Recent work 1 on the adsorption of the optical isomers of dibenzoyl tartaric acid indicated that their behaviour at the interface was identical but that marked differences were observed with the racemic mixture. It was proposed that this was due to the adsorption of D-L couples. The geometrically isomeric acids, maleic and fumaric, were studied by Korchinsky 2 by the electrocapillary method. Differences were observed which were compatible with the structure of the isomers. No detailed study has yet been made of the adsorption of isomeric ions. The present work was undertaken to investigate the properties of the mercury-aqueous solution interface when such ions are adsorbed.
Water was purified by triple distillation. Sodium maleate and sodium fumarate (British Drug Houses Ltd.) were recrystaliized from this water after treatment of the solutions with activated charcoal. The salt was dried finally in a desiccator over silica gel. Mercury was purified as described previously 3 . The reference electrode was a dip-type saturated calomel electrode. It was dipped into a 0.1 mol 1 "1 solution of salt (maleate or fumarate); contact with the working solution being then made in a glass tap. Capacities were measured using the bridge previously described 4 . No significant dispersion with frequency was observed. The point of zero charge was found using a streaming mercury electrode s .
The capacity curves for solutions containing 1 mol 1 "1 of each salt at 25Β°C are shown in Fig. 1. It is evident that the two anions yield capacity curves which differ considerably in shape in the region where specific adsorption of the anions would be expected. In the presence of fumarate the capacity shows a large 'hump' close to the point of zero charge followed on the positive side by a deep minimum. This behaviour is similar to that shown by perchlorate or hexafluorophosphate ion but the peak value of the capacity is higher. On the other hand in the presence of maleate ion the 'hump' is almost suppressed while the steep rise in capacity occurs at a potential 0.25 V more negative than in the presence of fumarate. Preliminary measurements of etectrocapillary curves suggest that the two isomers are adsorbed approximately equally. Thus the difference in the capacity curves must be due to the difference in the location of the adsorbed charge in the inner layer. General considerations suggest that the carbon-carbon axis in both anions is probably oriented parallel to the plane of the interface. In the fumarate ion the carboxyl groups will then both be close to the mercury surface while those of the