GUEST AUTHOR Richard A. Durstl Boston College Chestnut Hill, Massachusetts
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Textbook Errors,
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Mechanism of the Glass Electrode Response
Although glass membrane electrodes have been studied since the early twentieth century, the mechanism by which the electrode potential develops in response to hydrogen ion activity has only recently been clarified by critical experiments. Numerous theories of the glass electrode potential formation have been proposed. Among them, the most widely accepted were the adsorption-potential, the membrane-potential, and the phase-boundary-potential theories, all of which give satisfactory explanations of the glass electrode behavior. The adsorption-potential theory postulates an adsorbed layer of hydrogen ions on the glass surface causing a potential drop at the glass-solution interface, corresponding to the difference in chemical potential between the free and adsorbed ions. However, this theory predicts a change of less than 59 mv per pH unit change and cannot explain the glass electrode behavior in strongly acidic or basic solutions. In the membranepotential theory, the glass membrane is considered to be permeable to hydrogen ions, and the potential developed is due to the difference in diffusion rates of ions through the glass. This theory correctly predicts the behavior of the glass electrode in acidic and basic as well as neutral media. Although this mechanism is cited in a number of recently published textbooks as the explanation of the pH response of the glass electrode, the concept of an actual penetration through the glass membrane by substantial amounts of hydrogen ion has been definitely disproved by the work of HaugaardZ and the coulometric tracer experiments of Schwabe and D a h m ~ . It ~ was shown that hydrogen ions become bonded in the silicon-oxygen network of the gel layer and contribute little to the conGuest columns and suggestions of material suitable for them should he sent with as many details as possible, particularly with references to modern textbooks, to W. H. Eberhardt, School of Chemistry, Georgia Institute of Technology, Atlanta, Ga. 30332. Since the purpose of this column is to prevent the spread and continuation of errors and not to evaluate individual texts, the sources of error will not be cited. In order ta be presented, an error must occur in at leest two independent recent standard books. Present address: Chemistry Building, Room B326, National Bureau of Standards, Washington, D.C. 20234. a HAUGAARD, G., J. Phys. Chem., 45, 148 (1941). SCHWABE, K., AND DAHMS,H., Z . Elektrochem., 65, 518 11461 \
S C B ~ ~K., E AND , SUSCHHE,H. D., Sngew. Chem., 76, 39 (1964). SCAWABE, K., AND DAHMS,H., Z . Elektrochem., loe. eit. BATES,R. G., "Determination of pH, Theory and Practice," John Wiley & Sons, New York, 1964, Chap. 10.
duction of the slight current through the glass, while most of the current appears to be carried by the more labile sodium ions. At the present time, the most generally accepted theory of the glass electrode mechanism is based on an ion exchange equilibrium occurring a t the solution-glass boundary. The phase-boundary-potential (or Donnanpotential) theory assigns the development of the glass electrode potential to a thermodynamic equilibrium between the electrolyte solution and the glass. However, since the potential is established much faster than could be expected from a true phase equilibrium, it must be assumed that only the gel layer of the glass directly participates in the eq~ilibrium.~Indeed, it has been shown5 that the cationic exchange only occurs in the external part of the gel layer, which does in fact act as a semipermeable membrane to hydrogen ions, and the inner regions of the glass have little effect on the potential formation. Using the mechanism in which alkali metal ions in the silicon-oxygen network of the glass exchange with ions in the test solution, the pH response and the acid and base errors of glass electrodes can be explained! Qualitatively, the negative acid error is due to the migration of the acid anions into the gel layer and/or a change in the activity of the water in the gel layer thereby affecting the hydrogen ion activity. The gradual dissolution of the outermost layer of glass may also account for the error in strongly acid solution by preventing the formation of a steady-state Donnan potential. The positive alkaline error is due to the partial exchange of cations, other than hydrogen ion, between the pH sensitive surface layer of the glass and the basic solution. In general, the alkaline error will be large when the test solution contains a cation in common with the glass, or a cation of the same group but of smaller atomic number. In summary, the glass electrode can be considered to exhibit an overall potential which is the sum of several independent potentials: (a) the potential of the inner reference electrode, (b) the potential developed a t the inner surface of the glass membrane, (c) the asymmetry potential, and (d) the pH responsive potential a t the external glass membrane surface. The potentials (a) and (b) are constant and only dependent on the particular solution contained in the glass electrode envelope. The asymmetry potential is somewhat variable and attributed to differences in thc inner and outer surfaces of the glass membrane, e.g., uneven stwin in the glass or differences in the chemical composition of the glass. In using the glass electrode, the potentials Volume 44, Number 3, March 1967
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( a ) , (b), and (c) are normally canceled by standardization of the glass electrode in a buffer solution. The remaining pot,ential difference developed in response to the pH of the test solution is given by the equation: RT 2.3RT A& = - ( A h am+) = 5
5
(APE)
i.e., 0.05915 v / p H unit at 25'C, except in very acid or alkaline solutions as noted above. Thus, according to the presently accepted theory of the glass electrode, it is an ion emhange process in the gel layer of the glass
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membrane producing a phase boundary potential that determines the pH response of the electrode and not a diffusion of hydrogen ions through the entire glass membrane. For a more detailed discussion of the glass electrode and the development of the above theories, the reader is referred to a recent review by Schwabe and Suschke? An earlier review by Eisenman8on cation selective glass electrodes may also be of interest. 7
SCRWABE, K., AND SUSCRKE, H. D., Angew. C h a . , loc. eit. EISENMAN, G., Bwphya. J., 2, 259 (1062).
