| NaH2PO4 | monosodium hydrogen phosphate. |
| Na2HPO4 | disodium hydrogen phosphate. |
| Na3PO4 | normal sodium phosphate. |
These salts are sometimes called respectively primary, secondary, and tertiary phosphates. They may be prepared by bringing together phosphoric acid and appropriate quantities of sodium hydroxide. Phosphoric acid also forms mixed salts, that is, salts containing two different metals. The most familiar compound of this kind is microcosmic salt, which has the formula Na(NH4)HPO4.
Orthophosphates. The orthophosphates form an important class of salts. The normal salts are nearly all insoluble and many of them occur in nature. The secondary phosphates are as a rule insoluble, while most of the primary salts are soluble.
3. Pyrophosphoric acid (H4P2O7). On heating orthophosphoric acid to about 225° pyrophosphoric acid is formed in accordance with the following equation:
2H3PO4 = H4P2O7 + H2O.
It is a white crystalline solid. Its salts can be prepared by heating a secondary phosphate:
2Na2HPO4 = Na4P2O7 + H2O.
4. Metaphosphoric acid (glacial phosphoric acid) (HPO3). This acid is formed when orthophosphoric acid is heated above 400°:
H3PO4 = HPO3 + H2O.
It is also formed when phosphorus pentoxide is treated with cold water: