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How to understand the response mechanism of ion-selective electrodes
Talanta Talanta
44 (1997) 1505P1508
How to understand the response mechanism of ion-selective electrodes Ernii Pungor Institute for General and Analytical Chemistry, Technical University, Budapest, Hungary Received
16 July 1996; received
in revised
form 23 October
1996; accepted
13 November
1996
Abstract The present study breaks with the earlier mechanism of electrode potential on basis of experimental investigations and theoretical considerations. It rejects that the transport through the membrane produces the electrode potential and definitely proves that the electrode potential is created via surface chemisorption; i.e., the electrode potential is produced by a surface react&. The reaction centres can be acid-base groups or complex formation groups (e.g., valinomycin or other alkaline earth metal complexing ligands). 0 1997 Elsevier Science B.V. Keywords:
Chemisorption;
Ion-selective
electrodes;
Surface
reaction;
1. History Nearly a century has passed since the time when it was found at some sorts of glass that they gave an electrochemical sign with respect to the acidity of the solution [l]. There was not found any explanation for this surprising discovery in the earlier theories. It was not clear, how a solid phase, at which any electron transitional reaction necessary to the potential response interpreted in case of metal phases cannot come true, gave an electrode potential response. The researchers wished to find the answer for explanation by the first imaginable approach from a reaction of quite other type. In the experiment of Donnan [2], in which he separated two solutions-one of them was protein solution, the other was sodium-chloride solution-by a membrane, electrode potential was measurable, and it 0039-9140/97/$17.00
0 1997 Elsevier Science B.V. All rights
PIISOO39-9140(96)02183-2
reserved.
Working
mechanism
of HE-s
could be interpreted by supposition of equilibrium of charges and by supposition of ion equilibrium through the membrane. The entirely interpretable, clear picture of the Donnan reaction was adapted to the interpretation of the potential appearing on the glass electrode supposing appearance of charge transfer through the glass membrane. The external layer of the glass screen is swollen by the water solution in case of appropriate glass and through it was supposed the ion transport determining the potential. This transport idea has remained in the interpretation of glass electrodes with the difference that on the basis of precise layer analysis has been evident that the charge transporters can be only sodium ions, ion crystals. In the 1930s a new change appeared in the interpretations in the way that Nicholski [3] introduced the term of ion exchange for the phenomenon observed by Lengyel and Blum [4]
1506
E. Pungor / Talanta 44 (1997) 1.505-1.508
potential given to high level of sodium ions in comparison to hydrogen ions. Nicholski interpreted it by that at certain ion concentration proportions the electrode responds to the activity of the formally not potential determining ionic species. By generalizing this idea the literaturefirst of all the educational literature-garbled this totally clear idea of the selectivity coefficient. The generalizing meant that it saw the basis of the effect of ion-selective electrodes in the ion exchange. Naturally, the Nicholski equation can be put down formally for the whole response function, and a wide domain of them, but in its mechanism the role of ion exchange gives only effect in the domain, where the two types of ions already disturb the response potential of each other. Even today there are in technical books equations attributing potential to the ion exchange, leaving aside that there is not any charge difference between the two sides of the equation being to put down. Although, if in a reaction new charges are not created or lost, then new potential response cannot emerge.
