A kinetic method for the determination of zinc(II) in presence of a large excess of cadmium(II) by the different dissociation rate of their complexes with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine

Summary Acid-dissociation reaction of the cadmium(II) complex of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphme (H₂tspp) proceeds about 10¹² times as fast as that of the zinc(II) complex. This provides the basis for a kinetic determination of zinc(II) in presence of a large excess of cadmium (II). The absorbance at soret-band (421 nm) of Zn(II)(tspp) was used to monitor the reaction. At 5.0×10^–2M hydrogen ion concentration, Cd(II)(tspp) dissociates completely to Cd²⁺ in 1 s and only the reaction associated with Zn(II)(tspp) is observed during the reaction time from 30 s to 5 min. Zinc(II) concentration <10^–6M is determined in the presence of 10^–2M cadmium(II). The molar absorption coefficient is 2.7×10⁵M^–1 cm^–1. Iron(III), aluminum(III) and cobalt(II) being masked sodium tartrate, this method is highly selective and is free from interferences of most substances usually encountered. The method was applied to determine zinc(II) in tap water and in cadmium(II) sulfate.