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In a previous paper (Loureiro a Janz, 1942) we compared the iodometric and oxalic acid titres of permanganate solutions under various conditions, and verified that the values obtained in titrating small samples of oxalic acid in the usual way in hot dilute acid solution may differ appreciably from those yielded by the permanganate-iodide-thiosulphate reaction. This merely confirms a well-known fact on which there is a considerable literature (see McBride, 1912; and Fowler & Bright, 1935), namely, that when excess permanganate is used to oxidize oxalic acid and the excess permanganate is determined iodometrically, the reaction is not stoichiometric. We further ascertained that a closer agreement with theory is obtained when the reaction is carried out in strong acid solution and in the presence of added manganese sulphate. A solution containing 35% H2SO3 and 5% MnSO . 4H2O reacts with oxalic acid at room temperature and with KI in exactly equivalent proportions. This makes it possible to replace the usual titration to the 'pink end-point' by the more exact and sensitive iodometric estimation of unreduced permanganate through the iodide-thiosulphate reaction, or colorimetrically by means of a photoelectric colorimeter. This greater sensitivity may eventually permit a reduction of the size of the sample required for

Iodometric and colorimetric methods for the estimation of calcium in serum based on the use of an improved permanganate solution