chalcopyrite using various auxiliary analytical methods for the identification of the reaction products25,26 and the cathodic reduction of chalcopyrite27 have been investigated by different authors. Antonijevic et al.28 used natural monocrystalline chalcopyrite for potentiometric titration in water. Mihajlovic et al.23,29 used natural monocrystalline chalcopyrite for potentiometric acid-base and redox titration in some nonaqueous solutions. The facts that all of these natural monocrystallines have low electrical resistance and that they are easy to construct have increased interest in the implementation of these electrodes in place of glass membranes. These electrodes are of great practical importance, since they are solid and unbreakable. Proceeding from the fact that acetonitrile and propionitrile are good solvents for electrochemical acid- base investigations, in this work, propionitrile was applied for the coulometric determination of bases with hydrogen ions generated by the oxidation of anode depolarizers (dihydroxy, trihydroxy phenols, and some of esters of gallic acid). The m-cresol and 3-methoxy phenol we used in acetonitrile and propionitrile as cathodic depolarizers and as the medium for the generation of lyate ions in the coulometric determination of acids. No data are reported in the literature on its application for coulometric generation lyate ions in these solvents. For the potentiometric detection of the end point, monocrystalline pyrite (FeS2) and chalcopyrite (CuFeS2) were used and the results compared with those obtained with a glass electrode. Experimental Reagents All the investigated depolarizers and titrated bases were of p.a. purity, Merck or Fluka. Before use, the liquid bases were dried over fused potassium hydroxide and then distilled under reduced pressure. The concentration of the solutions of bases was checked by titration with H+ ions generated by the oxidation of hydrogen dissolved in palladium. All the employed acids were of p.a. purity, Merck and Fluka. Before use, the acids were standardized Simić et al. 711Vol. 22, No. 4, 2011 with tetraethylammonium hydroxide (TBAH) in methanol. Acetonitrile and propionitrile were purified before use by a procedure described in the literature.30,31 As the conducting salt in acetonitrile and propionitrile for coulometric determination of bases, a 0.25 mol L-1 solution of sodium perchlorate was used. As the conducting salt in solvents for coulometric determination of acids, a 0.1 mol L-1 tetrabutylammonium perchlorate was used. Apparatus and electrodes The apparatus employed for the coulometric titration of bases was described previously.7 The current source was a voltage and current stabilizer. The anode and cathode compartments were separated by a G-4 sintered glass disc. The volume of the investigated solution was 20.00 mL and that of the catholyte 5.00 mL. Platinum spirals of surface area 25 mm2 were used as the anode and cathode. A conventional glass electrode (G 200 B, Radiometer) was used as the indicator electrode and a modified saturated calomel electrode (401 Radiometer, Copenhagen) was used as the reference electrode. The modification of the normal SCE has been done by complete replacement of the inner solution with the saturated solution of potassium chloride in corresponding solvent. This modification has been done to decrease the liquid junction potential between inner solution of the SCE and investigated solution. The glass electrode was conditioned in the appropriate solvent 48 h before use. The experiments were carried out with either a sample of natural pyrite or a chalcopyrite crystal from the Veliki Krivelj copper mine (Bor, Serbia). Chemical analyses of the minerals showed that the pyrite contained 44.3% Fe, 52.5% S, and 0.6% Cu and the chalcopyrite 27.1% Fe, 34.2% S, and 33.2% Cu. The indicator chalcopyrite electrode was prepared in the following manner:23 A quadratic piece of chalcopyrite (a = 0.5 cm) was used as the electrode material. The chalcopyrite electrode was made by polishing the chalcopyrite crystal with diamond paste, and the best polished side was used as the working surface of the electrode. A narrow glass tube was fixed with glue to the other side of the electrode and then filled with mercury. One end of a copper wire was immersed in the mercury and this device was mounted into a wider glass tube (ø = 1 cm) which was then cemented with a cold sealing mass based on methyl methacrylate. After solidification of this mass, the working surface of electrode