A method of oxidizing Cr+3 for the determination of trivalent chromium.

The chromium of most chromium compounds exists in the form of triand hexa-valences. Application of a chemical method of determination of the former is difficult because the former is far more inactive than the latter. For example, neither an extraction method by organic solvents or a colorimetric method with diphenylcarbazide is suitable to determine Cr+3, because Cr+3 does not react with diphenylcarbazide and other chelating agents such as APDC and DDTC etc. Therefore, a method of oxidizing Cr+3 to Cr+6 is necessary for the determination of Cr+3.1) The author proposed the following method by which Cr+3 was oxidized to Cr+6 in a water-soluble state without difficulty. The sample solution containing Cr+3 was made acidic by adding H2SO1, and then a small quantity of a strong oxidizing agent was added to it. Powdery MnO2 or PbO2 was used for the oxidizing agent in the present experiment. Then the solution was boiled for a while. The oxidizing reaction proceeded easily by the above procedure, and Cr+3 changed to Cr+6 rapidly as follows: