The determination of iron in organic substances: Micro-colorimetry as ferric thiocyanate. II. Potentiometric titration with titanous sulphate.

IN certain experiments on iron metabolism, involving the iron-containing protein of egg-yolk, lecitho-vitellin, we planned to make a large number of iron estimations. To conserve our small supply of protein, the preparation of which had involved considerable time and labour, we desired to cut down the size of the sample analysed to the smallest amount on which an accurate iron determination could be made. The author and colleagues [1930] had already found that the iron content of egg-yolk protein could be accurately determined by the method of Kennedy [1927]. The samples then analysed contained about 0 05 mg. iron, an amount somewhat greater than that which we now wished to estimate. The results of a critical study of Kennedy's procedure as the basis of a micro-colorimetric method are recorded in the first part of this paper. The second part is concerned with the determination of small amounts of iron in biological materials by potentiometric titration with titanous sulphate.