The reaction of 3,8,13,18-tetramethyl-21H,23H-porphine-2,7,12,17-tetrapropionic acid or coproporphyrin-I ( CPI ) with cadmium(II) was studied spectrophotometrically and its kinetic and equilibrium constants were determined. The influence of temperature on the reaction rate was also studied. At different ratios of [ CPI ]/[ Cd(II) ] two types of complex were formed: CdII ( CPI ) and ( CdII )2( CPI ); an investigation of the solution properties and the mechanism of aggregation of ( CdII )2( CPI ) at different pH were performed. It is verified that cadmium(II) accelerates the reaction of the incorporation of manganese(II) into CPI by the substitution of cadmium(II) with manganese(II) in the CdII ( CPI ) complex; the kinetics and mechanism of this substitution reaction at alkaline pH were studied. A sensitive kinetic method for the determination of cadmium(II) at ppb levels has been established; the molar absorptivity and the Sandell's sensitivity (for A = 0.001) of the recommended procedure at 458 nm and 300 s after the start of the reaction are (6.175 ± 0.026) × 105 1 mol−1 cm−1 and (1.820 ± 0.008) × 10−4 μg cm−2 respectively.
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