Dec., 1942
NOTES
3033
of the validity of the technique and results of the excellent work of Hornibrook, Janz and Gordon is intended. B Y EDGAR REYNOLDS SMITH AND JOHN KEENANTAYLOR From the dimensions of the electrodes and the An aging effect with freshly prepared silver- time and current used for coating with silver chlosilver chloride electrodes was first noted by Mac- ride, it appears that the thickness of the coatings Innes and Parker,' who found it to be always in and the magnitude of the current density used by the same direction. This effect was investigated Smith and Taylor were, respectively, about by Smith and Taylor,2 who attributed its origin 13.4and 5.6 times as large as those used by Hornito concentration-polarization and made tests, brook, Janz and Gordon. The latter point out the results of which harmonized with this explana- that concentration differences tend to disappear tion. When silver electrodes are coated electro- with exp ( - r 2 k t / x 2 ) , where k is the diffusion conlytically with silver chloride, the solution within stant, t the time, and x the distance from the the pores of the silver chloride becomes more plane where the concentration is constant. The dilute than the surrounding solution and these time required for the exponential to fall to a given electrodes, freshly prepared, therefore, act as value increases as the square of the distance, x , cathodes toward electrodes previously aged in the which can be taken as the thickness of the layer of solution, since the latter have had the electrolyte silver chloride. Hornibrook, Janz and Gordon within their pores replenished. Because the aging found no indication of aging after twenty-five behavior is always in the same direction and, minutes in one experiment and after forty minutes for relatively thick coatings of silver chloride, in another. If the time of aging with their relaamounts a t least to several tenths of a millivolt, tively thin layer was of the order of ten minutes, it appears to be an effect dzerent from the it would escape detection, while with a layer 13 fluctuating bias of a few hundredths of a millivolt times as thick, about twenty-eight hours would frequently observed after the electrodes have ap- elapse before the aging was over, and the effect parently come to equilibrium. If sufficient time would be readily found. is allowed, the freshly prepared electrodes attain Present conditions of work make it unjustifiable the same average potential as the aged electrodes. for us to spend the time required to show exThe time required for this equilibrium to be at- perimentally how the aging period of freshly pretained depends on several factors, among which pared silver-silver chloride electrodes varies from are the amount of silver chloride deposited per a negligible time for a very small thickness of unit area, i. e., the thickness of the layer through silver chloride deposited a t a low current density which the diffusion must occur, and the current to an appreciable time for a thicker layer deposited density at which the layer is formed. The smaller a t a higher current density. Our previous work, the current density of silver chloride formation, however, has shown the latter, and the results of the nearer is the concentration within the pores Hornibrook, Janz and Gordon not only show the of the layer kept to equilibrium with the surround- former, but are what would reasonably be exing solution and consequently the smaller is the pected on the basis of our explanation of the aging aging effect. effect. In some recent work, Hornibrook, Janz and Because of the various factors, such as film Gordon3 made experiments, the results of which thickness, current density, temperature, concenthey consider to contradict the conclusions of tration and agitation of the solution, which affect Smith and Taylor. Their results, however, in- the time of aging, different investigators using stead of being contradictory to these conclusions different conditions of preparation of their are in agreement with what is to be expected for silver-silver chloride electrodes will find marked thin films of silver chloride formed a t low current differences in the magnitude and duration of densities. It should be emphasized that no dis- the aging effect. In any case, however, it is pute as to the facts is involved and no criticism important to recognize the possibility of the (1) D. A. MacInnes and K. Parker, THISJOURNAL, 37, 1445 occurrence of the effect and to make sure it has (1915). been eliminated from the results of the final (2) E.R.Smith and J. K . Taylor, J.Research h'afl. Buy. Standards, 20,837 (1939). measurements. Change in Potential of Silver-Silver Chloride Electrodes with Time
(3) W.J. Hornibrook, G . J. Janz and A. R. Gordon, THIS JOURNAL, 64, 514 (1942).
WASHINGTON,
D. C.
RECEIVED SEPTEMBER 10, 1942
