Phosphate in the synthetic detergents is pointed out as a main facter to make sea water and lake water rich nutrition. Lime and alum are generally used as precipitating agents in order to remove phosphates in drainage. The principal of phosphate removal is based on the formation of insoluble phosphates with calcium or aluminum ion in the precipitating agents. There are many reports which treat the removal of orthophosphates in drainage, but the phosphates are added into synthetic detergents in the form of condensed phosphates, mainly as tripolyphosphate. Therefore, the stability of sodium tripoly-, pyro-and orthophosphate solutions in pH 9.0 containing calcium ion were studied by using chemical analysis, X-ray analysis, IR absorption analysis and paperchromatography. The results obtained are summarized as follows :
In sodium tripolyphosphate solution, calcium ion of mole ratio Ca²⁺/P₂O₅ below 0.5 was stabilized as complex ion [CaP₃O₁₀] ^3-, whereas the complex decomposed with precipitating insoluble phosphate in the solution of mole ratio above 0.75. The phosphate was identified as Ca₄H₂ (P₃O₁₀) ₂ by X-ray analysis.
In the case of pyro-and orthophosphate solution, the stability decreased as the concentration of calcium ion increased and precipitated Ca₃H₂ (P₂O₇) ₂·H₂O in pyrophosphate solution and Ca₅ (PO₄) ₃OH in orthophosphate solution.
Except of the tripolyphosphate solution of mole ratio below 0.5, the order of stability of solutions containing calcium ion was as follows, ortho->tripoly->pyro-.

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α-Hemihydrates could be obtained by heating calcium sulfite in 1 to 7N H₂SO₄ solution for 5 to 120 minutes at boiling temperature, with stirring and blowing air through the solution.
This process gave needle like α-hemihydrates of about 200 to 500 μm long. By adding a small amount of Al₂ (SO₄) ₃ to the solution as a catalyzer, the crystals of α-hemihydrates became prismatic or tabular.

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