Phosphate Guide

The phosphate ion is a polyatomic ion with the empirical formula PO3−4 and a molar mass of 94.973 g/mol. It consists of one central phosphorus atom surrounded by four oxygen atoms in a tetrahedral arrangement. The phosphate ion carries a negative three formal charge and is the conjugate base of the hydrogen phosphate ion, HPO2−4, which is the conjugate base of H2PO−4, the dihydrogen phosphate ion, which in turn is the conjugate base of H3PO4, phosphoric acid. It is a hypervalent molecule (the phosphorus atom has 10 electrons in its valence shell). Phosphate is also an organophosphorus compound with the formula OP(OR)3. A phosphate salt forms when a positively-charged ion attaches to the negatively-charged oxygen atoms of the ion, forming an ionic compound. Many phosphates are not soluble in water at standard temperature and pressure. The sodium, potassium, rubidium, caesium and ammonium phosphates are all water soluble. Most other phosphates are only slightly soluble or are insoluble in water. As a rule, the hydrogen and dihydrogen phosphates are slightly more soluble than the corresponding phosphates. The pyrophosphates are mostly water soluble.

In dilute aqueous solution, phosphate exists in four forms. In strongly-basic conditions, the phosphate ion (PO3−4) predominates, whereas in weakly-basic conditions, the hydrogen phosphate ion (HPO2−4) is prevalent. In weakly-acid conditions, the dihydrogen phosphate ion (H2PO−4) is most common. In strongly-acid conditions, aqueous phosphoric acid (H3PO4) is the main form....