Automatic photometric titration of surface active agents based on two-phase titration; III. Determination of soap using dichlorofluorescein as indicator

Abstract

Experimental conditions were studied for the determination of fatty acids by two-phase automatic photometric titration. The salts of fatty acids were back-titrated with standard sodium dodecylsulfate after addition of excess Benzethonium chloride in water-chloroform medium. An automated titrator equipped with porous PTFE (Teflon) membrane separator (previous publications, literatures 1 and 2) was used. Using the method proposed, determination of long-chain fatty acids was satisfactory, while short-chain acids generally gave lower end point (lower recoveries) due to their incomplete extraction. Addition of 14 % 1-propanol into the aqueous solution was effective to improve the recovery of short-chain fatty acids. Among indicators tested (Bromocresol Green and several fluorescein derivatives), 2', 7’ -dichlorofluorescein proved to be satisfactory in applicability to wider range of fatty acids and in sharp end-point indicator. Under the recommended conditions, recoveries were 0, 26.3, and 85.4 % for C6, Cg, C10 acids, respectively; those of the longer-chain fatty acids were quantitative. However, a soap containing a mixture of short-chain fatty acids was successfully analyzed by using the modified molecular weights which were obtained from the experimentally determined recovery of each C6~C10 acid. Typically titration time required was 15~20 min per sample, and the relative standard deviation of determination was about 0.3 %. © 1984, The Japan Society for Analytical Chemistry. All rights reserved.