Investigation of the reaction mechanism of the mercurimetric determination of benzylpenicillin.

The assay of different penicillins in the European Pharmacopoeia was carried out by mercurimetric titration with potentiometric determination of the end-point. The consecutive formation of reaction products during titration was followed by reversed-phase liquid chromatography (LC) and ultraviolet detection. In one experiment the titration was carried out with 14C-labeled benzylpenicillin and the reaction was followed with LC coupled to radiochemical detection. The identity of the intermediates and final reaction products was deduced from their retention times in comparison with reference products obtained by independent chemical transformation of benzylpenicillin. This allowed one to define for the first time the complete reaction scheme. This involves the isomerization of the natural penicilloic acid followed by decarboxylation, which has not been reported previously. At the end of the titration, only benzylpenilloaldehyde and a 1:1 complex of mercury and penicillamine were present in the solution.