NOTES
694
obtained with a charge initially containing 0.5 ill CoS04, 1.9 NHa/Co++, 0.33 (NIT&SO4, and 100 g./L cobalt metnl aced, or conditions where the Co++-H2-H oauilibriuma revcnts quantitative rcduction of C o + + to mPtnl. No colfoidxl grnphit,ewas added t o either run. +
Discussion The principal K-W arguments for their mechanism appear to be: (1) linear reductionrates“wou1d not be expected if the rate-controlling step takes place a t the surface of the growing cobalt met(a1particle”; (2) the rediiction rate was “virtually independent of the nature of the cobalt metal deposit produced”; (3) the reduction rate was proportional to the concentration of colloidal graphite added and was extremely small in the absence of the graphite; and (4)the excellent agreement between the data and their mechanism. Regarding (l), this writer has observed linear reduction rates during 80% of the reaction in the reduction of Ni++ by Hz from aqueous solutions containing nickel metal seed powder but, having no salt precipitate or colloidal graphite.6 However, these linear rates were proportional t80the surface area of the metal particles present. No reduction could be observed in identical runs without seed, indicating negligible homogeneous nucleation of metal particles under the conditions tested. Therefore, the rate-controlling step in the seeded runs was a t the surface of the growing nickel metal particles although linear reduction rates were observed, contrary to K-W’s assumption. This writer does not have strictly similar data for the Co++-Ha reaction, but the assumption in (1) is in question. It may also be noted that K-W analyzed for cobalt in a sample after any cobalt metal had been removed with a magnet. They therefore apparently analyzed the total Co++ in a sample (solution plus salt) rather than the Co++ concentration in the solution alone. Although K-W did not mention the formation of a basic cobalt salt in their work, this writer’s results (curve A in Fig. 1) suggest that with typical K-W conditions the Co++ conceptration in solution remains roughly constant during a large portion of the reduction because the basic salt dissolves, tending to mainhin the c o + + concenihtion in solution a t its equilibrium value. (The early increase in Co++ concentration in each curve in Fig. 1 is attributed to the increased equilibrium solubility of the basic salt due to the formation of (NH&S04 during the reduction.) Therefore, a constant rate of disappearance of total Co++ during the early part of a run would a t first glance be not unexpected since growth kinetics are probably concerned with solution composition if anything. However, this writer has observed that with conditions similar to those used by K-W the growing cobalt metal particles were intimately mixed with, and sometimes imbedded in, the salt precipitate during the early part of reduction. Growth kinetics in such a case is difficult to interpret. Further, linear reduction rates per se when the initial mole ratio of NH3/Co++ is le% than about 2 (K-W’s Fig. 1) is not supported by the present work. In curve B in Fig. 1 of this note, the reduction rate decreased markedly because of approach to equilibrium. (6) W, Q, Courtney, in preparation.
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Regarding (2), the “nature” of the metal deposit which I
