Salt effect on the solubility of mer-(+)-tris(L-alaninato)cobalt(III)

Abstract

The effect of NaF, NaCl, NaBr, NaI, NaNO3, CH3COONH4, NaH2PO4, and (NH4)2SO4 on mer-(+)-tris(L-alaninato)cobalt(III) (mer-(+)-[Co(L-ala)3]) was examined by determining the solubility of the chelate. While the solubility of mer-(+)-[Co(L-ala)3] (salting-in effect) increased with the addition of NaCl, NaBr, NaI, and NaNO3, the solubility (salting-out effect) decreased with the addition of NaF, CH3COONH4, and NaH2PO4. In low salt concentration (NH4)2SO4 caused slight salting-in, but in salt concentration >0.5 mol dm-3 (NH4)2SO4 brought about salting-out. The magnitude of the salting-in effect increased in the order of NaCl < NaNO3 < NaBr < NaI. The standard enthalpy (Δ Hos) and entropy (Δ Sos) of dissolution for mer-(+)-[Co(L-ala)3] were estimated from the temperature dependence of the solubility. When Δ Hos was plotted against T Δ Sos , a single-correlation straight line was obtained for the salts that brought about the salting-in effect. On the basis of these findings, the origin of the salting-in effect was discussed. Furthermore, while NaF, NaCl, and NaBr showed larger salting-in effect on fac-tris(glycinato)cobalt(III) (fac-[Co(gly)3]) than on mer-(+)-[Co(L-ala)3], NaI greatly decreased the solubility of fac-[Co(gly)3]. At the present stage, an explanation for this anomalous salting-out effect of NaI cannot be presented.Key words: salt effect, solubility, thermodynamic parameters of dissolution, mer-tris(L-alaninato)cobalt(III), fac-tris(glycinato)cobalt(III).