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Capacitance Potential
MICROCHEMICAL JOURNAL ARTICLE NO.
59, 323–325 (1998)
MJ981578
Capacitance Potential K. L. Cheng1 Department of Chemistry, University of Missouri—Kansas City, Kansas City, Missouri 64110 Received December 23, 1997; accepted January 9, 1998 Based on their different mechanisms, several simple tests are presented for identifying two types of potentiometry: Faradaic and non-Faradaic To understand the electrode potential, mechanism is emphasized over commonly misused thermodynamics. That the improper term reduction potential be changed to capacitance potential for calomel and Ag/AgCl is suggested. © 1998 Academic Press Key Words: Faradaic; non-Faradaic; capacitance; potential; redox; half-cell; mechanism.
INTRODUCTION There are two types of potentiometry: (1) Faradaic potentiometry based on redox reactions and (2) non-Faradaic potentiometry based on charges or ion adsorption onto a dielectric or semiconductor that can hold the charges. The former is normally a cell composed of two half-cells with a salt bridge. The latter is a capacitor against another capacitor or a reference electrode requiring a connecting wire. The definitions of a cell and a capacitor have been given (1). Non-Faradaic potentiometry does exist, but little has ever been reported. Many electrodes and reactions in the area of non-Faradaic potentiometry have been misidentified as Faradaic potentiometry, for instance, the pH glass electrode, the potential of which is derived from a capacitor, not from a half-cell. Also, the Nernst equation based on redox equilibrium has been misused (1). Most ion selective electrodes (ISEs) are capacitors, not half-cells as commonly believed (1). It is important to distinguish between them because their potential mechanisms are different. This paper tries to offer a few simple tests to identify whether an electrode is a half-cell or a capacitor. DISCUSSION A half-cell requires a salt bridge to transfer ions to maintain an ionic balance between two half-cells. The function of the salt bridge is to permit ions to flow from one solution to the other. A capacitor, as distinguished from a half-cell, has no ions to be transferred; consequently, there is no need for a salt bridge. A connecting wire is sufficient for the electron transfer. Certainly, a salt bridge would serve the same purpose. The use of a connecting wire for measurement of potential is a simple test to distinguish a capacitor from a half-cell. For a half-cell, the electrode shows no potential difference when it is dipped in the solution at different depths, as the electrode is used to conduct the current flow and the potential depends on the concentrations of oxidants and reductants in separate solutions. For example, pH measurement with an antimony electrode is a redox reaction, displaying the same potential at different depths (2). On the other hand, the potential of 1
To whom correspondence should be addressed.
323 0026-265X/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
324
K. L. CHENG TABLE 1 Difference in Potential Measurements Condition
Half-cell
Capacitor
Salt bridge Metal wire connection instead of salt bridge Add oxidant or reductant Dipping electrode at different depths Capacitance measurement Dielectric or semiconductor Ion adsorption effect Discharge and recharge Stirring (5) Like ISE
Required No Potential change No potential change No capacitance No No No potential change No potential change No
Not required Yes No potential change Potential change Yields capacitance Yes Yes Potential change Potential change Yes
a capacitor comes from the charge density of the electrode surface. Its potential varies as a function of the contact between the electrode surface and the solution (1). This has been demonstrated with the lead ISE and pH glass electrode (1). The capacitance of a pH glass electrode has been measured in both acid and basic solutions (3). A half-cell, in contrast, does not yield any capacitance. Many half-cells are connected to a metal, and capacitors are dielectrics or semiconductors that can hold the charges. Chemical capacitors have special characteristics that can hold both cations and anions. They are called zwitterionic capacitors. Only the net charges are counted. Chemical capacitors have functional groups at their surface that can adsorb either cations or anions or both. For instance, a pH glass electrode adsorbs H1 in an acidic solution to show a positive potential with increasing acidity and a more negative potential with increasing basicity. The Na1 and OH2 ions can be adsorbed on the surface of the glass simultaneously in an alkaline solution (2). The Ag/AgCl reference electrode shows different potentials in contact with different concentrations of KCl solution because different amounts of chloride ions are adsorbed onto the Ag/AgCl surface. The Ag/AgCl reference electrode is a capacitor, not a half-cell (4). Furthermore, a capacitor holding charges may be discharged by grounding; its potential may be slowly recharged due to readsorption of ions on the electrode surface. CONCLUSION We have two types of potentiometry, namely, Faradaic potentiometry and non-Faradaic potentiometry. In the past we have neglected the latter. It is important to recognize which type of potentiometry we are dealing with and its potential origin. Then we know the potential mechanisms of electrodes which are of vital importance to us in understanding the reactions and interpretations. This paper provides some simple tests to distinguish the two types of potentiometry (see Table 1). The potentials of calomel and Ag/AgCl reference electrodes, improperly included in the reduction potential list (6 – 8), may be renamed capacitance potentials because there is no redox in their reactions.
