Phase boundary potentials of absorbed films on metals.—Part II.—On the behaviour of iodine on platinum

We have noted that both gold and platinum on exposure to air are immediately covered with a film of chemi-adsorbed oxygen. On treatment of a platinum surface with an oxidizing agent such as hot chromic acid or on heating in air to a high temperature, and subsequent rapid washing with distilled water, ethyl alcohol and petrol ether, the platinum is found to have acquired a high air-metal potential difference, the magnitude of which is dependent on the mode and intensity of the oxidation. Furthermore, these high values are found to be transitory in character and decay with time. The rate of decay is found to be accurately exponential in character, and a few of the unimolecular velocity constants derived from the equationd (ΔV) /dt = —k ΔV ork = 1 /t log ΔVt / ΔV0 are given in Table I. It appears probable that we are dealing with the slow unimolecular decay of an unstable oxide of platinum. Indeed with strong oxidizing agents the initial rate of change in phase boundary potential is relatively rapid and the complete curve can be separated into two exponential curves suggesting the existence of another but very unstable higher oxide of platinum. The values ofk are, however, quite irreproducible, as shown by Table I, and the complete study of this decay and the determination of a heat or heats of activation, as for iodine on platinum, was not undertaken.