Ionic polymerisation as a means of end-point indication in non-aqueous thermometric titrimetry. Part VI. The determination of thiols

Abstract

Alkyl and aryl thiols have been determined in the presence of carboxylic acids and phenols by means of acid-base catalytic thermometric titrimetry. Two titrations are carried out, with acrylonitrile and acetone as the end-point indicators. With the former indicator, thiol groups are not determined, so that the difference between the titration values obtained by using the two methods of end-point indication is a measure of the thiol content.

The thiol content of 2-mercaptothiazoline, 4,6-dihydroxypyrimidine-2-thiol (2-thiobarbituric acid), purine-6-thiol and 2-mercaptobenzimidazole can be determined by the same procedure. In the titration of 2-thiohydantoin, 4-hydroxypyrimidine-2-thiol (2-thiouracil), 2-mercaptobenzoxazole and 2-mercaptobenzothiazole, however, both end-point methods give the same titration value. These apparently anomalous results can be explained if it is accepted that the last four heterocyclic thiols exist in the thione tautomeric form in dimethylformamide solution. Some thioamides also titrate as acids, and differences between titration values obtained by using the two methods of end-point indication can again be attributed to thione-thiol tautomerism.

Thiols can be determined conventiently in amounts down to 0·01 mequiv, i.e., about 2 mg of dodecane-1-thiol, with 0·1 M titrants. In instances when the acrylonitrile method can be used for the direct determination of the thiol function, 0·001 M titrant can be used and the lower level of determination is then about 0·0001 mequiv.