A capillary electrophoretic reactor with an electroosmosis control method for measurement of dissociation kinetics of metal complexes

The solvolytic dissociation rate constants of 1:2 complexes of Al³⁺ and Ga³⁺ with an azo dye ligand, 2,2'-dihydroxyazobenzene-5,5'-disulfonate (DHABS, H₂L^2-), have been evaluated with a capillary electrophoretic reactor (CER) system. This CER system is based on the fact that metal complexes encounter an overwhelming force to dissociate when apart from the ligand by CE resolution. Treatment of a capillary with a slightly acidic buffer solution, e.g., pH 5, reduces the double-layer potential (ζ) of the inner silica wall. Owing to slow relaxation of the deprotonation equilibria of superficial silanol groups known as the pH hysteresis, this ζ potential can be actually retained during the electrophoresis of the metal complexes in question with a neutral buffer at pH 7.0. This method enables one to manipulate migration times, namely, residence times in a capillary tube, from 5 to 90 min, depending on the prescribed conditioning pH, without changing any other operation conditions such as buffer composition and electric field strength. The excellent performance of the CER is exemplified by the accurate estimation of the dissociation degree of the complexes. The dissociation degree-time profiles for the complexes are quantitatively described using both internal and external standards; the very inert complex of [Co(III)L₂]^5- for the peak signal standardization and methyl orange for the injection volume correction. The solvolytic dissociation rate constants of the 1:2 complexes of Al³⁺ and Ga³⁺ ions with DHABS [AlL₂]^5- and [GaL₂]^5- into the 1:1 ones have been determined as (4.9 ± 1.0) x 10^-4 and (3.7 ± 0.3) x 10^-3 s^-1 at 303 K, respectively.