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Salinity measurements with polyaniline matrix coated wire electrodes
www.elsevier.nl/locate/elecom Electrochemistry Communications 1 (1999) 271–273
Salinity measurements with polyaniline matrix coated wire electrodes ´ C.S. de Freitas, Walter M. de Azevedo Flamarion B. Diniz *, Katia ´ ´ ´ ´ Laboratorio de Eletroquımica, Departamento de Quımica Fundamental, CCEN-UFPE, Cidade Universitaria, 50670-901 Recife, PE, Brazil Received 19 May 1999; received in revised form 11 June 1999; accepted 11 June 1999
Abstract Potentiometric measurements were carried out with a wire electrode coated with a mixture of polyaniline and polystyrene. The salinity in water is obtained by measuring the potential difference between the coated wire electrode and an AgNAgCl, saturated KCl reference electrode. The salinity of a sample is obtained by the interpolation of its potential in the calibration curve obtained with KCl solutions of known concentrations. The results indicated that it is possible to measure the salinity in sea water, brackish water and drinking water in the range of 0.010 to 75‰. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Salinity; Ion exchange; Polyaniline; Potentiometry; Coated wire electrodes
1. Introduction Most applications of polyaniline deal with its electrochemical activity and with electronics, a smaller amount being dedicated to its ion exchange properties [1–4]. A particularly interesting example is the use of polyaniline modified glassy carbon particles as the stationary phase in ion exchange chromatography [5–8]. In its protonated form, polyaniline is a salt with a positive charge delocalized over the polymer chain, interacting with a negative charge coming from counter ions. Hence, the polymer may behave as an ion exchange material. Burgmayer and Murray [9,10] have indeed demonstrated that this is the case for polypyrrole. A similar behavior for polyaniline was verified in a previous paper of our group [11] and by other authors [12–16] in recent years. This paper presents the results of potentiometric measurements carried out with a wire electrode coated with a mixture of polyaniline and polystyrene. This mixture was used to give mechanical resistance to the polyaniline obtained in powder form. Measurements of salinity is a complex subject that has a long history [17]. A comprehensive review of this subject can be found in the work of Lewis [18]. Very briefly, it can be seen that, in the past, the salinity in water was determined by hydrometric and argentometric methods. In recent years, conductivity and density methods have been used because of * Corresponding author. Tel.: q55-81-271-8440; fax: q55-81-271-8442; e-mail: [email protected]
their high sensitivity and precision [19]. This work opens up the way for development of a new method for measuring salinity of water, i.e., potentiometry, and shows also a new application for polyaniline.
2. Experimental Polyaniline was synthesized by chemical oxidation of 0.1 M aniline solutions in 1 M HCl, employing 0.01 M K2Cr2O7 as an oxidizing agent as described in the literature [20,21]. After the synthesis the polymer was thoroughly rinsed with 1 M HCl until a clear rinse was obtained. The polymer was allowed to dry inside an evacuated desiccator. The wire electrodes were coated with polystyrene dissolved in chloroform and mixed with polyaniline in different proportions. The resulting slurry was brushed on platinum wire electrodes and allowed to dry in air. No specific control of the thickness of the coating was attempted. The measurements were carried out using a digital potentiometer (Digimed model DMPH-2) with readings precise to 1 mV. The coated wire electrodes were stored for 4 days in 0.1 M solutions of KCl before measurements were carried out. In this way reproducible measurements were obtained in a few seconds after exposure to the electrolyte in the range of concentration investigated (74.56 to 7.5=10y4‰). The concentration was expressed as parts per thousand (‰). The investigated lifetime was longer than 6 months. After that
1388-2481/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII S 1 3 8 8 - 2 4 8 1 ( 9 9 ) 0 0 0 5 7 - 0
Thursday Jul 08 03:09 PM
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 67
272
F.B. Diniz et al. / Electrochemistry Communications 1 (1999) 271–273
time, no further measurements were carried out, but if needed, the electrode could easily be coated again. The salinity was obtained by measuring the potential difference between the coated wire electrode and an AgNAgCl, saturated KCl reference electrode with a digital potentiometer. During the measurements the solution was maintained under agitation by means of a magnetic agitator; however, the use of agitator could be suppressed. Distilled deionized water and analytical grade reagents were used in all experiments. Fig. 2. Slope of linear portions of the potentiometric response of a polyaniline matrix coated wire electrode vs. radius of the anions in the salts. TFAcstrifluoroacetate, PTSsp-toluenesulfonate.