was polished to a high glow. The electrode was then rinsed with distilled water and alcohol, and dried on air after which it was ready for use. A pyrite indicator electrode was prepared in a similar way. The pyrite and chalcopyrite electrodes were made by polishing a pyrite (chalcopyrite) crystal with Al2O3 and the best polished was used as the working surface of the electrode. The potential was measured during the titration by means of a “Iskra” pH meter MA 5740. The current- potential curves (anode and cathode) were recorded on a Polarographic analyzer PA2. Potentiometric measurements Stationary potential measurements of the electrodes were carried out in a series of p-toluenesulfonic acid in the concentration range of 0.1-0.001 mol L-1. The potential of the pyrite (chalcopyrite) electrode with time was followed in a temperature-controlled cell (25 ± 0.1˚C). The ionic strength of the solution was maintained with 0.05 mol L-1 tetrabutylammonium perchlorate. The potential values determined in this way were used to calculate of the slopes. The change in the potential of the pyrite (chalcopyrite) electrode with time was followed in the required solvent. This indicator electrode was coupled with a modified SCE as the reference electrode. Potentiometric end point detection The supporting electrolyte was added to a certain level into the cathode compartment of the vessel and a platinum spiral was immersed in it; a titrated supporting electrolyte solution was poured into the anode compartment up to the same level and the depolarizer was added. A Pt anode and an electrode couple (glass-SCE, CuFeS2-SCE, FeS2-SCE) were immersed in the investigated solution. During the determination of acids, the investigated acid was added into the cathode compartment and after the current was switched on, lyate ions were generated. The solution was vigorously stirred with a magnetic stirrer during the titration. The potential was measured after each addition of lyate ions, at 2-3 min interval. The end point was determined by the classical method from the second derivative or the Gran method.32 Several samples can be determined successively in the same supporting electrolyte. The procedure for the determination of bases was the same as for the determination of acids, except the generation of acids (hydrogen ions) was performed in the anode compartment of the electrolysis vessel. Results and Discussion Mechanism of the indicator pyrite (chalcopyrite) electrode Pyrite and chalcopyrite are sensors which potential depends on many factors: the nature of the oxidant, the Application of Pyrite and Chalcopyrite Electrodes for the Acid-Base Determinations in Nitriles J. Braz. Chem. Soc.712 pH value, the temperature, the nature and concentration of the present cations, anions and other chemical species. The electronic structure of pyrite indicates its low-spin complex with d2sp3 hybridization. The hybrid orbitals are occupied with electrons, which makes pyrite a non- reactive compound. Pyrite, FeS2, is a mineral that possesses characteristic electric and magnetic properties. It is sparingly soluble in water and it catalytically reduces oxygen present in the solution. The reduction of oxygen causes the anionic dissolution FeS2 to Fe2+ and SO42– following the equation:33 (1) Fe2+ ions in an oxidative environment are oxidized to Fe3+ ions. The hydrolysis of iron cations can be described by the following equation: (2) The formation of hydroxide according to equation (2) forms a film on the surface of the pyrite crystals, which actually represents a hydroxysulfide/metal electrode (Fe(OH)k(n-k)+/FeS2), which potential is defined by the equation: (3) Equation (3) shows that the potential of the electrode depends on the activity of H+ ions when used in both water and non-water environment that contains weak acids or bases. Similar to pyrite, the mineral chalcopyrite, CuFeS2, is also sparingly soluble in water and can only be dissolved in the presence of strong oxidizing agents, thus generating products of different composition. The oxidation can be represented by the equations:34-38 (4) (5) (6) (7) (8) (9) From these equations, in addition to elementary sulfur and sulfates, Fe2+, Fe3+ and Cu2+ cations are also products of the oxidation, which hydrolyze. As iron cations are hydrolyzed at lower pH values than copper ones, only the hydrolysis of iron cations will be considered in the further text. The hydrolyzed iron cations influence the formation of a hydroxysulfide / metal layer, Fe(OH)k(n-k)+/CuFeS2, on