How do we calculate the pH of a buffer that is composed of HPO_4^(2-) and H_2PO_4^(-)?

1 Answer
anor277
Apr 13, 2017

The buffer equation tells us that pH=pK_a+log_10([[HPO_4^(2-)]]/[[H_2PO_4^(-)]]), but here pK_a=7.20 for H_2PO_4^(-).

Explanation:

A buffer is formed by mixing appreciable quantities of a weak acid and its conjugate base. Such a mixture keeps the pH of the solution tolerably close to the pK_a of the weak acid.

Now this site gives pK_a=7.20 for the following reaction:

H_2PO_4^(-) + H_2O(l) rightleftharpoonsHPO_4^(2-),

And thus solutions which contain tolerably equal concentrations of dihydrogen phosphate, and biphosphate, should have pH reasonably close to 7.20.

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