CAPACITANCE POTENTIAL
325
REFERENCES 1. Cheng, K. L. Microchem. J., 1990, 42, 5–24. 2. Cheng, K. L. Proceedings, 31st IUPAC Congress, Anal. Chem. Div., pp. 171–197 (1987). 3. Cheng, K. L. pH glass electrode and its mechanism. In Electrochemistry, Past and Present (J. T. Stock and M. V. Orma, Eds.), ACS Symposium Series No. 390 pp. 286 –302. Am. Chem. Soc., Washington, DC, 1989. 4. Cheng, K. L.; Temsamani, K. R. Paper presented at the ACS National Meeting, Dallas, TX, March 1998. 5. Chin-I Huang, C-I.; Huang, H. J.; Cheng, K. L. in Advances in the Applications of Membranes—Mimetic Chemistry (T. F. Yen, R. D. Gilbert, and J. H. Fendler, Eds.), pp. 227–240. Plenum, New York, 1994. 6. Rieger, P. H. Electrochemistry, p. 452. Prentice–Hall, Englewood Cliffs, NJ, 1987. 7. Milazzo, G.; Caroli, S. Tables of Standard Electrode Potentials, pp. viii–xiii. Wiley, New York, 1978. 8. Bard, A. J.; Parson, R.; Jordon, J. Standard Potentials in Aqueous Solution, pp. 1–7. Marcel Dekker, New York, 1985.
59, 323–325 (1998)
MJ981578
Capacitance Potential K. L. Cheng1 Department of Chemistry, University of Missouri—Kansas City, Kansas City, Missouri 64110 Received December 23, 1997; accepted January 9, 1998 Based on their different mechanisms, several simple tests are presented for identifying two types of potentiometry: Faradaic and non-Faradaic To understand the electrode potential, mechanism is emphasized over commonly misused thermodynamics. That the improper term reduction potential be changed to capacitance potential for calomel and Ag/AgCl is suggested. © 1998 Academic Press Key Words: Faradaic; non-Faradaic; capacitance; potential; redox; half-cell; mechanism.
INTRODUCTION There are two types of potentiometry: (1) Faradaic potentiometry based on redox reactions and (2) non-Faradaic potentiometry based on charges or ion adsorption onto a dielectric or semiconductor that can hold the charges. The former is normally a cell composed of two half-cells with a salt bridge. The latter is a capacitor against another capacitor or a reference electrode requiring a connecting wire. The definitions of a cell and a capacitor have been given (1). Non-Faradaic potentiometry does exist, but little has ever been reported. Many electrodes and reactions in the area of non-Faradaic potentiometry have been misidentified as Faradaic potentiometry, for instance, the pH glass electrode, the potential of which is derived from a capacitor, not from a half-cell. Also, the Nernst equation based on redox equilibrium has been misused (1). Most ion selective electrodes (ISEs) are capacitors, not half-cells as commonly believed (1). It is important to distinguish between them because their potential mechanisms are different. This paper tries to offer a few simple tests to identify whether an electrode is a half-cell or a capacitor. DISCUSSION A half-cell requires a salt bridge to transfer ions to maintain an ionic balance between two half-cells. The function of the salt bridge is to permit ions to flow from one solution to the other. A capacitor, as distinguished from a half-cell, has no ions to be transferred; consequently, there is no need for a salt bridge. A connecting wire is sufficient for the electron transfer. Certainly, a salt bridge would serve the same purpose. The use of a connecting wire for measurement of potential is a simple test to distinguish a capacitor from a half-cell. For a half-cell, the electrode shows no potential difference when it is dipped in the solution at different depths, as the electrode is used to conduct the current flow and the potential depends on the concentrations of oxidants and reductants in separate solutions. For example, pH measurement with an antimony electrode is a redox reaction, displaying the same potential at different depths (2). On the other hand, the potential of 1
To whom correspondence should be addressed.
