How do we make a constant-pH solution

We often need to prepare a solution having a constant pH. Such solutions are vital in the cosmetics industry, as well as when making foodstuffs and in the more traditional experiments performed by the biologist and physical chemist. 

To make such a solution, we could calculate exactly how many moles of acid to add to water, but this method is generally difficult, since even small errors in weighing the acid can cause wide fluctuations in the pH. Furthermore, we cannot easily weigh out one of acid oxides such as NO. Anyway, the pH of a weak acid does not clearly follow the acid s concentration (see p. 254). 

The Henderson-Hasselbach equation, Equation (6.50), relates the pH of a buffer solution to the amounts of conjugate acid and conjugate base it contains  

We follow the usual pattern here by making a buffer with a weak acid HA and a solution of its conjugate base, such as the sodium salt of the respective anion, A-. 

In some texts, Equation (6.50) is called the Henderson-HasselbaLch equation. 

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