2. New phenomena In the meantime several new electrodes were developed, and these were wished to be interpreted by the above principles, too. Transport through phase, as determinative process of potential was disclaimed by several experiences. One of the basic experiences is the response time figure determinable on the electrode investigation [5]. It was measurable that the response time of ion-selective electrodes is in general so short (20-40 ms) that this time is sufficient only for penetration onto the surface of the electrode through the fluid adhering to the surface of the electrode. Consequently, if we can speak about a kind of transport at such response times, it can contain in no way the ion transfer through the membrane phase. A further result of experiments is that we separate space by a potassium-selective electrode so that on one side of the given surface potassium salt solution, on the other side distilled water can be found and considering the electrode membrane thickness as well as the ion transport being to
calculate by the diffusion coefficient applied for one membrane phase, potassium ion does not appear on the water side of the membrane even during 8-10 times longer time, than could be calculated on the basis of the diffusion coefficient. A further observation was, which we made with iodide membrane so that we prepared the electrode in a ‘sandwich’ form. We applied aluminium, platinum or silver, on the two sides iodide membrane electrodes. The speed of the potential response of such system was the same as that of the ‘non-sandwich’ membrane, and the E, value of the electrode remained the same. We have proved by investigations of late years that the type of ions effecting potential of the glass electrode does not enter into the glass, but it takes part only in chemisorption on its surface [6]. Consequently it lies cross very fundamentally to the adaptation of analogies with the Donnan reaction. We made experiments with help of reflection infrared spectrum [7], how deep does potassium ion penetrate into the potassium electrode. These penetration experiments of potassium ion in presence of very strongly lyophobic anion the depth of penetration is about 5-10 nanometre. Measurements of glass electrode by SIMS technique indicated the same depth adding that the depth distribution of gold atomized to the electrode was the same as that of the silver ion used at the experiment. ._At the same time, if the lipophilic anion together with potassium is present, then the potassium ion can enter into the inside of the phase with help of a complexing agent. In this case at a high concentration of lyophobic anions we come to a condition, when the electrode is measuring the activity data of the lyophilic anions instead of that of potassium (Donnan maximum). The observations concerning glass electrodes and electrodes with complexing agent can be attributed to the same experimental phenomena, namely to the chemisorption. In connection with chemisorption first of all the investigations of silver halides pointed out that by linkage of own ions onto the surface of the crystal a large electric field is emerging, which can considerably change the characteristics of the molecules adsorbed onto
E. Pungor jTalanta
the surface (constants of acid-base equilibrium, redox potential, face solubility product) and these data are measurable. Similarly the so-called related ions also establish adequate electric fields after their chemisorption [8]. In case of membranes with ion structure, the selectivity coefficients regarding the electrodes cannot only be interpreted, but also numerically defined on this basis [9]. In the new time of literature some authors are following the results described above. I have to emphasize the works of Umezawa et al. [IO], P. Bilhlmann et al. [ll], who approached by the ATR-IR and SHG analyses the same phenomena, which we got earlier as results of ATT-IR. Another question of ion-selective electrodes was analysed by Bakker et al. [12] and he interpreted the electrode selectivity coefficient. I would like to emphasize, too, that the selectivity coefficient bears only in that field an interpretation, where the disturbance function of the analysed ions arises. Therefore the equation, which can be written down without disturbing ion, can bear an interpretation only formally for the whole equation. At the same time Bakker et al. [13] analysed the interfacial potential method in details, too. Specially remarkable is the publication, in which Buck et al. [14] deals with the transport mechanism. This publication is not understandable, because the potential through the membrane was described on a way, which was never controlled, but it was only assumed without any basis. The results until now give that chemisorption developments on the surface of the membrane on the solution side, and hereby charge separation comes into being on the electrode and solution side. We demonstrated that the resistance of the membrane can be even infinitely high, but it changes the possibility of polarization only to such an extent that the input impedance of the measuring instrument will not be lower than the value of the measuring electrode system, about a 10 OOO-fold value. Accordingly, the chemisorption will take place on the area of interface of the membrane electrode, the potential will emerge here, and the trajectories cannot be given through the membrane phase.
1507
44 (1997) 1505-1508
3. Interpretation
of the phenomena
Specially remarkable is the result measured by us with help of valimomycin and bis crown ether membranes. It came out that the measured potential data were the same in case of both components. The energetics, which can be determined by the complex former, would have been different, if it had depended on the two complex formers. The energetics function is in connection with the charge separation energetics of the salt in the solution, and it can be expressed with help of the Gibbs function. Accordingly, in case of lypophilic anion the chemisorption does not allow the penetration of the ion determining the potential into the membrane phase and in case of ion crystals it is from the beginning impossible that ion transport effected possibly by defect structure of the crystal plays a role in formation of electrode potential. The reaction itself producing potential is based upon charge separation in which the chemisorption regarding the given ion is bound on the electrode surface and the counter-ion is staying in the solution. We have to search for the energetic picture belonging to the establishment of the potential of the ion-selective electrode in the chemical potential of the ionic side in the solution, so we can put down, as follows: - AG=nFE where AG is the chemical potential change, n is the charge of ion, F is the Faraday constant, E is the potential The approach of Guggenheim established for ion-selective electrodes does not stand the proof. On the one hand the Guggenheim equation puts down two parameters changing in the same way and hereby the equation becomes fundamentally indefinable, because innumerable electrochemical potentials and galvanopotentials, which are, however, connected with each other, belong to the same chemical potential. On the other hand at the same time, if the components determining the potential can be found only on the surface of the membrane at the chemisorption, then there is no way to count with the potential data in the inside of the membrane.