the surface of the CuFeS2 crystals. This layer actually represents an electrode the potential of which is described by equation (3) for both an aqueous and non-aqueous environment. Characteristics of the indicator pyrite (chalcopyrite) electrode In order to apply an ion selective electrode as a sensor for use in quantitative measurements, a stable potential in both acid and base environments, a relatively short response time, and a long life-time must be ensured. Potential of the electrodes The stationary potential of a pyrite and a chalcopyrite electrode in acetonitrile and propionitrile was measured by direct potentiometry at 25 ± 0.1 °C in a freshly prepared 0.05 mol L-1 solution of p-toluenesulfonic acid in the appropriate solvents. All measurements were performed in the presence of a background electrolyte of constant ionic strength (0.05 mol L-1 tetrabutylammonium perchlorate) in order to minimize the effect of streaming and diffusion potentials in the streaming sample solution. In the both of the investigated solutions, a stable potential was attained in less than 4-5 min. Slope of the potential response of the electrodes The potential of the electrodes were determined using a series of p-toluenesulfonic acid in the concentration range of 0.1-0.001 mol L-1 in acetonitrile and propionitrile in a temperature-controlled cell (25 ± 0.1°C). The ionic strength of the solutions was maintained with 0.05 mol L-1 tetrabutylammonium perchlorate. It was found that the electrodes show sub-Nernst dependence. Since the electrodes exhibit sub-Nernst dependence, those cannot be used for measuring the pH of a solution. However, the potential of the electrodes as indicator electrodes are very stable with respect to time: hence, those can be successfully applied to the titration of acids and bases in acetonitrile and propionitrile as the solvents. Response time of the electrodes The response time of a pyrite and a chalcopyrite electrode was determined by recording the time elapsed before a stable potential value was attained after the pyrite (chalcopyrite) electrode and the reference electrode (modified SCE) were immersed in calibration solutions Simić et al. 713Vol. 22, No. 4, 2011 from highly acidic (0.05 mol L-1 of p-toluenesulfonic acid) to highly basic (0.05 mol L-1 TBAH) solutions. From the acidic (p-toluenesulfonic acid) to the basic region (TBAH), the change of the electrode potential for the pyrite electrode ranged from –190 to +415 mV (acetonitrile) and for the chalcopyrite electrode ranged from –365 to +496 (acetonitrile). Therefore, the potential changes at the TEP (titration end point) for the chalcopyrite electrode were greater than those for the pyrite electrode (Table 1). The response time for the pyrite was 12 s and for the chalcopyrite electrode was 11 s in the investigated solvents. Long-term stability (lifetime) and repeatability The lifetime of the electrodes was determined by raising the potential values of the calibration solution (p-toluenesulfonic acid) and plotting the calibration curves for a period of of 1 year. The slope of the electrodes remained constant. When the electrodes are not used for titrations, these are kept in a dry place protected from dust. Before the next use, the electrodes are kept in the investigated solvent for half an hour. However, if the electrodes had been used frequently and for a long time, it is necessary to rub the crystal pyrite (chalcopyrite) with aluminum oxide, wash the electrodes and continue with use. In order to establish the efficiency of use of the electrodes in potentiometric titrations and the repeatability of the results obtained, the titration of p-toluenesulfonic acid (acetonitrile) was selected as a model and it was repetitively carried out for five times and the end point was monitored by using pyrite electrode. The results obtained for the titration p-toluenesulfonic acid (acetonitrile) were shown in Figure 1. The relative standard deviation (RSD) for the end point determination of titration was found to be 0.5% (Table 2). Coulometric-potentiometric determinations of acids using pyrite and chalcopyrite as indicator electrodes For the coulometric determination of acids, lyate ions can be generated either by direct cathodic reduction at a platinum cathode or by the reduction of the solvents with a high current efficiency and by the reduction of suitable electroreactive compounds. The condition for the application of electroreactive compounds (cathodic depolarizer) in the solution is that