was proceeding through the chlorine intermediate as one would predict. The addition of cupric ion (final concentration, 0.03F) caused the reaction to speed up satisfactorily; no adverse effects of the cerous salt were noticed. Thus large amounts of chloride interfere in the ceric titration of iron, but one can easily use chlorine as titrant in the presence of cerous if the requisite amount of copper is present as a catalyst. If the sample originally contains a large amount of chloride, then there would be no point in adding cerous ion to the solution at all.Test Method Using Standard Samples.With the above-mentioned interferences in mind, four Bureau of Standards samples were chosen, and their iron content determined by this method. The silver reductor was used and a parallel blank run in all cases; the blank was negligible except in the case of the ferrosilicon.Fluorspar. Standard Sample No.

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ond, the rise to the diffusion plateau with eathodically increasing potential would not be as steep as in the present case.Third, the decay of current with time would be much slower at the foot of the diffusion plateau. CONCLUSIONSThe potential-current-time surface with its associated equation gives excellent qualitative and in most cases quantitative agreement with more rigorous treatments for various voltammetric techniques. It may prove useful in the evaluation of. proposed new techniques for which exact theoretical treatments are difficult. Moreover, experience indicates that it is a valuable teaching aid. The fundamental similarities among various voltammetric processes as well as their differences become apparent with the aid of the surface. These similarities seem to escape many students when they deal only with the more abstract algebraic equations describing the methods. ,

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