Abstract
The potentiometric titration of some phenothiazine drugs in chloroform is described. A standard solution of bromine in chloroform was used as titrant and the end-points were detected with a small platinum indicator electrode, and an Ag/AgCl electrode in 0.01 M Et 4NCl in chloroform as reference. A fast redox reaction occurs during the titrations and the phenothiazine ring is oxidized to the phenothiazonium free radical (P +). The low dielectric constant of the medium favours the formation of (P +Br −) ion-pairs or (P +Br − 3) ion-pairs on further addition of bromine, which has no influence on the position of the first end-point. The method was used for the determination of promethazine, chlorpromazine, trifluoperazine and thioridazine in pharmaceutical preparations after extraction into chloroform. The relative standard deviation for the determination of 5 mg of all these phenothazines was about 1%.
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