The phosphate ion is a polyatomic ion with the empirical formula PO3−
4 and a molar mass of 94.97 g/mol. It consists of one central phosphorus atom surrounded by four oxygen atoms in a tetrahedral arrangement. The phosphate ion carries a −3 formal charge and is the conjugate base of the hydrogen phosphate ion, HPO2−
4, which is the conjugate base of H
2PO−
4, the dihydrogen phosphate ion, which in turn is the conjugate base of H
3PO
4, phosphoric acid. 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.
Aqueous 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 (H
2PO−
4) is most common. In strongly acidic conditions, trihydrogen phosphate (H
3PO
4) is the main form.