Electromotive force studies of electrolytic dissociation. Part 12.—Dissociation constants of some strongly ionising acids at zero ionic strength and 25°C

Abstract

A glass electrode–AgCl cell, measured to ±0.01 mV, has been used in evaluations of pK values at 25°C of four acids in the range pK 1–2.2. The practical work consisted of calibrating the cell with dilute HCl followed by additions of stock solutions containing the acid ligands. Three of the acids have been previously studied with Pt, H₂–AgCl cells so critical comparisons are possible.

Attempts have been made to reduce the dependence of the pK values at zero ionic strength (when these are < 2.5) upon the value of the ion-size parameter ρ used in the Debye–Hückel activity coefficient expression. The present calculations are based upon the use of a second parameter Q. Round values of this at different values of have been calculated from some HCl, salt data. A second but smaller feature which has been taken into account is the observations of Bates that standard electrode potentials (E°) vary slightly with ρ if E° is derived by linear analysis. By using these two factors, (method C of table 3) the pK against ρ dependence is markedly diminished. Also, by using the ρ and Q parameters appropriate to HCl solutions, pK values close to the averages of those obtained by method C are obtained.