Several rate-accelerating effects on the oxidation of ferroin by Cr(VI) in sulfuric acid medium are reported. In all cases, these effects are observed in the earlier portions of the reaction profile and no catalyic cycle appears to be associated with them. Conversion of the rate-modifying species to an inactive form by destruction or, mainly, by complexation with Cr(lll) seems to account for the lack of catalytic cycle. Judicious choice of reaction conditions allows the determination (by initial rate measurements) of microgram amounts/milliliter of oxalic acid, citric acid, vanadium(IV), arsenic(lll), chromium(VI), hexacyanoferrate(lll), and mg/ml of molybdenum(VI).
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