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.