3. Results and discussion A comparison of the potentiometric response of the coated wire electrodes in a series of solutions of different salts was carried out and the results are given in Fig. 1. It is seen that for salts of small anions there are no significant changes in the slopes of the linear portions of the curves. The length of the linear portion varies from salt to salt. In Fig. 2 the relationship between the slopes obtained in Fig. 1 against the radius of the anion in the electrolyte is shown. It can be seen that the slopes are not dependent on anion size for radii smaller than 240 pm. In other words, these results indicate that the coated wire electrode has a similar selectivity for anions with radii below 240 pm. This behavior is different from that obtained with membranes of polyaniline and is presented elsewhere [11]. There, a better selectivity was observed. The ions employed (with anions of radius smaller than 240 pm) are those commonly found in sea water, brackish water and drinking water. Thus, this electrode can measure the total concentration of these ions in water or, in other words, its salinity. An exception to this behavior is observed for sulfate anions, because of their double charge. The effect of the thickness of the coating material on the potentiometric response was shown as a small variation (about 2 mV) on the linear and angular coefficients of the straight segment of the potentiometric line. However, a given electrode gave very reproducible results upon repetitive measurement. The measurements of salinity were carried out in solutions of KCl and in sea water, brackish water and drinking water
Fig. 1. Plots of potential vs. log(concentration of salts) for a polyaniline matrix coated wire electrode. TFAcstrifluoroacetate, PTSsp-toluenesulfonate.
Thursday Jul 08 03:09 PM
Table 1 Ions a present in drinking water, brackish water and sea water Composition Drinking water S/‰
Brackish water
Sea water
Naq Kq Mg2q Ca2q Sr2q Cly NO3y HCO3y SO42y PO43y
8.38=10y3 6.4=10y4 1.28=10y3 6.1=10y4 – 1.266=10y2 1.30=10y3 6.08=10y3 1.86=10y3 –
8.93=10y1 – 8.31=10y1 1.082 – 3.206 – 1.46=10y1 3.280 –
9.584 3.70=10y1 2.430 4.05=10y1 1.3=10y2 20.979 4=10y3 1.56=10y1 2.500 9=10y3
Total
3.281=10y2
9.438
36.450
a
The concentration of these ions (salinity) is expressed as parts per thousand (‰).
that were reproduced in the laboratory in agreement with the compositions given in the literature (Table 1). The sea water was prepared employing only the 10 most important components [22,23]. Salinity is defined as the total amount of solid material, in grams, contained in 1 kg of sea water [23]. In this paper we express salinity in parts per thousand (grams of total salts per liter of water). Fig. 3 shows a linear relationship between potential and log(salinity), in the interval Ss0.01 to Ss75‰. Thus, the calibration can be made by measuring the potential in just two concentrations of KCl that are within that interval. The salinity of a sample is obtained by the interpolation of its potential in the calibration curve. The potentials obtained with this coated wire electrode originate from the diffusion of anions through ion exchange in the polyaniline matrix [11]. These potentials are related to the concentration of anions in solution, which is related to the concentration of total dissolved salts. The linear relationship between the potentials and the concentrations obeys a Nernst-like equation where the slope of the straight line should have a slope of 59 mV if the electrode were exclusively selective to monovalent anions. The smaller slope presented
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 67
F.B. Diniz et al. / Electrochemistry Communications 1 (1999) 271–273
273
alternative to conductometric methods requires extensive studies in order to achieve comparative accuracy, in a similar way to the conductometric method in the past.
