Determination of the two-phase equilibrium constants of copper(II)-modified silica gels used in liquid chromatography

Abstract

Silica gels modified with copper(II) are currently used in ligand-exchange liquid chromatography. The main species present at the surface of the stationary phase is shown to be (SiO) 2Cu(NH 3) 2(H 2O) x. Measurements of the ammonia concentration in the mobile and stationary phases are combined with computer-assisted calculations to determine the following apparent equilibrium constants: for SiOH + NH 3 ⇌ SiO −NH + 4, log K = −0.2; for (SiO) 2Cu + NH 3 ⇌ (SiO) 2Cu(NH 3), log β 1 = 2.8; and for (z.tbnd;SiO) 2Cu + 2NH 3 ⇌ (SiO) 2Cu(NH 3) 2, log β 2 = 4.0. A distribution diagram of the different species present at the surfac of copper-modified silica gel is given. Under the usual operating conditions for ligand-exchange chromatography (pNH 3 < 1), more than 60% of the silanol groups which fix copper(II) have a silicate structure with two moles of ammonia per mole of copper(II).