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

Before any outside acid or base is added, the pH of a buffer mixture of a weak acid and its salt is given by the same expression that we derived previously from the equilibrium constant:

The pH of the buffer solution is determined by the pKa of the acid used and the ratio of basic form (salt, A-) to the acidic form (acid, HA). To prepare the buffer mixture, [A-] can be assumed to be equal to the total concentration of the salt, and [HA] equal to the total concentration of the weak acid used.

If the ratio of salt to acid is 1: 1, then the pH will be the same as the pKa, but one obviously can obtain any desired pH close to pKa by altering the salt-acid ratio of the solution as it is made up.

Example. The second dissociation of phosphoric acid has a pKa near 7.0:

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