Acetylation as a means of end-point indication in the catalytic thermometric titration of bases

Abstract

The acid-catalysed acetylation of alcohols and phenols with acetic anhydride is used to indicate the end-point in the titration of tertiary amines and metal carboxylates with 0.1, 0.01 and 0.001 M perchloric acid in acetic acid.

Mixtures of acetic anhydride with butan-1-o1, 2-methylpropan-2-ol, 4-hydroxy-4-methylpentan-2-one, 3,5-dimethylphenol, pyrocatechol, hydroquinone or quinhydrone are all satisfactory end-point indicators for the determination of pyridine and quinoline derivatives, tertiary aliphatic amines and metal carboxylates, but the acetic anhydride-quinhydrone mixture is superior to the other mixtures for the determination of the very weak bases, antipyrine, caffeine, theophylline and urea. A colour change occurs at the end-point when quinhydrone is the hydroxylic component.

The sharpness of the end-point inflection can be improved by adding dichloromethane, nitromethane or propylene carbonate to the sample solution.

Sample sizes down to about 0.000 1 mequiv can be determined with 0.001 M titrant solution. Precisions are usually better than 0.5% with 0.1 M and 1.0% with 0.01 and 0.001 M titrants.

The influence of intra- and intermolecular hydrogen bonding of the hydroxyl groups in the alcohol and phenol reagents on end-point sharpness is discussed.