Colorimetric determination of potassium by Folin-Ciocalteu phenol reagent

A method for the colorimetric estimation of manganese by reduction of the phosphomolybdicphosphotungstic acid phenol reagent has been described (Abul-Fadl, 1948). Cobalt was also found to give a similar colour reaction in the presence (but not in the absence) of amino-acids. Conversion of cobalt into the cobaltinitrite did not affect its response to this reaction. As methods for the estimation of potassium in biological fluids depend on precipitation as cobaltinitrite, and subsequent estimation of one of the constituents of the precipitate, it was thought that this colour reaction might be useful in this connexion. Amongst colour reactions hitherto used for the microdetermination of potassium -are those of Doisy & Bell (see Briggs, 1923), and of Looney & Dyer (1942). Both reactions are based on the formation of an azo compound by the nitrite radicals of cobaltinitrite. Theoretically, those methods which determine directly a stable constituent ofthe precipitate, e.g. cobalt, are preferable to those which depend upon the unstable nitrite radical. Breh & Gaebler (1930) described a method in which cobalt is determined as the thiocyanate. This method involves the precipitation of potassium as the -silver cobaltinitrite complex which is more sparingly soluble than the customary K2NaCo(NO2)6. The method is, however, more troublesome to execute and the colour reaction involved is not entirely satisfactory, since it depends upon such factors as temperature, ethanol concentration, etc. Jacobs & Hoffman (1931) introduced a reaction between alkaline ferrocyanide and cobalt compounds in the presence of choline hydrochloride, which gives a stable green colour.