A Micro Method for the Determination of Potassium as Iodoplatinate *-

The standard and most widely accepted method for the determination of potassium depends upon its precipit,ation as potassium chloroplatinate. The modification called the Lindo-Gladding procedure (1) is official with the Association of Official Agricultural Chemists. Methods based upon the precipitation of potassium as potassium of cobalti-nitrite (2), have been very useful in determination of potassium in the blood and serum when the technique described (3) is rigidly adhered to, but under slightly different conditions, or for material of varying salt composition, are questionable. This criticism applies also to modifications involving either the gasometric (4) or calorimetric (5) determination. The cobalti-nitrite precipitate is not of constant composition (6). The method about to be presented depends upon the precipitation of the potassium chloroplatinate in the ash of physiological material and its subsequent determination by calorimetric or titrimetric procedure or by both. The quantitative determination depends upon the conversion of potassium chloroplatinate to potassium iodoplatinate by the addition of potassium iodide. Because of the deep rich wine color of the iodoplatinate in solution, amounts as small as 0.1 mg. of potassium in this form can be estimated by calorimetric comparison with known amounts of potassium chloroplatinate also treated in this way. The principle was used in 1903 by Cameron and Failyer (7) for the determination of potassium in water. The presence of iodine in the compound permits t,he use of the