A modification of Duboscq-Pellin colorimeter for bicolorimetric work as in colorimetric pH determination without buffer mixtures.

Abstract

A colorimeter in which the combined color of two solutions is used as the standard is required for some purposes. The best illustration is the colorimetric determination of pH without standard buffer mixtures, where the color of the indicator in the unknown solution is matched with the combined color of an acid solution and an alkaline solution of the same indicator. Knowing the ratio of the depths of the two color solutions of the same concentration or the ratio of the concentrations of the solutions of the same depth, the pH of the unknown solution which matches with that of the combination can be calculated by means of an equation derivable from the mass action law and the theory of indicators or by referring to a curve determined experimentally.

Various devices have been used with more or less success for superimposing the colors by Bjerrum 1 , Barnett and Chapman, 2 Barnett and Barnett, 3 Gillespie, 4 , 5 and Myers. 6 But since the Duboscq-Pellin colorimeter is more widely used than any other type of colorimeter, on account of the excellent optical system it possesses, it seems desirable to modify this instrument so that it can be used for bicolorimetric as well as for monocolorimetric work. Such a modification would also avoid the unnecessary duplication of colorimeters.

The modification here described consists essentially in placing on one side of the colorimeter above the original stage a new cup, which is carried on a new stage fitted with an independent rack, pinion and vernier. Thus while one side of the instrument remains unchanged the other is fitted with two cups, each with its own adjustment and vernier, the upper of which is an entirely new addition and requires a new slit in the brass back of the colorimeter.