Potentiometric titration of phenothiazine compounds in chloroform and its use in pharmaceutical analysis

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 0.01M Et(4)NCl 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%.