Ionic polymerisation as a means of end-point indication in non-aqueous thermometric titrimetry. Part II. The determination of organic acids

Abstract

A method for the thermometric titration of organic acids in non-aqueous solution, involving the ionic polymerisation of acrylonitrile to indicate the end-point, has been evaluated for a range of mono- and polyacidic compounds, including phenols, triazine derivatives and tannic acid. In addition to acrylonitrile, methyl acrylate and dimethyl itaconate can be used as monomers for the end-point indication. A number of titrant-catalysts have been examined, including tetra-n-butylammonium hydroxide, n-butyllithium and potassium hydroxide.

The precision of the method ranges from about 0·5 per cent. with 0·1 M titrant, to 2·7 per cent., with 0·001 M titrant, by using the manual and semi-automatic methods described in Part I.

Sample sizes down to about 0·0001 mequiv, e.g., about 10 µg of benzoic acid, which corresponds to 100 p.p.m. of benzoic acid in the volumes of sample solution titrated, can be determined with 0·001 M titrant. Calibration graphs show the volume of titrant and mass of sample to be linearly or almost linearly related in the range 0 to 2 ml of titrant.

Comparison of the titration values obtained by using the acetone method and ionic polymerisation with potassium hydroxide and tetra-n-butylammonium hydroxide solutions as titrants enables one to differentiate between acidic groups of different ionic strengths in the 10 to 12 pK_a region.