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/H
3PO
4 solutions (mol ratio 2) into 0.5mo
l/
l CoC
l2 solution. The composition of the four hydrate was influenced considerably by aging time and changed in CoHPO
4·nH
2O (spherical crystal with about 30μm, violet) at the early stage of the reaction, CoHPO
4.2H
2O (platy crystal with about 8×0.5μm, pale purplish pink) after 2 days, CoHPO
4·3/2H
2O (platy crystal with about 80×10μm, light purplish pink) after 10 days and new phase of CoHPO
4·3/4 H
2O (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) CoHPO
4·nH
2O 256°C-Amorphous phase 600°C-α-Co
7P
2O
7 2) CoHPO
4·2H
2O about 130°C (first step) -CoHPO
4 250°C (second step) -Amorphous phase 594°C-α-Co
7P
2O
7 3) CoHPO
4·3/2 H
2O 253°C-CoHPO
4·1/2H
2O (new phase) 308°C-α-Co
7P
2O
7 4) CoHPO
4·3/4H
2O 363°C-α-Co
7P
2O
7
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