1508
E. Pungor / Talanta 44 (1997) 1505- 1508
The question arises that the Guggenheim approach is only then true, when we can put down the relation on the external layer of the membrane, as well as on the water phase and the area of interface closely connected with them. If the ion determining potential can be forced through the membrane to pass the phase in form of ion transport, it is natural, if we apply an external electric field, which breaks down the potentials being established on the surfaces of the electrode, in this case the current intensity is adequate to the resistance of the electrode. Naturally, in this case the same component, which effects the chemisorption, as reagent can transport charges through the phase. Acknowledgements
The paper is devoted to the birthday of my friend Professor G. Baiulescu, who made great progress for Analytical Chemistry in Romania.
References [I] F. Haber and Z. Clemensiewitz, Z. Phys. Chem., 67 (1909) 385. [2] F.G. Donnan, Z. Electrochem, 17 (1911) 572. [3] B.P. Nicholsky, Acta Physicochem. USSR, 7 (1937) 797. [4] B. Lengyel and E. Blum, Trans. Faraday Sot., 30 (1934) 461. [5] K. T&h, I. Gavaller and E. Pungor, Anal. Chim. Acta, 57 (1971) 131. [6] E. Pungor, Electroanalysis, 8 (1996) 348. [7] K. Toth, E. Lindner, E. Pungor, E. Zippel and R. Kellner, Fresenius Z. Anal. Chem., 331 (1988) 448. [8] E. Pungor, Acta Chim. Hung., 12 (1957) 265. [9] E. Pungor and E. Hollos-Rokosinyi, Acta Chim. Hung., 27 (1961) 63. [lo] K. Umerawa, X.M. Lin, S. Nishizawa, M. Sugawara, Y. Umezawa, Anal. Chimi. Acta 282 (1993) 247. [11] P. Btihlmann, S.Y.K. Tohda, K. Umerawa, S. Nishizawa and Y. Umezawa, Electroanalysis 7 (1995) 811. [12] E. Bakker, R.K. Meruva, E. Pretsch and M.E. Mayerhoff, Anal. Chem., 66 (1994) 3021. [13] E. Bakker, M. Nagele, U. Schaller and E. Pretsch, Electroanalysis, 7 (1995) 817. [14] R.P. Buck, Tal M. Nahir, Vasile V. Cosofret, E. Lindner and M. Erdosy, Anal. Proc. Including Anal. Comm., 31 (1994) 301.
44 (1997) 1505P1508
How to understand the response mechanism of ion-selective electrodes Ernii Pungor Institute for General and Analytical Chemistry, Technical University, Budapest, Hungary Received
16 July 1996; received
in revised
form 23 October
1996; accepted
13 November
1996
Abstract The present study breaks with the earlier mechanism of electrode potential on basis of experimental investigations and theoretical considerations. It rejects that the transport through the membrane produces the electrode potential and definitely proves that the electrode potential is created via surface chemisorption; i.e., the electrode potential is produced by a surface react&. The reaction centres can be acid-base groups or complex formation groups (e.g., valinomycin or other alkaline earth metal complexing ligands). 0 1997 Elsevier Science B.V. Keywords:
Chemisorption;
Ion-selective
electrodes;
Surface
reaction;
1. History Nearly a century has passed since the time when it was found at some sorts of glass that they gave an electrochemical sign with respect to the acidity of the solution [l]. There was not found any explanation for this surprising discovery in the earlier theories. It was not clear, how a solid phase, at which any electron transitional reaction necessary to the potential response interpreted in case of metal phases cannot come true, gave an electrode potential response. The researchers wished to find the answer for explanation by the first imaginable approach from a reaction of quite other type. In the experiment of Donnan [2], in which he separated two solutions-one of them was protein solution, the other was sodium-chloride solution-by a membrane, electrode potential was measurable, and it 0039-9140/97/$17.00
0 1997 Elsevier Science B.V. All rights
PIISOO39-9140(96)02183-2
reserved.