they are reduced quantitatively before the reduction of the other components occurs. The first alkalimetric titrations in non-aqueous solvents employed a strong base as titrant that was obtained by the reduction of water at a platinum electrode.39 Tetrabutylammonium bromide and tetrabutylammonium perchlorate as the supporting electrolyte were used to generate hydroxide ions by the reduction of water, which had been added to the solution in a concentration of up to 1% (m/v).40 Few data in the literature report the application of organic compounds, which on reduction produce a strong base. Water can be used as a cathodic depolarizer (in concentrations of up to 0.5% (m/v)), although it has an adverse effect on the conditions of the titration. In this study, a procedure was developed for the direct coulometric generation of strong bases by reduction of the organic compounds m-cresol and 3-methoxy phenol in nitrile media. m-Cresol and 3-methoxy phenol are reduced with ease and the equivalent amount of lyate ions separate at the platinum cathode. Such generated ions can be used as the strong bases for the determination of acids. The Table 1. Potential jumps (mV) at the end-point in the coulometric-potentiometric titrations of acids in acetonitrile and propionitrile Solvent Titrated acid glass-SCE CuFeS2-SCE FeS2-SCE Acetonitrile p-Toluensulphonic 155 158 118 5-Sulphosalicylic 135 130 108 Oxalic acid 120 107 110 Propionitrile p-Toluensulphonic 132 95 75 5-Sulphosalicylic 145 130 78 Oxalic acid 120 98 105 Figure 1. Five titration curves of p-toluenesulfonic acid in acetonitrile obtained by using m-cresole and pyrite as indicator electrode. Application of Pyrite and Chalcopyrite Electrodes for the Acid-Base Determinations in Nitriles J. Braz. Chem. Soc.714 reduction of m-cresole and 3-metoxy phenole according to the equations: (10) (11) In order to establish the approximate potentials of the reduction of the components of the employed system of nitrile-0.05 mol L-1 TBAP (tetrabutylammonium perchlorate), voltametric curves were recorded under the same conditions as those used for the determination of the acids (Figure 2). From the figure, it can be seen that m-cresol is reduced in acetonitrile at a potential of about –0.9 V, which is about 0.7 V more positive than the reduction potential of the other components. p-Toluenesulfonic, trichloracetic, oxalic and 5-sulfosalicylic acids were titrated with m-cresol ions generated at the cathode. Also, on the basis of its structure, it would be expected that, like m-cresol, 3-methoxy phenol would be reduced and generate an equivalent amount of lyate ions. Current-potential curves Table 2. Results of coulometric titrations of acids in acetonitrile and propionitrile with lyate ions obtained by the reduction of m-cresole, and 3-metoxy phenole with potentiometric end-point detection; I = 5 mA Solvent Depolarizer Titrated acid Taken (mg) Recovery (%) Acetonitrile m-Cresole p-Toluensulphonic 8.75 100.3 ± 0.3 (a) 8.75 99.1 ± 0.5 (b) 8.75 98.7 ± 0.7 (c) 5-Suphosalicylic 11.54 100.9 ± 0.2 (a) 11.54 100.7 ± 0.9 (b) 11.54 98.8 ± 0.5 (c) Trichloracetic 10.29 99.5 ± 0.4 (a) 10.29 99.1 ± 0.9 (b) 10.29 101.3 ± 0.6 (c) Oxalic acid 9.24 99.6 ± 0.5 (a) 9.24 100.7 ± 0.7 (b) 9.24 98.6 ± 0.5 (c) Propionitrile 3-Metoxy phenole p-Toluensulphonic 7.31 100.6 ± 0.8 (a) 7.31 101.8 ± 0.9 (b) 7.31 100.4 ± 0.8 (c) 5-Suphosalicylic 9.67 98.7 ± 0.6 (a) 9.67 101.8 ± 0.9 (b) 9.67 99.4 ± 0.8 (c) Trichloracetic 8.13 100.7 ± 0.8 (a) 8.13 98.4 ± 0.8 (b) 8.13 98.4 ± 0.7 (c) Oxalic acid 6.82 101.5 ± 0.7 (a) 6.82 99.6 ± 0.4 (b) 6.82 98.9 ± 0.6 (c) (a) Glass-SCE; (b) FeS2-SCE; (c) CuFeS2-SCE. Figure 2. Change in cathodic potential with current density in acetonitrile: 1) solvent; 2) trichloracetic acid; 3) 5-sulphosalicylic acid; 4) 3-metoxy phenole; 5) m-cresole. Simić et al. 715Vol. 22, No. 4, 2011 recorded for 3-methoxy phenol, acetonitrile, and the titrated acids showed that the reduction potential of 3-methoxy phenol was higher than those of the other components in the solutions (Figure 2). Accordingly, it can be seen that m-cresol and 3-methoxy phenol were reduced at a much more positive potential than those of the titrated acids and solvent used; thus, the condition for the application of these compounds as a cathodic depolarizers is satisfied. The results of the determinations of 20-100 milliequivalent of acid are shown in Table 2. The end point detection was performed potentiometrically with a glass electrode, a pyrite and a chalcopyrite as the indicator