323 0026-265X/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
324
K. L. CHENG TABLE 1 Difference in Potential Measurements Condition
Half-cell
Capacitor
Salt bridge Metal wire connection instead of salt bridge Add oxidant or reductant Dipping electrode at different depths Capacitance measurement Dielectric or semiconductor Ion adsorption effect Discharge and recharge Stirring (5) Like ISE
Required No Potential change No potential change No capacitance No No No potential change No potential change No
Not required Yes No potential change Potential change Yields capacitance Yes Yes Potential change Potential change Yes
a capacitor comes from the charge density of the electrode surface. Its potential varies as a function of the contact between the electrode surface and the solution (1). This has been demonstrated with the lead ISE and pH glass electrode (1). The capacitance of a pH glass electrode has been measured in both acid and basic solutions (3). A half-cell, in contrast, does not yield any capacitance. Many half-cells are connected to a metal, and capacitors are dielectrics or semiconductors that can hold the charges. Chemical capacitors have special characteristics that can hold both cations and anions. They are called zwitterionic capacitors. Only the net charges are counted. Chemical capacitors have functional groups at their surface that can adsorb either cations or anions or both. For instance, a pH glass electrode adsorbs H1 in an acidic solution to show a positive potential with increasing acidity and a more negative potential with increasing basicity. The Na1 and OH2 ions can be adsorbed on the surface of the glass simultaneously in an alkaline solution (2). The Ag/AgCl reference electrode shows different potentials in contact with different concentrations of KCl solution because different amounts of chloride ions are adsorbed onto the Ag/AgCl surface. The Ag/AgCl reference electrode is a capacitor, not a half-cell (4). Furthermore, a capacitor holding charges may be discharged by grounding; its potential may be slowly recharged due to readsorption of ions on the electrode surface. CONCLUSION We have two types of potentiometry, namely, Faradaic potentiometry and non-Faradaic potentiometry. In the past we have neglected the latter. It is important to recognize which type of potentiometry we are dealing with and its potential origin. Then we know the potential mechanisms of electrodes which are of vital importance to us in understanding the reactions and interpretations. This paper provides some simple tests to distinguish the two types of potentiometry (see Table 1). The potentials of calomel and Ag/AgCl reference electrodes, improperly included in the reduction potential list (6 – 8), may be renamed capacitance potentials because there is no redox in their reactions.
CAPACITANCE POTENTIAL
325
REFERENCES 1. Cheng, K. L. Microchem. J., 1990, 42, 5–24. 2. Cheng, K. L. Proceedings, 31st IUPAC Congress, Anal. Chem. Div., pp. 171–197 (1987). 3. Cheng, K. L. pH glass electrode and its mechanism. In Electrochemistry, Past and Present (J. T. Stock and M. V. Orma, Eds.), ACS Symposium Series No. 390 pp. 286 –302. Am. Chem. Soc., Washington, DC, 1989. 4. Cheng, K. L.; Temsamani, K. R. Paper presented at the ACS National Meeting, Dallas, TX, March 1998. 5. Chin-I Huang, C-I.; Huang, H. J.; Cheng, K. L. in Advances in the Applications of Membranes—Mimetic Chemistry (T. F. Yen, R. D. Gilbert, and J. H. Fendler, Eds.), pp. 227–240. Plenum, New York, 1994. 6. Rieger, P. H. Electrochemistry, p. 452. Prentice–Hall, Englewood Cliffs, NJ, 1987. 7. Milazzo, G.; Caroli, S. Tables of Standard Electrode Potentials, pp. viii–xiii. Wiley, New York, 1978. 8. Bard, A. J.; Parson, R.; Jordon, J. Standard Potentials in Aqueous Solution, pp. 1–7. Marcel Dekker, New York, 1985.