Spectrophotometric determination of ultramicro amounts of zinc with α,β,γ,δ-tetrakis-(1-methylpyridinium-3-yl)porphine

Abstract

A sensitive and practical method for the determination of zinc at parts per 10⁹ levels with α,β,γ,δ-tetrakis(1-methylpyridinium-3-yl)porphine [T(3-MPy)P or H₂L⁴⁺] has been developed. Zinc(II) reacts rapidly with T(3-MPy)P to form a 1:1 complex at pH 10.5, the T(3-MPy)P remaining is converted into its copper(II) complex by adding a small excess of copper(II) at this pH after the zinc(II) complex has been formed, the zinc(II) complex is decomposed by acidifying the solution and the absorbance of the Soret band of the resultant H₄L⁶⁺ equivalent to zinc is measured. The spectrophotometric sensitivity for 0.001 absorbance and the relative standard deviation of the procedure are 0.186 ng cm^–2 and 0.48%, respectively. Adsorption of T(3-MPy)P and its complexes on the walls of glassware in alkaline media did not interfere with the determination because the absorbance measurement was carried out in an acidic medium. Interferences from cations were removed by the combined use of masking agents and solvent extraction with dithizone. Anions usually encountered did not interfere with the determination. The proposed procedure has been applied to the determination of zinc in tap water.