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

Abstract

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 Pb^II(tspp) as a reaction intermediate; Pb^II(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 (λ_max. 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.