In electroless plating and water treatment, water-soluble phosphides such as sodium hypophosphite and sodium phosphite can be removed by adding calcium oxide to form the corresponding insoluble calcium salts, which can then be largely removed through methods such as precipitation. However, it should be noted that not all calcium salts are insoluble precipitates; these soluble calcium salts require other chemical methods for removal. Below is a brief explanation of the solubility data for several commonly encountered calcium salts.

Calcium hypophosphite: 16.7 g/L (room temperature)

Calcium sulfate: 0.3 g/L (20℃), 0.16 g/L (100℃)

Calcium phosphite: sparingly soluble (room temperature)

Calcium citrate: 0.09 g/L (18℃)

Calcium hydrogen phosphite: soluble (room temperature)

Calcium tartrate: 0.019 g/L (18℃), 0.03 g/L (25℃)

Calcium malate: 0.9 g/L (18℃), 0.85 g/L (25℃)

Calcium hydrogen malate: 8.5 g/L (45℃), 32.2 g/L (57℃)

Calcium phosphate: 0.002 g/L (room temperature)

From the above data, we can see that the higher the temperature, the greater the solubility of calcium salts, and the solubility of acidic calcium salts is greater than that of ortho-calcium salts. Therefore, when removing excess phosphorus from water, it is advisable to add as much calcium oxide as possible. However, this can lead to an increase in the pH value of the water. Therefore, even with an excess of calcium oxide, it is necessary to lower the pH value of the water in order to ultimately achieve the effect of reducing phosphorus content in water treatment.