A comparison of platinum and palladium hydrogen-electrodes in aqueous solutions of acid potassium phthalate

Abstract

A study was made of the reproducibility of hyd rogen electrodes of various types and of silver-sHver-chloride electrodes of the thermal-electrolytic type in aqueous solutions of phthalates, in order to learn whether phthalates are reduced to hexahydrophthalic acid and the electromotive force thereby changed. If so, glass electrodes would have to be used to determine the pH valu es of solut ions of phthalates. This type of study is necessary for all reducible materials. E lectro-motive-force measurements of the electrode combinations were made for various periods of t ime. The potentials of hydrogen electrodes prepared with platinum sponge under a variety of conditions constantly increased with time !:nd fre-quently were erratic, whereas those made with palladium sponge under different condit ions remained remarkably constant for periods of 35 hours. Even after 75 hours, the potentials increased only 0.3 mv-which corresponds to an increase of only 0.005 in pH. Palladium or platinum electrodes having the same type of coating but of different ages agreed in potential after 2 hours in phthalate solu-tions. The ra te and the magnitude of the increase in potential for the plati-num electrodes after approximately 2 hours depend mainly upon the thickness of the metallic sponge. The characteristics of the palladium electrodes were practically independent of the thickness of the metallic sponge ; of the current density used in the electrolysis; of the concentration, composition, acidity, and conductance of the plating solution ; and of the concentration, composition, pH, and buffer capacity of the phthalate solutions in which the electrodes were used. The difference in the behavior of platinum and palladium hydrogen-elect rodes may be caused by their different catalytic activity.