A BUFFER SOLUTION FOR COLORIMETRIC COMPARISON

Abstract

In the use of standard buffer solutions for calorimetric comparison where more than a restricted range of reaction is required, it has been necessary in the past to make use of several solutions. The author has developed a system requiring but two stock solutions and covering a range of from pH 2.2 to pH 8.0 which approximately includes the limits of reaction for arable soils and physiological media. The materials used are as follows: 0.2 M disodium phosphate1 and 0.1 M citric acid, combined in such volumes as to make 20 cc. of the mixture. The disodium phosphate employed was recrystallized three times. A 0.2 M solution was prepared by titration against I&Cl, using methyl orange as indicator. Although the titration endpoint was not distinct, the error in the resulting mixtures was usually not appreciable, seldom exceeding 0.01 pH. On account of the variable water of crystallization content of the phosphate salt2 the stock solution was standardized by titration in order to have a reproducible method. Upon exposure to the air for a period of 2 weeks the water of crystallization of disodium phosphate is reduced to 2 molecules. If, after exposure, a quantity of the salt is kept on hand in a closed container and the correct weight of the salt required to produce the proper concentration of stock solution determined by titration, it is possible at any time to make up the stock solution by simply weighing out the salt. The citric acid was recrystallized at least twice before using.