Some limiting factors in the use of picramate as a measure of reduction

Abstract

This report pertains to the colored solutions formed by the reduction of picric acid. The color is not due to a single substance but to a mixture of several colored substances. In solutions where the color is due to some one substance, there is a simple and definite ratio between the concentrations and the color values. When the color is due to two or more substances a definite ratio holds for differences in concentrations only within relatively narrow limits. The colorimetric determination of picrate reduction mixtures is therefore limited. It is thought that this limitation can in part be counteracted by isolating the most characteristic component of the color. This is done by extracting the products after the reduction of picrates with immiscible organic solvents, shaking this extract with an alkaline solution. The alkaline solution assumes a color which is qualitatively very like the original reduction mixture. The number of colored constituents has been reduced. The color values of the reduction mixture and the isolated color component are affected by change in temperature. One degree centigrade decreases the color value one per cent. Salts, cane sugar and alkalies increase the color value. These tests were made by diluting picramate mixtures with molar solutions and comparing them with a standard prepared in the same manner as the picramate mixtures which were tested. In half-molar solutions there is but a slight error for sodium chlorid; for the sugar it came to 10 per cent., and for potassium carbonate to 12 per cent. Molar solutions gave an error of 17 per cent. in the case of salt, 14.5 per cent. for sugar, and 16 per cent. for potassium carbonate. Sodium hydroxide at this concentration caused an error of 8 per cent.