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When selecting a buffer for a specific pH, the optimal pKa range should be within one pH unit of the desired pH. This is based on the principle that the effectiveness of a buffer is maximized when the ratio of the acid to its conjugate base is close to 1:1, which occurs when the pKa of the buffering system is similar to the desired pH.

When the pKa is within one unit of the pH you wish to maintain, the buffering capacity is enhanced, allowing the solution to resist changes in pH upon the addition of small amounts of acid or base. This range ensures that the concentrations of the acid and its conjugate base are present in sufficient proportions to effectively neutralize added acids or bases.

Choosing a pKa that is too far from the target pH, such as below 2 for higher pH solutions or significantly above the desired pH, will result in a less effective buffering system, leading to larger fluctuations in pH even with small additions of strong acids or bases. Therefore, the choice of a buffer system with a pKa within one unit of the desired pH offers optimal buffering capacity and stability.