EQUILIBRIUM AND STRUCTURAL STUDIES OF SILICON(IV) AND ALUMINUM(III) IN AQUEOUS-SOLUTION .31. AQUEOUS COMPLEXATION BETWEEN SILICIC-ACID AND THE CATECHOLAMINES DOPAMINE AND L-DOPA

Three component equilibria between H+, silicic acid, and the catecholamines dopamine and L-DOPA were studied by means of potentiometric (glass electrode) measurements in 0.6 M Na(Cl) medium at 25-degrees-C. Experimental data cover the ranges 2 less-than-or-equal-to -log[H+] less-than-or-equal-to 9.5, 0.002 less-than-or-equal-to B less-than-or-equal-to 0.005 M; 0.005 less-than-or-equal-to C less-than-or-equal-to 0.016 M, and 1 less-than-or-equal-to C/B less-than-or-equal-to 6 (B and C stand for the total concentration of Si and catecholamine, respectively). In acidic and near-neutral solutions no interactions were registered while, in slightly alkaline solutions, both systems are characterized by a formation of mononuclear tris-complexes SiL3 containing pyrocatechol-type bonds. The species forming at -log[H+] almost-equal-to 7.5 have fully protonated amine side-chains which, at somewhat higher -log[H+] values, start to deprotonate. Simultaneously, a decomposition of the octahedrally coordinated complex is occurring and the hydrolytic species SiO(OH)3- becomes increasingly important. With dopamine, the equilibrium constants characterizing the system are log K (Si(OH), + 3H3L+ half arrow right over half arrow left Si(HL)3+ + 2H+ + 4H2O) - 9.70 +/- 0.03 and log K (Si(OH)4 + 3H3L+ half arrow right over half arrow left SiL(HL)2+3H+ + 4H2O) - 19.33 +/- 0.03. The corresponding equilibrium reactions with L-DOPA are log K (Si(OH)4 + 3H3L half arrow right over half arrow left Si(HL)32+ 2H+ + 4H2O) - 10.08 +/- 0.05 and log K (Si(OH)4 + 3H3L half arrow right over half arrow left SiL(HL)23- + 3H+ + 4H2O) -19.35 +/- 0.07. Data were analyzed using the least-squares computer program LETAGROPVRID. The octahedral oxygen coordination around the silicon atom in these complexes has been verified by using the Si-29 NMR method. Via model calculations it is shown that a minimum of approximately 0.004 M catecholamine is needed to double the aqueous solubility of quartz.