On the Accuracy of Volhard Method for the Determination of Manganese

Critical study was made on the causes of errors in determining manganese in steel after Volhard method. Solid zinc oxide have a tendency to adsorb manganese ion. Therefore, a large excess of it should be avoided in zinc oxide separation. Potassium permanganate was stable in boiling hot solution at pH 7.08 which was obtained by saturation with zinc oxide. Fading in pink of potassium permanganate observed near the end point of the titration is attributed, therefore, to the consumption of it by manganese ions adsorped on manganese dioxide. Manganese dioxide formed by titration was charged negatively. In the absence of any other positive ion, it adsorbed manganous ions and coagulated. Accordingly low results were apt to be obtained. On the contrary, in the presence of large excess of zinc salt, manganese dioxide adsorbed chiefly zinc ions and manganese ions were left free. Thus satisfactory results were obtained. Adsorption of manganese ion on manganese dioxide and their reaction with potassium permanganate were studied potentiometrically.