Acknowledgements Financial support for this work by FINEP is gratefully acknowledged. K.C.S. de F. acknowledges a Ph.D. scholarship from CAPES.
References Fig. 3. Potential vs. salinity plot of a polyaniline matrix coated wire electrode.
here indicates the interference of other ions; however, the potentials measured in the samples of water were very close to the KCl calibrating solution of the same concentration. This can be justified by the fact that the compositions of these samples are rich in chloride ions. With respect to precision and errors it must be said that a good sensitivity of the potential measurements (0.01 mV or better) is required in order for this method to compete with the conventional conductometric method, since the potential is not directly related to salinity, but to its logarithm. Finally, it should be added that wires coated with electrochemically synthesized polyaniline yielded no reproducible potentiometric responses under these experimental conditions. This is probably due to the very high permeability of this hydrophilic material.
4. Conclusions Potentiometric measurements with a polyaniline/polystyrene coated wire electrode showed that it is possible to measure the salinity of sea water, brackish water and drinking water in the interval of Ss0.01 to Ss75‰. These salinity measurements present an application of the ion exchange properties of polyaniline and they represent as innovation the use of the potentiometric technique instead of electric conductivity or density measurements that are used at the present time. Obviously, the utility of this type of electrode as an
Thursday Jul 08 03:09 PM
[1] A.A. Syed, M.K. Dinesan, Synth. Met. 36 (1990) 209. ¨ J. Electrochem. Soc. 138 [2] C. Barbero, M.C. Miras, O. Haas, R. Kotz, (1991) 669. [3] H. Nakao, T. Nagaoka, K. Ogura, Bunseki Kagaku 45 (1996) 189. ¨ O. Haas, J. Electroanal. Chem. 437 [4] C. Barbero, M.C. Miras, R. Kotz, (1997) 191. [5] T. Nagaoka, M. Fujimoto, H. Nakao, K. Kakuno, J. Yano, K. Ogura, J. Electroanal. Chem. 350 (1993) 337. [6] T. Nagaoka, M. Fujimoto, H. Nakao, K. Kakuno, J. Yano, K. Ogura, J. Electroanal. Chem. 364 (1994) 179. [7] T. Nagaoka, K. Kakuno, M. Fujimoto, H. Nakao, J. Yano, K. Ogura, J. Electroanal. Chem. 368 (1994) 315. [8] T. Nagaoka, H. Nakao, K. Tabusa, J. Yano, K. Ogura, J. Electroanal. Chem. 371 (1994) 283. [9] P. Burgmayer, R.W. Murray, J. Am. Chem. Soc. 104 (1982) 6139. [10] P. Burgmayer, R.W. Murray, J. Phys. Chem. 88 (1984) 2515. [11] F.B. Diniz, K.C.S. de Freitas, W.M. de Azevedo, Electrochim. Acta 42 (1997) 1789. ¨ J. Electrochem. Soc. 144 [12] C. Barbero, M.C. Miras, O. Haas, R. Kotz, (1997) 4170. [13] T. Nagaoka, H. Nakao, T. Suyama, K. Ogura, Analyst 122 (1997) 1399. [14] S. Winkels, M.M. Lohrengel, Electrochim. Acta 42 (1997) 3117. [15] H. Nakao, T. Nagaoka, K. Ogura, Anal. Sci. 13 (1997) 327. [16] K. Koziel, M. Lapkowski, E. Vieil, Synth. Met. 84 (1997) 91. [17] W. Dittmar, Chall. Rep. Phys. Chem. 1 (1884) 1. [18] E.L. Lewis, IEEE J. Oceanic Eng. 5 (1980) 3. [19] Standard Methods for the Examination of Water and Wastewater, vol. 1, American Public Health Association, Washington, DC, 1989, pp. 2, 61–62. ` A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met. 36 [20] E.M. Genies, (1990) 139. [21] A.A. Syed, M.K. Dinesan, Talanta 38 (1991) 815. [22] R.A. Horne, Marine Chemistry, Wiley-Interscience, New York, 1969. [23] M. Grant Gross, Oceanography — A View of the Earth, PrenticeHall, Englewood Cliffs, NJ, 1972.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 67
Salinity measurements with polyaniline matrix coated wire electrodes ´ C.S. de Freitas, Walter M. de Azevedo Flamarion B. Diniz *, Katia ´ ´ ´ ´ Laboratorio de Eletroquımica, Departamento de Quımica Fundamental, CCEN-UFPE, Cidade Universitaria, 50670-901 Recife, PE, Brazil Received 19 May 1999; received in revised form 11 June 1999; accepted 11 June 1999
