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2 February, 05:02

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?

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  1. 2 February, 05:30
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    (a) No

    (b) pH 2

    Explanation:

    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
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