electrodes and with a modified saturated calomel electrode as the reference. The results given in the Table show that the current efficiency was within 98.4-101.8%, with a good reproducibility. The titration curves of p-toluenesulfonic, trichloracetic, oxalic, and 5-sulfosalicylic acids in acetonitrile and propionitrile with the application of the electrode couples FeS2-SCE and CuFeS2-SCE are shown in Figures 3 and 4. In the coulometric titration of, for example, p-toluenesulfonic acid with the application of the FeS2-SCE electrode pair, the rise of potential at the EP was 118 mV, whereas under the same conditions but applying the CuFeS2-SCE electrode pair, the rise of the potential at the EP was 158 mV (Table 1). Using a glass indicator electrode under all conditions, a slightly higher increase of potential (155 mV) was obtained at the EP. In propionitrile as the solvent, using coulometrically generated lyate ions for the titration of the same acid, the increase in the potential at the EP was 75 mV and 95 mV for the electrode pairs FeS2-SCE and CuFeS2-SCE, respectively. As in the previous case, a higher increase of the potential was achieved by application of the glass electrode during the titration of the above mentioned acid (Table 1). Using propionitrile as solvent, with the FeS2 electrode as the indicator electrode, the rises of the potential at the EP were slightly higher compared to those obtained using the CuFeS2 electrode. Also, Table 1 shows that in all cases in which the sulfide electrodes were employed, the increases of the potential at the EP were slightly smaller when compared those obtained using a glass electrode. Although the highest potential jumps were obtained with the glass electrode, a glass electrode has a limited useful life because organic solvents dehydrate its membrane. On the other hand, the potential during the titration was very stable and rapidly established when the sulfide electrodes (FeS2 and CuFeS2) were applied. Coulometric-potentiometric determinations of bases using pyrite and chalcopyrite as indicator electrodes Dipolar aprotic solvents affect not only the solubility of substances and their acid-base properties but they also enlarge and extend significantly the potential region for possible measurements. For this reason, these solvents can be used more successfully as a medium for the investigations of numerous compounds than water. Under certain conditions, some suitable electroreactive compounds are oxidized quantitatively with the generation of acids (H+ ions) in the propionitrile as solvent at the positively polarized platinum anode. In previous studies, it was demonstrated that H+ ions can be generated by the oxidation of hydrogen dissolved in palladium and also by the oxidation of some organic compounds with a low oxidation potential.3,6,41-45 Some dihydroxy and trihydroxy phenols and some esters of gallic acid can be applied for the coulometric generation of acids (hydrogen ions) in some dipolar aprotic solvents, such as acetonitrile, acetic acid-acetic anhydride, nitromethane, sulfolane and g-butyrolactone.6,41 As propionitrile is a Figure 3. The effect of the indicator electrode on the shape of the end-point inflection in the coulometric-potentiometric titration of acids in acetonitrile by using m-cresole: a) FeS2-SCE; b) CuFeS2-SCE; 1) trichloracetic; 2) oxalic; 3) p-toluenesulfonic; 4) 5-sulfosalicylic acid; I = 5 mA. Figure 4. The effect of the indicator electrode on the shape of the end- point inflection in the coulometric-potentiometric titration of acids in propionitrile by using 3-metoxy phenole: a) FeS2-SCE; b) CuFeS2-SCE; 1) trichloracetic; 2) oxalic; 3) p-toluenesulfonic; 4) 5-sulfosalicylic acid; I = 5 mA. Application of Pyrite and Chalcopyrite Electrodes for the Acid-Base Determinations in Nitriles J. Braz. Chem. Soc.716 good solvent and as the literature contains little data on its application to coulometric determinations, it was considered of interest to investigate the possibility of applying m-dihydroxy benzene and some gallic acid esters for the coulometric generation of acid. These depolarizers have low oxidation potentials, much lower than those of other components present. In order to establish whether the used compounds could be applied for the generation of acids in the investigated solvent, IE curves for the solvents, bases, indicators and anode depolarizers were recorded (Figure 5). The IE curves were recorded under the same conditions as those employed for the coulometric-potentiometric