Working
mechanism
of HE-s
could be interpreted by supposition of equilibrium of charges and by supposition of ion equilibrium through the membrane. The entirely interpretable, clear picture of the Donnan reaction was adapted to the interpretation of the potential appearing on the glass electrode supposing appearance of charge transfer through the glass membrane. The external layer of the glass screen is swollen by the water solution in case of appropriate glass and through it was supposed the ion transport determining the potential. This transport idea has remained in the interpretation of glass electrodes with the difference that on the basis of precise layer analysis has been evident that the charge transporters can be only sodium ions, ion crystals. In the 1930s a new change appeared in the interpretations in the way that Nicholski [3] introduced the term of ion exchange for the phenomenon observed by Lengyel and Blum [4]
1506
E. Pungor / Talanta 44 (1997) 1.505-1.508
potential given to high level of sodium ions in comparison to hydrogen ions. Nicholski interpreted it by that at certain ion concentration proportions the electrode responds to the activity of the formally not potential determining ionic species. By generalizing this idea the literaturefirst of all the educational literature-garbled this totally clear idea of the selectivity coefficient. The generalizing meant that it saw the basis of the effect of ion-selective electrodes in the ion exchange. Naturally, the Nicholski equation can be put down formally for the whole response function, and a wide domain of them, but in its mechanism the role of ion exchange gives only effect in the domain, where the two types of ions already disturb the response potential of each other. Even today there are in technical books equations attributing potential to the ion exchange, leaving aside that there is not any charge difference between the two sides of the equation being to put down. Although, if in a reaction new charges are not created or lost, then new potential response cannot emerge.
2. New phenomena In the meantime several new electrodes were developed, and these were wished to be interpreted by the above principles, too. Transport through phase, as determinative process of potential was disclaimed by several experiences. One of the basic experiences is the response time figure determinable on the electrode investigation [5]. It was measurable that the response time of ion-selective electrodes is in general so short (20-40 ms) that this time is sufficient only for penetration onto the surface of the electrode through the fluid adhering to the surface of the electrode. Consequently, if we can speak about a kind of transport at such response times, it can contain in no way the ion transfer through the membrane phase. A further result of experiments is that we separate space by a potassium-selective electrode so that on one side of the given surface potassium salt solution, on the other side distilled water can be found and considering the electrode membrane thickness as well as the ion transport being to
calculate by the diffusion coefficient applied for one membrane phase, potassium ion does not appear on the water side of the membrane even during 8-10 times longer time, than could be calculated on the basis of the diffusion coefficient. A further observation was, which we made with iodide membrane so that we prepared the electrode in a ‘sandwich’ form. We applied aluminium, platinum or silver, on the two sides iodide membrane electrodes. The speed of the potential response of such system was the same as that of the ‘non-sandwich’ membrane, and the E, value of the electrode remained the same. We have proved by investigations of late years that the type of ions effecting potential of the glass electrode does not enter into the glass, but it takes part only in chemisorption on its surface [6]. Consequently it lies cross very fundamentally to the adaptation of analogies with the Donnan reaction. We made experiments with help of reflection infrared spectrum [7], how deep does potassium ion penetrate into the potassium electrode. These penetration experiments of potassium ion in presence of very strongly lyophobic anion the depth of penetration is about 5-10 nanometre. Measurements of glass electrode by SIMS technique indicated the same depth adding that the depth distribution of gold atomized to the electrode was the same as that of the silver ion used at the experiment. ._At the same time, if the lipophilic anion together with potassium is present, then the potassium ion can enter into the inside of the phase with help of a complexing agent. In this case at a high concentration of lyophobic anions we come to a condition, when the electrode is measuring the activity data of the lyophilic anions instead of that of potassium (Donnan maximum). The observations concerning glass electrodes and electrodes with complexing agent can be attributed to the same experimental phenomena, namely to the chemisorption. In connection with chemisorption first of all the investigations of silver halides pointed out that by linkage of own ions onto the surface of the crystal a large electric field is emerging, which can considerably change the characteristics of the molecules adsorbed onto
E. Pungor jTalanta
the surface (constants of acid-base equilibrium, redox potential, face solubility product) and these data are measurable. Similarly the so-called related ions also establish adequate electric fields after their chemisorption [8]. In case of membranes with ion structure, the selectivity coefficients regarding the electrodes cannot only be interpreted, but also numerically defined on this basis [9]. In the new time of literature some authors are following the results described above. I have to emphasize the works of Umezawa et al. [IO], P. Bilhlmann et al. [ll], who approached by the ATR-IR and SHG analyses the same phenomena, which we got earlier as results of ATT-IR. Another question of ion-selective electrodes was analysed by Bakker et al. [12] and he interpreted the electrode selectivity coefficient. I would like to emphasize, too, that the selectivity coefficient bears only in that field an interpretation, where the disturbance function of the analysed ions arises. Therefore the equation, which can be written down without disturbing ion, can bear an interpretation only formally for the whole equation. At the same time Bakker et al. [13] analysed the interfacial potential method in details, too. Specially remarkable is the publication, in which Buck et al. [14] deals with the transport mechanism. This publication is not understandable, because the potential through the membrane was described on a way, which was never controlled, but it was only assumed without any basis. The results until now give that chemisorption developments on the surface of the membrane on the solution side, and hereby charge separation comes into being on the electrode and solution side. We demonstrated that the resistance of the membrane can be even infinitely high, but it changes the possibility of polarization only to such an extent that the input impedance of the measuring instrument will not be lower than the value of the measuring electrode system, about a 10 OOO-fold value. Accordingly, the chemisorption will take place on the area of interface of the membrane electrode, the potential will emerge here, and the trajectories cannot be given through the membrane phase.
