Buffer Solutions

In Figure above, we see that the pH changes most slowly in the vicinity of the half-equivalence point, the buffer region. Small additions of base (or acid) to a solution containing comparable amounts of an acid and its conjugate base change the pH only slightly; the solution is said to be buffered against pH changes. Consider 100 mL of a solution containing 0.1 M CH₃COOH and 0.1 M CH₃CO₂^-, pH = 4.75. Addition of 10^-3 mol of HCl would change the ratio [CH₃CO₂^-]/[CH₃COOH] from 0.1/0.1 to 0.09/0.11 and the pH would change from 4.75 to 4.66. We could, of course, prepare 100 mL of a pH 4.75 solution by diluting HCL to 1.78 × 10^-5 M. However, addition of 10^-3 mol of HCl would reduce the pH to 2.0, a much bigger change than for the buffer solution.

Example

How many moles of NaH₂PO₄ and Na₂HPO₄ should be used to prepare 250 mL of a solution with a total phosphate concentration of 0.10 M and pH = 7.00?

The relevant equilibrium is the second proton transfer step for phosphoric acid,