Phosphate - Chemical Properties

Chemical Properties

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 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. 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 (H₂PO−
4) is most common. In strongly acidic conditions, trihydrogen phosphate (H₃PO₄) is the main form.

• H₃PO₄
• H₂PO−
  4
• HPO2−
  4
• PO3−
  4

More precisely, considering the following three equilibrium reactions:

H₃PO₄ H+ + H₂PO−
4

H₂PO−
4 H+ + HPO2−
4

HPO2−
4 H+ + PO3−
4

the corresponding constants at 25°C (in mol/L) are (see phosphoric acid):

(pK_a1 2.12)

(pK_a2 7.21)

(pK_a3 12.67)

The speciation diagram obtained using these pK values shows three distinct regions. In effect H₃PO₄, H₂PO−
4 and HPO2−
4 behave as separate weak acids. This is because the successive pK values differ by more than 4. For each acid the pH at half-neutralization is equal to the pK value of the acid. The region in which the acid is in equilibrium with its conjugate base is defined by pH ≈ pK ± 2. Thus the three pH regions are approximately 0–4, 5–9 and 10–14. This is idealized as it assumes constant ionic strength, which will not hold in reality at very low and very high pH values.

For a neutral pH as in the cytosol, pH=7.0

so that only H₂PO−
4 and HPO2−
4 ions are present in significant amounts (62% H₂PO−
4, 38% HPO2−
4 Note that in the extracellular fluid (pH=7.4), this proportion is inverted (61% HPO2−
4, 39% H₂PO−
4).

Phosphate can form many polymeric ions such as diphosphate (also known as pyrophosphate), P₂O4−
7, and triphosphate, P₃O5−
10. The various metaphosphate ions (which are usually long linear polymers) have an empirical formula of PO−
3 and are found in many compounds.