The measurement of ionic activities in solution with electrodes containing ion‐exchange‐resin membranes

It is shown that an electrode made from a membrane (actually a short length of 2‐mm.‐diameter rod) of a cation‐exchange resin of the sulphonated‐phenol type and containing a reference electrolyte of fixed concentration, gives steady and reproducible e.m.f. values when immersed in solutions of the same electrolyte of other concentrations. Between certain limits of concentration, different for every electrolyte, the e.m.f. values are those predicted by the Nernst equation when the activity functions are put equal to the mean ionic activities of cation and anion in the electrolyte solution. The upper limit of conformity with the Nernst equation is determined, at least qualitatively, by the ratio of the mobilities of cation and anion within the resin phase, as predicted by the Meyer & Sievers and the Teorell equations. With some limitations the electrode can be used to measure unknown ionic activities in solution in a manner similar to that of the glass electrode, and the activity coefficients of potassium ferrocyanide in solutions of 0·02–0·001M are measured in this way. Unlike the glass electrode for the hydrogen ion, the resin electrode is not specific for any one ion, and measurements with solutions containing two cationic species show the effect of the contributions from both species to the total e.m.f.