Solubility of blue CoAl2O4 ceramic pigments in water and diethylene glycol media

The chemical stability of blue CoAl2O4 ceramic pigment in aqueous and non-aqueous suspension was investigated. Distilled water and diethylene glycol (DEG) were used as media. The isoelectric points (IEP) for the commercial and synthesised CoAl2O4 pigment were determined to be 4.9 and 8.5, respectively. The IEP shifts toward acidic pH values for the commercial pigment with respect to the synthesised pigment due to the existence of the quartz phase. In water, the cobalt ion concentration was low (2.5 mg dm−3), and this concentration did not change with time. However, the aluminium ion concentration was initially high and then decreased to ∼10 mg dm−3 at pH 9 and 11 due to the precipitation of Al3+ ions as an Al(OH)3 phase. In DEG, the dissolution of pigment particles with higher ion concentrations (>800 mg dm−3) was more significant than that of the aqueous medium, implying that CoAl2O4 pigment has insufficient chemical stability in DEG medium with respect to aqueous suspensions.