Change in Composition of Tricobalt Phosphate Octahydrate in Aqueous Disodium Hydrogenphosphate Solution

In order to modify hue of cobalt phosphate as violet pigment, Co₃ (PO₄) ₂·8H₂O was immersed in aqueous disodium hydrogenphosphate solution under various conditions such as temperature (50-96°C), concentration (0.2-2.0 mol/l) and pH (4.3-10.0). Co₃ (PO₄) ₂·8H₂O was changed to a new compound with vivid purple or blue color on further immersing. The composition and particle properties of the new compounds were investigated by means of chemical analysis, thermal analysis (TG-DTA), X-ray diffraction, measurement of hue, paper chromatographic analysis, infrared absorption and scanning microscope. The composition and particle properties of the new compounds were summarized as follows;<BR>1) NaCoPO₄·H₂O : vivid bluish purple color (complementary wavelength λc; 568.9 nm, excitation purity Pe; 81.0%). Dodecahedral crystal (s. g.; 2.78 g/cm³).<BR>NaCoPO₄·H₂O 190-260°C→NaCoPO₄-I'490°C→_??_NaCoPO₄-I 720°C_??_h-NaCoPO₄.<BR>2) Na₃Co₉ (PO₄) ₇·10H₂O : light bluish purple color (λc; 565.2 nm, Pe; 21.9%). needle crystal (s. g. ; 3.14 g/cm³).<BR>Na₃Co₉ (PO₄) ₇·10H₂O180°C→Amor.525°C→l-Na₃Co₉ (PO₄) ₇765°C→Na₃Co₉ (PO₄) ₇.<BR>3) 2Na₂HPO₄-Co₃ (PO₄) ₂·8H₂O : strong purplish blue color (λc, 568.5 nm, Pe; 63.8%). Glossy cubical crystal (s. g.; 2.87 g/cm³). The crystal changed to transparent paste by kneading linseed oil.<BR>4) Unknown compound : light reddish purple color (λc, 554.0 nm, Pe; 45.0%). Plate crystal (s. g. ; 2.87 g/cm³).From above results, the new compounds and their burned products of vivid hue were more effective for an oil painting pigment. Moreover the burned product with skeleton structure of mother crystal could be expected as inorganic filler.