Estimation of Small Stability Constants in Aqueous Solution. The Chromium(III)--Bromide System.

Weak complex formation in the chromium(III)-bromide system has been studied by spectrophotometric measurements and by ion-exchange high-performance liquid chromatography (HPLC). The equilibria were studied at 25-degrees-C in weakly acidic LiBr solutions up to almost-equal-to 11 M. The semi-thermodynamic constant K1 = [Cr(H2O)5Br2+]a(H2O)/[Cr(H2O)6(3)+][Br-]gamma-c(LiBr), was found by HPLC to have the value 0.0024 +/- 0.0004 M-1 in equilibrium solutions containing from 4.5 to 9.5 M LiBr. The uncorrected concentration constant varied in the same equilibrium solutions from 0.0128 to 1.02 M-1. From the red shifts of the absorption spectra per ligand taken up the following values were estimated for the semi-thermodynamic constants: K1 almost-equal-to 0.0030, K2 almost-equal-to 0.00035 and K3 almost-equal-to 0.00004 M-1. The hydrolysis of the dichloro and dibromo complexes in 1 M acidic solutions is more than 100 times faster than the hydrolysis of the monohalogeno complexes. This fact was used to prepare pure solutions of the monohalogeneo ions which could be precipitated as Recoura's chloro- and bromosulfates.