Kinetic spectrophotometric determination of traces of tungsten(VI) by the catalytic reaction of chlorpromazine with hydrogen peroxide. A mechanistic study

A catalytic spectrophotometric method for the determination of traces of tungsten(VI) is proposed. Chlorpromazine (CP) is oxidized by hydrogen peroxide to form a colorless sulfoxide. The reaction is accelerated by trace amounts of tungsten(VI) and can be followed by measuring the increase in absorbance at 344 nm. Since the initial slope R0 of the absorbance/time curves increases with an increase in the tungsten(VI) concentration, R0 is used as the parameter for tungsten(VI) determination. Under the optimum experimental conditions (8.2 × 10−4 M chlorpromazine, 0.42 M sulfuric acid, 0.010 M hydrogen peroxide, 40 °C), tungsten(VI) can be determined in the range 2 μg 1−1–10 mg 1−1. The relative standard deviations are 0.8, 6.0 and 6.8% for 60, 10 and 5 μg 1−1, respectively. This method was successfully applied to a determination of tungsten(VI) in hot spring water samples. According to results of a kinetic study, a mechanism is proposed which leads to the following rate equation: Rcat0 = ([W(VI)][H+][H2O2]2[CP] + [W(VI)][H+]2[H2O2]2[CP])(P + Q[H+] + S[H+][H2O2] + T[H+][H2O2]2).