Kinetic method for the determination of nanogram amounts of lead(II) using its catalytic effect on the reaction of manganese(II) with 5,10,15,20-tetrakis(4-sulphonatophenyl)porphine
Lead(II) accelerates the reaction of manganese(II) with 5,10,15,20-tetrakis(4-sulphonatophenyl)porphine (H2tspp). The kinetics and mechanism of this reaction were studied at pH 6–8.7, 25 °C and l= 0.1 (NaNO3). Lead(II) rapidly forms PbII(tspp) as a reaction intermediate; PbII(tspp) is 7200 times as reactive as the free base porphyrin (H2tspp) in the reaction with manganese(II). Lead(II) concentrations as low as 10–7 mol dm–3 can be determined from the decrease in absorbance at 413 nm (λmax. of H2tspp) at a fixed time after the start of the reaction of manganese(II) with H2tspp. After the separation of lead(II) from iron and silicate by solvent extraction using n-decanoic acid, the proposed method is highly selective and free from interference from most substances usually encountered in the samples studied if carried out in the presence of cyanide as a masking agent for cadmium(II). The molar absorptivity and Sandell's sensitivity calculated from the calibration graph 15 min after the start of the reaction are 2.75 × 105 mol–1 dm3 cm–1 and 0.75 ng cm–1, respectively. The method was applied to the determination lead(II) in rain and drain water.