Mass Transfer to a Rough Rotating Cylinder

Abstract

Cd(OH)3-and Cd(OH)4-predominate for OH-concentrations greater than 0.5N while Visco and Sonner (10) argue that Cd(OH)4-is not encountered even in 15N KOH. Adsorption of either negative ion at the original valence would be expected to start near the zpc (zero point of charge) here estimated at 0.72V [based on zpc =-0.11 vs. N.H.E. at pI~I 11.2, Ref. (15) ] and to increase with increasing potential. The reverse dependence of Qc~ upon U was actually found. Adsorption as the neutral atom might be expected to yield a semilogarithmic or linear (if heat of adsorp-tion decreases with coverage) dependence of Qcd upon U. This may be reflected at the low and high potential ends of Fig. 6. The arrest at intermediate potentials may be due to a tendency to form a surface alloy of fixed composition. The decline of Qcd at high potentials may alternatively reflect a competition with an-odic film formation. Cadmium ion adsorption and electroreduction of oxygen .-From Fig. 6 we see that Qcd = 0 for U ~ 1.2V and hence Cd ion adsorption ought not to affect a platinum-catalyzed oxygen electrode operating near the reversible potential (1.23V). However, practical oxygen electrodes tend to operate at 0.9V or less, as in Wagner's experiment (1). Beginning at 0.9V, in the presence of dissolved cadmium, Ocd would tend to rise toward the intermediate value reflected by Qcd in Fig. 6. That coverage would cause a drop in voltage (increase in oxygen overvoltage) and an increased tendency to adsorb cadmium ions until the highest values of Qcd and the potential of the counterelectrode is reached. This was actually observed in a laboratory cadmium-oxygen battery cell (1).