250. Differential capacity measurements in aqueous and methanolic ammonium chloride, ammonium fluoride, and hydrochloric acid in relation to anion adsorption

Abstract

Differential capacities were measured in O.~N-NH,CI and -NH,F and in 1.0-, 0-3-, and O.~-N-HCI, in water and methanol. For the first two electrolytes, in both solvents, earlier results were conhned, and the method usually followed in interpreting the minimum capacity observed with increasing cathodic polarisation is discussed. In aqueous and methanolic hydrochloric acid solutions, reduction of hydrogen ions prevented extension of the measurements, and no capacity minima were observed within the cathodic potential range corresponding to ideal polarisation. The electrocapillary curves of mercury in the latter solutions were obtained. From the rate of change of the interfacial tension with the chemical potential of HCl, surface excesses of chloride ions were calculated for aqueous and methanolic 0.3~-hydrochloric acid, and the results compared.GRAHAME reported measurements of the differential capacity of the electrical double layer between mercury and methanolic salt solutions of ammonium chloride, fluoride, and nitrate. The outstanding features of the curves obtained for 0-1N-solutions are, first, the complete disappearance of the hump characteristic of the curves obtained in the corresponding aqueous solutions; secondly, the relatively low minimum capacity value, Cmin., of 9.0 pF/cm.2 observed on the cathodic potential side compared with that observed in aqueous solutions, viz. ,2 154--16.0 yF/cm.2. At potentials less cathodic than that corresponding to Cmin., the curves diverged because of the specific adsorption of anions (Cl-), but they coincided almost exactly at more cathodic potentials. Because of this similarity in behaviour, and by comparison with that observed in aqueous solutions, was considered to indicate complete repulsion of any specifically adsorbed charge from the metal surface. Grahame pointed out that conclusive evidence could be obtained from the rate of change of the differential capacity with chemical potential, but the necessary data, the activity coefficient values in methanol, are lacking. From surface-tension measurements in aqueous and methanolic hydrochloric acid solutions, Parsons and Devanathan had shown that anions are less readily repelled from a negatively charged mercury surface in methanol than in water. In methanol positive adsorption of the C1ions even at potentials corresponding to hydrogen evolution was reported, a result apparently not in accordance with Grahame's conclusion.We measured differential capacities in 0.1N-aqueous and -methanolic ammonium chloride and fluoride under conditions differing only in minor details from those used by

Properties of the Electrical Double Layer in Solutions of Electrolytes in Some Organic Solvents

“…It is well known that the differential capacity in solutions of some aliphatic alcohols, unless they do not have very low values of dielectric constant, can be measured by means of a similar method, as used by Grahame, and results are equally reproducible and easily obtained as in water (2)(3)(4)(5).…”