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I am currently leaning towards an experiment on how certain factors affect the capacity of a phosphate buffer, and am wondering if temperature in any way affects the capacity of the buffer. I know changing the temp will change the Ka of the acid, but am not sure if that changes capacity (how much it resists pH change) or just changes the pH range in which the buffer effective e.g. from 5-8 pH to 4-7 pH.

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If you follow this derivation of buffer capacity, the last equation shows you that buffer capacity is a function of $K_a$, so a change in the Ka will affect the buffer capacity. Also, since you know that $K_a$ is affected by temperature, that should lead you to realize that the actual chemical environment is different at different temperatures due to a different concentration of acid and weak base pair. The magnitude of effect maybe small, but your experiment can prove that one way or the other!

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