Spectrophotometric determination of zinc(II) and cadmiun(II) with methylthymol blue, oxine derivatives and cetylpyridinium chloride

Abstract

Zinc(II) or cadmium(II) rapidly forms a mixed complex with methylthymol blue(MTB), cetylpyridinium chloride(CPC) and 5-nitroso-8-quinolinol (NO-oxine) {for zinc(II)} or 2-methyl-8-oxine (CH3-oxine) {for cadmium(II)} in a weak basic medium. The absorbance of the cadmium(II) complex at 630 nm (λmax) and that of the zinc(II) complex at 620 nm (λmax) were constant over the pH range from 7.6 to 9.0 for 3 h. The molar ratio of metal, MTB, oxine derivative and CPC in the mixed complex was a 1:1:1:2 by Job's method and the molar ratio method. The calibration curves were linear up to 4.0 μg/ml of cadmium(II) and 3.0 μg /ml of zinc (II). Sandell's sensitivity was 0.005 μg/cm2 for cadmium(II) and 0.003 p.g/cm2 for zinc(II). The recommended procedure is as follows: To a sample solution containing≤40 μg of cadmium(II) {or <30 μg of zinc(II)}, 0.5 ml of 1.0 × 10-2 M CPC, 0.25 ml of 1.0 × 10-2 M NO-oxine (or CH3-oxine solution), 2.0 ml of a borate buffer (pH 8.2) and 1.0 ml of 1.0 × 10-3 M MTB were added. The mixture was diluted to 10.0 ml with water. After the solution was kept at (2025)°C for 5 min, the absorbance was measured at 630 nm {for cadmium(II)} or at 620 nm {for zinc(II)} against a reagent blank. Nickel(II), cyanide, sulfide and thiocyanate interfered. Aluminum (III) and thorium (IV) could be masked with sodium fluoride, and mercury(II) with 1-ascorbic acid.