Ionic polymerisation as a means of end-point indication in non-aqueous thermometric titrimetry. Part X. Acrylonitrile indicator reactions in the determination of acids

Abstract

The mechanism of the reaction marking the end-point when acrylonitrile is used as the indicator reagent in the thermometric titration of acids with solutions of potassium hydroxide and alkoxides in alkanols has been investigated. Two indicator reactions occur: cyanoethylation of alcohols and anionic polymerisation of the acrylonitrile. The relative contribution of the two reactions is shown to depend on the nature and concentration of alcohols in the titrand solution. Cyanoethylation is more important with respect to end-point sharpness, when primary and secondary alcohols are present, and polymerisation predominates in the presence of tertiary alcohols. Anomalous titration values obtained in the determination of thiols and sulphanilamide can be explained by the effect of the structure of the sample compound on the reversibility of its reaction with acrylonitrile.

The experimental findings are used to devise combinations of titrand solvents and titrants leading to “ideal” titration curves.