A phosphate, an inorganic chemical, is a salt of phosphoric acid. Inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry.

Chemical properties

In dilute aqueous solution, phosphate exists in four forms. In strongly-basic conditions, the phosphate ion (PO₄³⁻) predominates, whereas in weakly-basic conditions, the hydrogen phosphate ion (HPO₄²⁻) is prevalent. In weakly-acid conditions, the dihydrogen phosphate ion (H₂PO₄⁻) is most common. In strongly-acid conditions, aqueous phosphoric acid (H₃PO₄) is the main form.

Image:3-phosphoric-acid-3D-balls.png|

More precisely, considering the following three equilibrium reactions:

H₃PO₄ H^{+ + H₂PO₄−}

H₂PO₄⁻ H^{+ + HPO₄2−}

HPO₄²⁻ H^{+ + PO₄3−}

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

$K\_\{a1\}=\backslash frac\{12\}\{3\}\backslash simeq\; 6.92\backslash times10^\{-3\}$ (pK_a1 2.16)

$K\_\{a2\}=\backslash frac\{45\}\{6\}\backslash simeq\; 6.17\backslash times10^\{-8\}$ (pK_a2 7.21)

$K\_\{a3\}=\backslash frac\{78\}\{9\}\backslash simeq\; 4.79\backslash times10^\{-13\}$ (pK_a3 12.32)

For a strongly-basic pH (pH=13), we find

$\backslash frac\{10\}\{11\}\backslash simeq\; 7.5\backslash times10^\{10\}\; \backslash mbox\{\; ,\; \}\backslash frac\{12\}\{13\}\backslash simeq\; 6.2\backslash times10^5\; \backslash mbox\{\; ,\; \}\; \backslash frac\{14\}\{15\}\backslash simeq\; 2.14$

showing that only PO₄³⁻ and HPO₄²⁻ are in significant amounts.