The determination of manganese by the peroxidisulphate method

The colorimetric determination of manganese according to the periodate method requires a rather long period of heating for the full development of colour, especially in extremely dilute solutions. This fact led the author to investigate the possibilities of the faster peroxidi-sulphate method which for several reasons has been considered as rather unreliable.The extinctions found were compared to those of standard permanganate solutions.The optimum conditions of acidity were fixed. ALTEN AND WEILAND'S statement, that the presence of phosphoric acid prevents the precipitation of manganese dioxide, was verified.The oxidation could be carried out in the presence of chlorides and minute amounts of bromides and iodides if these ions were complexly compounded with mercuric ions. The concentration of the catalysing silver ions can conveniently be kept as low as 10-5 M which will eliminate troubles from silver chloride precipitates.Variation of the concentration of the peroxidisulphate, within wide limits, has no influence on the results.The oxidation may be carried out in the presence of certain amounts of organic matter.The permanganate is stable as long as there is an excess of peroxidisulphate and no reducing compounds are present; the decomposition rate of the peroxidisulphate under the prevailing conditions has been investigated.Amounts of manganese ranging from 100 to 0.1 μmoles (5.5-0.0055 mg) were determined in l00 ml of solution. At the higher manganese concentrations the relative error was about 1 % ; at the lowest concentrations the relative error was larger but the absolute error in no case exceeded 0.01 mmmol.From this investigation it can be concluded that the peroxidisulphate method is at least as accurate as the periodate method. It is more rapid and the fact it can be used in the presence of chlorides and organic matter makes it preferable for several purposes, e.g., for natural waters and soil extracts.