Verification of the analytical signal additivity for the spectrophotometric determination of the total content of Cu(II), Co(II), Ni(II), Pb(II), Zn(II) using indicator films

Abstract

Statistical 3s- and t-criteria are used for the verification of the absorbance additivity of the immobilized in gelatin films complexes of 4-(2-pyridylazo)-resorcinol (PAR) with metal ions Co(II), Ni(II), Cu(II), Zn(II), Pb(II), both in equal concentrations, and in different ratios of their molar concentrations over the entire range of metal contents. An Agfa photographic film for offset printing is used as a transparent polymeric sorbent. Silver halides are previously removed from the gelatin layer. The extraction of the reagent into the gelatin film layer is carried out from an aqueous solution of PAR (1 mmol/l) for 45 minutes; the films with the reagent are kept in a solution of metal salts during 15 minutes. The light absorption of the films with red metal complexes is measured versus to the yellow film with the reagent on the Hitachi U-2000 spectrophotometer and the KFK-3 photocolorimeter. Two types of solutions are prepared in the concentration range (0.25–6.5)·10-5 mol/l — solutions of individual metal salts and solutions containing five metal ions simultaneously. Method I: the concentration of metal ions in individual solutions (c) is set equal to the total metal concentration in their mixture (сΣ); the metal concentrations in the mixture are equal (сΣ/5). Method II: the concentrations of metal ions in individual solutions are set to the same as in their mixtures; the ratio of the concentrations of metals in the solutions of their mixture is varied. 3-5 samples of the modified films are immersed in the solutions, air dried and obtained the value of absorbance in the wavelength range 480-580 nm in 5 nm steps. The results of the measurements are averaged and the dispersions are calculated for the individual and summary signals. The analytical signal from the sum of metal complexes(АΣ) and the sum of analytical signals from individual metal complexes (ΣА) are compared. In the case of application of the 3s-criterion, the deviation from the additivity of the total signal is assumed to be statistically insignificant if the calculated differences (| AΣ – ΣА/n | or | AΣ – ΣА |), doesn’t exceed the tripled standard deviation of the total signal. In the case of Student's-test, the critical value (tcrit=t0.01,f) and the calculated on the basis of the experimental data criterion value are compared. It is found, that the analytical signal from the sum of metals, both in equal concentrations, and in different ratios of their molar concentrations, doesn’t have significant deviations from additivity in the entire range of the metal contents and wavelength range. The indicator films are used for spectrophotometric and visual-test determination of the total content of Cu (II), Co (II), Ni (II), Pb (II), Zn (II) in samples of natural water. The accuracy of the determinations is proved by atomic absorption spectroscopy.