Recalculation of the Latimer, Pitzer, and Slansky Absolute Electrode Potential— A Discussion of Its Operational Significance

Abstract

The Latimer, Pitzer, and Slansky (LPS) calculation of the absolute electrode potential of the standard calomel electrode (StdCE) involves a sequence of processes in which the metals Hg and Na appear. Let Hg be the ``first'' metal and Na the ``second'' metal. The substitution of 21 other metals M for Na as the second metal leads to an array of remarkably self-consistent values. With 16 uni- and divalent second metals M, the LPS potential of the StdCE falls in the range of +0.523 v to +0.693 v (with the Gibbs-Stockholm sign convention) with a mean of +0.599 v and a standard deviation of 0.043 v. The constancy of these values is shown to be thermodynamically reasonable. The LPS method is then generalized to obtain the absolute electrode potentials of 22 other standard electrodes (SMeE) by substituting 22 other ``first'' metals Me for Hg. The generalized LPS absolute potentials of the 22 SMeE's can be converted to that of the StdCE by (1) a simplest, (2) a more refined calculation procedure. The simplest generalized LPS potential of the StdCE is strongly dependent on the choice of the first metal Me, the more refined potential is independent of this choice and agrees with the values obtained above with Hg as first metal. A conceptual experiment is set up by which it is shown that the LPS potential or any of the generalized LPS potentials of the StdCE can be interpreted operationally as linear combinations of conceptually observable potentials differences, all potentials being taken in ``pieces of the same kind of metal'' (to use Gibbs' phrase). This experiment suggests that the LPS potential of the StdCE is equivalent to the measurable outer or volta potential difference in a free space gap between mercury and electrolyte, when the two phases are in electrical contact. The discrepancy between the LPS calculated value and Klein and Lange's experimental value for this volta PD is discussed, and an experimental value of —98.3 kcal obtained for the standard free energy of hydration of the sodium ion.