Sulfate buffer? Your laboratory is out of materials to make phosphate buffer and you are considering using sulfate to make a buffer instead. The pKa values for the two hydrogens in H2SO4 are -10 and 2. (a) Will this approach work for making a buffer effective near pH 7? (b) Around what pH might a sulfate-based buffer be useful?

A buffer is most useful when the amounts of the conjugate acid and conjugate base are in a 1:1 ratio. The Henderson-Hasselbalch equation can be used to relate the pH of a buffer to ratio of the conjugate acid-base pair:

pH = pKa + log([A⁻]/[HA])

To create a sulfate buffer at pH 7, the ratio of conjugate base to conjugate acid needs to be (using the second pKa value)

pH = pKa + log([A⁻]/[HA])

7 = 2 + log([A⁻]/[HA])

5 = log([A⁻]/[HA])

[A⁻]/[HA] = 10⁵

The ratio is nowhere near 1:1, so the buffer will not be effective.

(b) Since, a buffer is most effective at a 1:1 ratio of the conjugate acid-base pair, the pH should equal the pKa:

pH = pKa + log([A⁻]/[HA])

pH = pKa + log(1)

pH = pKa = 2

Thus, a sulfate-based buffer might be useful at pH 2