Quick questions on Buffer solutions: TCE Chemistry (Tasmania)

Buffer capacity is the amount of acid or base a buffer can absorb before the pH changes sharply. It is greatest when the two components are present in roughly equal, high concentrations. Once one partner is largely used up, the buffer fails and the pH swings rapidly.

To make a buffer at a particular pH, choose a weak acid whose $pK_a$ is close to the desired pH, then adjust the ratio of acid to conjugate base. Because $\text{pH} = pK_a + \log_{10}\dfrac{[\text{base}]}{[\text{acid}]}$, equal amounts of the two give $\text{pH} = pK_a$, and shifting the ratio up or down moves the pH by at most about one unit before the buffer becomes ineffective. This is why each buffer system has a useful working range of roughly $pK_a \pm 1$.