Abstract Potentiometric measurements were carried out with a wire electrode coated with a mixture of polyaniline and polystyrene. The salinity in water is obtained by measuring the potential difference between the coated wire electrode and an AgNAgCl, saturated KCl reference electrode. The salinity of a sample is obtained by the interpolation of its potential in the calibration curve obtained with KCl solutions of known concentrations. The results indicated that it is possible to measure the salinity in sea water, brackish water and drinking water in the range of 0.010 to 75‰. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Salinity; Ion exchange; Polyaniline; Potentiometry; Coated wire electrodes
1. Introduction Most applications of polyaniline deal with its electrochemical activity and with electronics, a smaller amount being dedicated to its ion exchange properties [1–4]. A particularly interesting example is the use of polyaniline modified glassy carbon particles as the stationary phase in ion exchange chromatography [5–8]. In its protonated form, polyaniline is a salt with a positive charge delocalized over the polymer chain, interacting with a negative charge coming from counter ions. Hence, the polymer may behave as an ion exchange material. Burgmayer and Murray [9,10] have indeed demonstrated that this is the case for polypyrrole. A similar behavior for polyaniline was verified in a previous paper of our group [11] and by other authors [12–16] in recent years. This paper presents the results of potentiometric measurements carried out with a wire electrode coated with a mixture of polyaniline and polystyrene. This mixture was used to give mechanical resistance to the polyaniline obtained in powder form. Measurements of salinity is a complex subject that has a long history [17]. A comprehensive review of this subject can be found in the work of Lewis [18]. Very briefly, it can be seen that, in the past, the salinity in water was determined by hydrometric and argentometric methods. In recent years, conductivity and density methods have been used because of * Corresponding author. Tel.: q55-81-271-8440; fax: q55-81-271-8442; e-mail: [email protected]
their high sensitivity and precision [19]. This work opens up the way for development of a new method for measuring salinity of water, i.e., potentiometry, and shows also a new application for polyaniline.
2. Experimental Polyaniline was synthesized by chemical oxidation of 0.1 M aniline solutions in 1 M HCl, employing 0.01 M K2Cr2O7 as an oxidizing agent as described in the literature [20,21]. After the synthesis the polymer was thoroughly rinsed with 1 M HCl until a clear rinse was obtained. The polymer was allowed to dry inside an evacuated desiccator. The wire electrodes were coated with polystyrene dissolved in chloroform and mixed with polyaniline in different proportions. The resulting slurry was brushed on platinum wire electrodes and allowed to dry in air. No specific control of the thickness of the coating was attempted. The measurements were carried out using a digital potentiometer (Digimed model DMPH-2) with readings precise to 1 mV. The coated wire electrodes were stored for 4 days in 0.1 M solutions of KCl before measurements were carried out. In this way reproducible measurements were obtained in a few seconds after exposure to the electrolyte in the range of concentration investigated (74.56 to 7.5=10y4‰). The concentration was expressed as parts per thousand (‰). The investigated lifetime was longer than 6 months. After that
1388-2481/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII S 1 3 8 8 - 2 4 8 1 ( 9 9 ) 0 0 0 5 7 - 0
Thursday Jul 08 03:09 PM
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 67
272
F.B. Diniz et al. / Electrochemistry Communications 1 (1999) 271–273
time, no further measurements were carried out, but if needed, the electrode could easily be coated again. The salinity was obtained by measuring the potential difference between the coated wire electrode and an AgNAgCl, saturated KCl reference electrode with a digital potentiometer. During the measurements the solution was maintained under agitation by means of a magnetic agitator; however, the use of agitator could be suppressed. Distilled deionized water and analytical grade reagents were used in all experiments. Fig. 2. Slope of linear portions of the potentiometric response of a polyaniline matrix coated wire electrode vs. radius of the anions in the salts. TFAcstrifluoroacetate, PTSsp-toluenesulfonate.