1507
44 (1997) 1505-1508
3. Interpretation
of the phenomena
Specially remarkable is the result measured by us with help of valimomycin and bis crown ether membranes. It came out that the measured potential data were the same in case of both components. The energetics, which can be determined by the complex former, would have been different, if it had depended on the two complex formers. The energetics function is in connection with the charge separation energetics of the salt in the solution, and it can be expressed with help of the Gibbs function. Accordingly, in case of lypophilic anion the chemisorption does not allow the penetration of the ion determining the potential into the membrane phase and in case of ion crystals it is from the beginning impossible that ion transport effected possibly by defect structure of the crystal plays a role in formation of electrode potential. The reaction itself producing potential is based upon charge separation in which the chemisorption regarding the given ion is bound on the electrode surface and the counter-ion is staying in the solution. We have to search for the energetic picture belonging to the establishment of the potential of the ion-selective electrode in the chemical potential of the ionic side in the solution, so we can put down, as follows: - AG=nFE where AG is the chemical potential change, n is the charge of ion, F is the Faraday constant, E is the potential The approach of Guggenheim established for ion-selective electrodes does not stand the proof. On the one hand the Guggenheim equation puts down two parameters changing in the same way and hereby the equation becomes fundamentally indefinable, because innumerable electrochemical potentials and galvanopotentials, which are, however, connected with each other, belong to the same chemical potential. On the other hand at the same time, if the components determining the potential can be found only on the surface of the membrane at the chemisorption, then there is no way to count with the potential data in the inside of the membrane.
1508
E. Pungor / Talanta 44 (1997) 1505- 1508
The question arises that the Guggenheim approach is only then true, when we can put down the relation on the external layer of the membrane, as well as on the water phase and the area of interface closely connected with them. If the ion determining potential can be forced through the membrane to pass the phase in form of ion transport, it is natural, if we apply an external electric field, which breaks down the potentials being established on the surfaces of the electrode, in this case the current intensity is adequate to the resistance of the electrode. Naturally, in this case the same component, which effects the chemisorption, as reagent can transport charges through the phase. Acknowledgements
The paper is devoted to the birthday of my friend Professor G. Baiulescu, who made great progress for Analytical Chemistry in Romania.
References [I] F. Haber and Z. Clemensiewitz, Z. Phys. Chem., 67 (1909) 385. [2] F.G. Donnan, Z. Electrochem, 17 (1911) 572. [3] B.P. Nicholsky, Acta Physicochem. USSR, 7 (1937) 797. [4] B. Lengyel and E. Blum, Trans. Faraday Sot., 30 (1934) 461. [5] K. T&h, I. Gavaller and E. Pungor, Anal. Chim. Acta, 57 (1971) 131. [6] E. Pungor, Electroanalysis, 8 (1996) 348. [7] K. Toth, E. Lindner, E. Pungor, E. Zippel and R. Kellner, Fresenius Z. Anal. Chem., 331 (1988) 448. [8] E. Pungor, Acta Chim. Hung., 12 (1957) 265. [9] E. Pungor and E. Hollos-Rokosinyi, Acta Chim. Hung., 27 (1961) 63. [lo] K. Umerawa, X.M. Lin, S. Nishizawa, M. Sugawara, Y. Umezawa, Anal. Chimi. Acta 282 (1993) 247. [11] P. Btihlmann, S.Y.K. Tohda, K. Umerawa, S. Nishizawa and Y. Umezawa, Electroanalysis 7 (1995) 811. [12] E. Bakker, R.K. Meruva, E. Pretsch and M.E. Mayerhoff, Anal. Chem., 66 (1994) 3021. [13] E. Bakker, M. Nagele, U. Schaller and E. Pretsch, Electroanalysis, 7 (1995) 817. [14] R.P. Buck, Tal M. Nahir, Vasile V. Cosofret, E. Lindner and M. Erdosy, Anal. Proc. Including Anal. Comm., 31 (1994) 301.