determination of bases. The employed compound oxidized at potentials about 0.2-0.8 V lower than the potential of the other components present in the solution. The difference between the oxidation potential of the depolarizer and those of the other compounds in the solution depends on the nature of the solvent and the depolarizer. When, in the course of coulometric titrations, the current is switched on, oxidation of the depolarizers occurs first, i.e., acid is generated, and oxidation of other components does not occur until there is a sufficient volume of the depolarizer. The oxidation esters of gallic acid according to the equation: (12) In order to determine whether the acids are generated quantitatively under the given conditions, standard solutions of the bases were titrated with potentiometric end point detection using electrode pair FeS2-SCE. When the pyrite electrode was applied as the indicator electrode in acetonitrile and propionitrile as solvents, in the course of the titration the potential was established for less than one minute, whereas in the vicinity of the equivalence point for about 1-2 min. The results of the determinations, given in Table 3, showed that at 5 mA the oxidation proceeded with 99.8-100.1% current efficiency with good reproducibility. At a current higher than 5 mA, the oxidation was not quantitatively and proceeded with a current efficiency of less than 99%. These results are similar with results presented in the papers by Vajgand et al.3,42,46 which show that the hydrogen ions obtained by coulometry is successfully apllied for determination of weak bases. Furthermore, the results obtained in the determination of the bases using pyrite indicator electrode (Table 3) deviated on average by 0.2-0.7% from those obtained with a glass electrode. It was concluded that the pyrite electrode examined in this research could replace a glass electrode in the titration of bases in the solvents tested. Conclusions The natural minerals pyrite and chalcopyrite, were used as the indicator electrode sensors for the determinations of some organic acids and bases in acetonitrile and propionitrile as solvents. Based on the results it may be concluded that the natural minerals pyrite and chalcopyrite can be successfully applied as indicator electrodes in such determinations. The potential during the titration and at the equivalence point were rapidly established. The sensors carrier are pyrite and chalcopyrite, the monocrystallines, which are chemically inert in all working mediums, so these electrodes are very suitable for such determinations. Therefore, the proposed electrodes can be a good alternative for a glass electrode in the investigated solvents. Figure 5. Change in anodic potential with current density in propionitrile: 1) solvent; 2) collidine; 3) 2,2 –bipyridine; 4) dodecyl galate; 5) butyl galate; 6) pyrocatehol. Table 3. Results of coulometric titrations of bases with hydrogen ions obtained by the oxidation of some organic compounds with potentiometric* end-point detection; I = 5 mA Solvent Depolarizer Titrated base Recovery (%) Propionitrile Pyrocatechol Tributylamine 99.9 ± 0.4 Collidine 100.1 ± 0.2 2,2–Bipyridine 99.9 ± 0.2 Ethyl galate Tributylamine 99.9 ± 0.3 Collidine 99.8 ± 0.4 2,2–Bipyridine 99.9 ± 0.3 Butyl galate Tributylamine 100.0 ± 0.3 Collidine 100.1 ± 0.3 2,2–Bipyridine 100.0 ± 0.2 Dodecyl galate Tributylamine 99.9 ± 0.7 Collidine 100.0 ± 0.6 2,2–Bipyridine 100.1 ± 0.3 *FeS2-SCE. Simić et al. 717Vol. 22, No. 4, 2011 Additionally, the use of the m-dihydroxybenzene, some gallic acid esters, m-cresol and 3-methoxy phenol as a source of hydrogen and lyate ions in the determination of acids and bases in acetonitrile and propionitrile as solvents, make it simpler than the classical potentiometric method. By means of this procedure the use of a standard acid (base) solution is avoided, and according to application of the depolarizers brings no foreign organic compounds into the solution being investigated. Acknowledgment This work is supported by the Ministry of Science and Technological Development of the Republic of Serbia (Project No. 172 036). References 1. Hanselman, R. B.; Streuli, C. A.; Anal. Chem. 1956, 28, 916. 2. Vajgand, V.; Mihajlović, R.; Mihajlović, Lj.; Joksimović, V.; Anal. Chim. Acta 1988, 212, 73. 3. Mihajlović, R.; Vajgand, V.; Jaksić, Lj.; Manetović, M.; Anal. Chim. Acta 1990, 229, 287. 4. Mihajlović, R.; Vajgand, V.; Talanta 1983, 30, 789. 5. 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