3. Results and discussion A comparison of the potentiometric response of the coated wire electrodes in a series of solutions of different salts was carried out and the results are given in Fig. 1. It is seen that for salts of small anions there are no significant changes in the slopes of the linear portions of the curves. The length of the linear portion varies from salt to salt. In Fig. 2 the relationship between the slopes obtained in Fig. 1 against the radius of the anion in the electrolyte is shown. It can be seen that the slopes are not dependent on anion size for radii smaller than 240 pm. In other words, these results indicate that the coated wire electrode has a similar selectivity for anions with radii below 240 pm. This behavior is different from that obtained with membranes of polyaniline and is presented elsewhere [11]. There, a better selectivity was observed. The ions employed (with anions of radius smaller than 240 pm) are those commonly found in sea water, brackish water and drinking water. Thus, this electrode can measure the total concentration of these ions in water or, in other words, its salinity. An exception to this behavior is observed for sulfate anions, because of their double charge. The effect of the thickness of the coating material on the potentiometric response was shown as a small variation (about 2 mV) on the linear and angular coefficients of the straight segment of the potentiometric line. However, a given electrode gave very reproducible results upon repetitive measurement. The measurements of salinity were carried out in solutions of KCl and in sea water, brackish water and drinking water
Fig. 1. Plots of potential vs. log(concentration of salts) for a polyaniline matrix coated wire electrode. TFAcstrifluoroacetate, PTSsp-toluenesulfonate.
Thursday Jul 08 03:09 PM
Table 1 Ions a present in drinking water, brackish water and sea water Composition Drinking water S/‰
Brackish water
Sea water
Naq Kq Mg2q Ca2q Sr2q Cly NO3y HCO3y SO42y PO43y
8.38=10y3 6.4=10y4 1.28=10y3 6.1=10y4 – 1.266=10y2 1.30=10y3 6.08=10y3 1.86=10y3 –
8.93=10y1 – 8.31=10y1 1.082 – 3.206 – 1.46=10y1 3.280 –
9.584 3.70=10y1 2.430 4.05=10y1 1.3=10y2 20.979 4=10y3 1.56=10y1 2.500 9=10y3
Total
3.281=10y2
9.438
36.450
a
The concentration of these ions (salinity) is expressed as parts per thousand (‰).
that were reproduced in the laboratory in agreement with the compositions given in the literature (Table 1). The sea water was prepared employing only the 10 most important components [22,23]. Salinity is defined as the total amount of solid material, in grams, contained in 1 kg of sea water [23]. In this paper we express salinity in parts per thousand (grams of total salts per liter of water). Fig. 3 shows a linear relationship between potential and log(salinity), in the interval Ss0.01 to Ss75‰. Thus, the calibration can be made by measuring the potential in just two concentrations of KCl that are within that interval. The salinity of a sample is obtained by the interpolation of its potential in the calibration curve. The potentials obtained with this coated wire electrode originate from the diffusion of anions through ion exchange in the polyaniline matrix [11]. These potentials are related to the concentration of anions in solution, which is related to the concentration of total dissolved salts. The linear relationship between the potentials and the concentrations obeys a Nernst-like equation where the slope of the straight line should have a slope of 59 mV if the electrode were exclusively selective to monovalent anions. The smaller slope presented
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 67
F.B. Diniz et al. / Electrochemistry Communications 1 (1999) 271–273
273
alternative to conductometric methods requires extensive studies in order to achieve comparative accuracy, in a similar way to the conductometric method in the past.
Acknowledgements Financial support for this work by FINEP is gratefully acknowledged. K.C.S. de F. acknowledges a Ph.D. scholarship from CAPES.
