ages from concentration polarization with a corrcsponding decrease in the accuracy n i t h wliich k , could be determined. The fact tliat k, decreases as the p H and concentration of sulfate ion increase may be due to increasing complexation of Fe+3 by sulfate. One can calculate from the Fe(II1)-sulfate complex-dissociation constants determined by Whiteker and Davidson (2 3) t h a t t h e relative amounts of Fe+3, FeS04+, and Fe(S04)2- present in the three solutions of Figure 1 are 7%-55%38%, 1%-29%-70%, and
