16. Ions and Equilibrium;
       Acids and Bases
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       Buffers and pH control

What should the ratio of K2HPO4 to KH2PO4 be to obtain a solution with a pH exactly 7.00?


A phosphate buffer solution at pH = 7.00 should be approximately two parts K2HPO4 to three parts KH2PO4. This is reasonable, because we want the solution to be a little on the acid side of the observed pKa, and therefore we need a little more of the acid form of the buffer pair, KH2PO4.

Suppose now that we begin with a buffer mixture of 1.00 mole liter-1 of KH2PO4 and 0.62 mole liter-1 of K2HPO4, and add to this buffer mixture 0.01 mole liter-1 of any strong acid. How much will the pH shift? The added acid will react with HPO42- and produce more H2PO4-, so to a good approximation we can write:


The 0.01-molar acid has managed to lower the pH only by 0.01 unit. lf there had been no buffer present, addition of 0.01-molar strong acid would have shifted the pH to 2.00, a change in acidity similar to that between water and lemon juice. With a buffer the effect of the addition of strong acid is negligible.

Whenever pH changes are harmful, buffers have obvious applications. The bloodstream is buffered by carbonic acid and bicarbonate ions:

Since the blood is to be maintained approximately one pH unit higher than the pKa for this equilibrium, the ratio of bicarbonate ion to carbonic acid must be approximately 10: 1. The reason a carbonate buffer and not a phosphate buffer is used in the bloodstream, even though the PKa2 of phosphate is closer to 7.4, is apparently because CO2 is already present as a by-product of respiration. lt is available for use in buffering without any special supply system being needed.

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