Spectrophotometric Studies of the Solution Equilibria and the Kinetics of the Substitution Reaction between Eriochrom Black T and Cobalt(II)-Ethylenediaminetetraacetate Chelate

Abstract

The solution equilibria and the kinetics of the substitution reaction between Eriochrom Black T (BT) and cobalt(II)-EDTA chelate were studied spectrophotometrically.At the pH range from 8.50 to 9.50, BT was found to form only 1 to I normal chelate CoD-with cobalt(II), the stability constant of which is 10 20.0 Kinetically, the substitution reaction was found to proceed through the following two simultaneous reaction paths, i)ii) and k1+ and k2+ were determined to be 0.23 l mol-1 sec-1 andIn the previous paper,1) the solution equilibria between Eriochrom Black T (BT) and zinc(II)-NTA or copper(II)-CyDTA chelate were studied spectrophotometrically, and the stability constants and the compositions of 1 to 1 chelates of BT with these metal ions were determined. The rate of substitution reaction between BT and zinc(II)-NTA or copper (II) -CyDTA chelates was very large and the reaction mixtures reached the equilibrium within 30 sec.In the case of the substitution reaction of BT and cobalt(II)-EDTA chelate, however, the reaction proceeds at a considerable rate. In the present paper, spectrophotometeric studies of the solution equilibria and of the kinetics of the substitution reaction between BT and cobalt(II)-EDTA chelate are presented. ExperimentalThe standard solution of cobalt(II) was prepared by the following procedure. Hot ammoniacal metal solution was treated with a 1 percent solution of dimethylglyoxime in ethanol and filtered carefully using a frittedglass funnel. To the filterate, an excessive amount of sodium hydroxide was added and the solution was heated nearly to boiling point to precipitate cobalt(II) ions as hydroxide. The resulting precipitate was filtered off, washed thoroughly with redistilled water, and dissolved in diluted perchloric acid solution. The concentration of cobalt(II) was then determined by titration with a standard EDTA solution.2) The solution of disodium salt of ethylenediaminetetraacetate (EDTA) was standardized against pure metallic zinc.As was described in the previous paper,1) BT was purified by employing the procedure described by Diehl and Lindstrom 3) and the purity was checked spectrophotometrically.