The formation process and thermal change of cobalt hydrogenphosphate were investigated by means of X-ray diffraction, thermal analyses (TG-DTA) and chemical analysis. Four hydrates of the phosphates were prepared by aging for about 30 days at 96°C of suspension which formed by dropping KOH/H3PO4 solutions (mol ratio 2) into 0.5mol/l CoCl2 solution. The composition of the four hydrate was influenced considerably by aging time and changed in CoHPO4·nH2O (spherical crystal with about 30μm, violet) at the early stage of the reaction, CoHPO4.2H2O (platy crystal with about 8×0.5μm, pale purplish pink) after 2 days, CoHPO4·3/2H2O (platy crystal with about 80×10μm, light purplish pink) after 10 days and new phase of CoHPO4·3/4 H2O (platy crystal with about 10×2μm, strong reddish purple) after 30 days.
The dehydration and condensation of four hydrates on heating were summarized as follows;1) CoHPO4·nH2O 256°C-Amorphous phase 600°C-α-Co7P2O7 2) CoHPO4·2H2O about 130°C (first step) -CoHPO4 250°C (second step) -Amorphous phase 594°C-α-Co7P2O7 3) CoHPO4·3/2 H2O 253°C-CoHPO4·1/2H2O (new phase) 308°C-α-Co7P2O7 4) CoHPO4·3/4H2O 363°C-α-Co7P2O7
