Note 531 — Chapter XIV · OXIDATION AND REDUCTION REACTIONS. I

The convention, adopted in the text, for the use of the positive and negative signs in expressing potentials, is that proposed by Luther (cf. Le Blanc's Lehrbuch der Elektrochemie (third edition), p. 212). The sign always denotes the character of the charge on the first component written in the subscript to ε. Thus, for a copper plate in contact with a solution of cupric sulphate, when C > K, the logarithm, log(C / K), has a positive value and εCu, CuSO4 is positive, which means that the metal will be positive, the electrolyte negative. For instance, for [Cu2+] = 1, εCu, CuSO4 is found to be +0.606 (see the table at the end of Chapter XV). εCu, CuSO4 = −εCuSO4, Cu′. By this use of the signs one is never in doubt as to their meaning. Unfortunately, widely different definitions of the signs have been used (cf. Le Blanc, Electrochemistry (1896), pp. 209, 219, and Lehfeldt, Electro-Chemistry (1904), p. 159). Care must be taken, in using the data of original papers, to be informed as to the definition used.

In accordance with the convention as to signs, adopted in this book, the ratio of concentrations (C / K), used in the logarithm of Nernst's formula, is the reciprocal of the ratio usually given. The change has been made in order that the algebraic signs of the values obtained from the application of the formula should be the same as those observed in the experimental arrangements, as demanded by the convention.