References Fig. 3. Potential vs. salinity plot of a polyaniline matrix coated wire electrode.
here indicates the interference of other ions; however, the potentials measured in the samples of water were very close to the KCl calibrating solution of the same concentration. This can be justified by the fact that the compositions of these samples are rich in chloride ions. With respect to precision and errors it must be said that a good sensitivity of the potential measurements (0.01 mV or better) is required in order for this method to compete with the conventional conductometric method, since the potential is not directly related to salinity, but to its logarithm. Finally, it should be added that wires coated with electrochemically synthesized polyaniline yielded no reproducible potentiometric responses under these experimental conditions. This is probably due to the very high permeability of this hydrophilic material.
4. Conclusions Potentiometric measurements with a polyaniline/polystyrene coated wire electrode showed that it is possible to measure the salinity of sea water, brackish water and drinking water in the interval of Ss0.01 to Ss75‰. These salinity measurements present an application of the ion exchange properties of polyaniline and they represent as innovation the use of the potentiometric technique instead of electric conductivity or density measurements that are used at the present time. Obviously, the utility of this type of electrode as an
Thursday Jul 08 03:09 PM
[1] A.A. Syed, M.K. Dinesan, Synth. Met. 36 (1990) 209. ¨ J. Electrochem. Soc. 138 [2] C. Barbero, M.C. Miras, O. Haas, R. Kotz, (1991) 669. [3] H. Nakao, T. Nagaoka, K. Ogura, Bunseki Kagaku 45 (1996) 189. ¨ O. Haas, J. Electroanal. Chem. 437 [4] C. Barbero, M.C. Miras, R. Kotz, (1997) 191. [5] T. Nagaoka, M. Fujimoto, H. Nakao, K. Kakuno, J. Yano, K. Ogura, J. Electroanal. Chem. 350 (1993) 337. [6] T. Nagaoka, M. Fujimoto, H. Nakao, K. Kakuno, J. Yano, K. Ogura, J. Electroanal. Chem. 364 (1994) 179. [7] T. Nagaoka, K. Kakuno, M. Fujimoto, H. Nakao, J. Yano, K. Ogura, J. Electroanal. Chem. 368 (1994) 315. [8] T. Nagaoka, H. Nakao, K. Tabusa, J. Yano, K. Ogura, J. Electroanal. Chem. 371 (1994) 283. [9] P. Burgmayer, R.W. Murray, J. Am. Chem. Soc. 104 (1982) 6139. [10] P. Burgmayer, R.W. Murray, J. Phys. Chem. 88 (1984) 2515. [11] F.B. Diniz, K.C.S. de Freitas, W.M. de Azevedo, Electrochim. Acta 42 (1997) 1789. ¨ J. Electrochem. Soc. 144 [12] C. Barbero, M.C. Miras, O. Haas, R. Kotz, (1997) 4170. [13] T. Nagaoka, H. Nakao, T. Suyama, K. Ogura, Analyst 122 (1997) 1399. [14] S. Winkels, M.M. Lohrengel, Electrochim. Acta 42 (1997) 3117. [15] H. Nakao, T. Nagaoka, K. Ogura, Anal. Sci. 13 (1997) 327. [16] K. Koziel, M. Lapkowski, E. Vieil, Synth. Met. 84 (1997) 91. [17] W. Dittmar, Chall. Rep. Phys. Chem. 1 (1884) 1. [18] E.L. Lewis, IEEE J. Oceanic Eng. 5 (1980) 3. [19] Standard Methods for the Examination of Water and Wastewater, vol. 1, American Public Health Association, Washington, DC, 1989, pp. 2, 61–62. ` A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met. 36 [20] E.M. Genies, (1990) 139. [21] A.A. Syed, M.K. Dinesan, Talanta 38 (1991) 815. [22] R.A. Horne, Marine Chemistry, Wiley-Interscience, New York, 1969. [23] M. Grant Gross, Oceanography — A View of the Earth, PrenticeHall, Englewood Cliffs, NJ, 1972.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 67