The fact that an alcoholic solution of picric acid can be used as a reagent for potassium has been frequently noted in the literature of analytical chemistry. There exists (as of 1929), however, no quantitative data regarding either the sensitivity of this reagent toward the potassium ion or the allowable concentrations of other ions that can be present without causing interference. The experiments described in this article were undertaken in order to determine these points and to find the optimum conditions for this reaction.