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Aqueous solutions containing surface-active ions can be titrated with an aqueous solution containing surface-active ions of opposite charge. The end point, or point at which the solution contains an equal number of positive and negative charges on surface-active ions, is manifest by a minimum in the surface tension (1) and by a maximum in turbidity (2). If a layer of chloroform and an ionized organic dye are present, the end point is marked by sudden migration of color from one liquid phase to the other (3). Bromphenol blue has given sharp end points in this titration (4) and we have adapted the procedure using this dye to the determination of the adsorption of anion-active detergents on several surfaces commonly washed. The procedure could also readily be adapted to measurement of the adsorption of cation-active detergents. A different type of test would be required for measuring the adsorption of nonionic detergents. McLaren (5) has measured the adsorption of a nonionic surface-active agent on wool, titrating the detergent with phosphomolybdic acid according to a procedure of Oliver and Preston (6). Above a pH of about 4.6, bromphenol blue exists in aqueous solutions as a salt whose negative ion is an intensely blue complex organic structure. In a system with two liquid phases, water and chloroform, it will be found in the water layer. If positively charged surface-active ions, such as those provided by cetylpyridinium bromide, are added, they combine with the negative dye ions and the combination migrates to the chloroform layer coloring it blue. If surface-active negative ions are present in the aqueous phase, the cetylpyridinium ions combine with these before combining with the dye ions, hence the amount of surface-active negative ions in an aqueous solution can be determined by adding chloroform and a little bromphenol blue, and adding a standardized solution of cetylpyridinium bromide until the blue color starts to migrate to the chloroform layer. The reactions are shown in Fig. 1. The reaction on the left hand of the diagram proceeds essentially to completion before the reaction on the right hand begins. Bromphenol blue is usually sold as the water-insoluble acid form. To make a 0.4 per cent solution of the indicator from this form, 1 g is added to a mortar along with 14.9 ml of 0.1 N sodium hydroxide solution. The solution and solid are ground together until the bromphenol blue dissolves and the solution is then diluted to 250 ml with distilled water.
Fessler, William A.
Allied Chemical and Dye Corp., Hopewell, Va.