Among the drawbacks of catalytic methods of analysis is their relatively low selectivity, which can be improved using adsorption preconcentration. By combining adsorption with the catalytic determination of components directly on adsorbents [1], we can not only to gain advantages of both selective preconcentration and highly sensitive catalytic methods but also develop test procedures for determining, for example, copper [2], vanadium(IV and V) [3], and other elements [1].The goal of this work was to develop procedures for determining Cd(II), Ni(II), and Zn(II) by the catalytic method based on their inhibitory effect on the oxidation of aromatic amines. Note that metals acting as inhibitors are rarely determined by catalytic methods (papers are known on the determination of Cd (II) [4], Hg(II) [5], Au(III) [6], and some other elements [7,8]). It was assumed that Cd(II), Ni(II), and Zn(II) be determined both in solutions and on the support surface.The inhibitory effect of Cd(II), Ni(II), and Zn(II) was observed in the previous study of the catalytic effect of manganese(II) on the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with periodate [9]. The reaction was performed at pH 6-7 and yielded an intensely colored blue-green product ( λ max = 650 nm), which turned orange upon deeper oxidation [10]. In this work, we studied the effect of Cd(II), Ni(II), and Zn(II) by observing a change in the concentration of an intermediate blue-green product, which was a more convenient analytical signal in the visual test procedures. EXPERIMENTAL Reagents and apparatus. TMB of analytical grade from Riedel-de-Haen Co. (Germany), 1-[(6-bromo-2-benzothiazolyl)azo]-2-naphthol (bromobenzothiazo, BBT), and NaIO 4 of high-purity grade were used without additional purification. Ethanolic solutions of TMB (2.5 × 10 -2 M), BBT (2 mg/L), and dimethylglyoxime (DMG) and an aqueous 4.3 × 10 -2 M solution of NaIO 4 were prepared by dissolving accurately weighed portions of reagents in ethanol and water, respectively. Solutions of Cd(II), Ni(II), and Zn(II) (1 g/L) were prepared using CdCl 2 · 2.5H 2 O of high-purity grade, chemically pure NiCl 2 · 6H 2 O, and a GSO no. 7770-2000 Certified Reference Material containing 1 g/L ZnCl 2 , respectively. The solutions were acidified to pH 1.8-1.9 with sulfuric acid of high-purity grade. Solutions with lower metal concentrations were prepared every three days by the successive dilution of stock solutions with water. The remaining organic and inorganic compounds of chemically pure or analytical grades were used without additional purification. The solutions were prepared as needed at least once a week by dissolving reagent portions in water. A borate buffer solution (pH 8-10) was prepared from H 3 BO 3 of analytical grade and chemically pure KOH or NaOH. Acetate buffer solutions (pH 3-6) were prepared from 0.1 M CH 3 COOH and CH 3 COONa 3 H 2 O (both of analytical grade) solutions. Phosphate buffer solutions (pH 6-8) were prepared from Na 2 HPO 4 and KH 2 PO 4 (both of high-purity grade). A TRIS-HCl ...
