Oxides of phosphorus. Phosphorus forms two well-known oxides,—the trioxide (P2O3) and the pentoxide (P2O5), sometimes called phosphoric anhydride. When phosphorus burns in an insufficient supply of air the product is partially the trioxide; in oxygen or an excess of air the pentoxide is formed. The pentoxide is much the better known of the two. It is a snow-white, voluminous powder whose most marked property is its great attraction for water. It has no chemical action upon most gases, so that they can be very thoroughly dried by allowing them to pass through properly arranged vessels containing phosphorus pentoxide.
Acids of phosphorus. The important acids of phosphorus are the following:
| H3PO3 | phosphorous acid. |
| H3PO4 | phosphoric acid. |
| H4P2O7 | pyrophosphoric acid. |
| HPO3 | metaphosphoric acid. |
These may be regarded as combinations of the oxides of phosphorus with water according to the equations given in the discussion of the characteristics of the family.
1. Phosphorous acid (H3PO3). Neither the acid nor its salts are at all frequently met with in chemical operations. It can be easily obtained, however, in the form of transparent crystals when phosphorus trichloride is treated with water and the resulting solution is evaporated:
PCl3 + 3H2O = H3PO3 + 3HCl.
Its most interesting property is its tendency to take up oxygen and pass over into phosphoric acid.
2. Orthophosphoric acid (phosphoric acid) (H3PO4). This acid can be obtained by dissolving phosphorus pentoxide in boiling water, as represented in the equation
P2O5 + 3H2O = 2H3PO4.
It is usually made by treating calcium phosphate with concentrated sulphuric acid. The calcium sulphate produced in the reaction is nearly insoluble, and can be filtered off, leaving the phosphoric acid in solution. Very pure acid is made by oxidizing phosphorus with nitric acid. It forms large colorless crystals which are exceedingly soluble in water. Being a tribasic acid, it forms acid as well as normal salts. Thus the following compounds of sodium are known: