Acid-base equilibria
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I need help understanding titration curves for diprotic acids such as tartaric acid. How do you know whether the point of inflexion is the first or second equivalence point? Thanks.
Ulex answers
This shouldn’t be a problem. Imagine taking a solution of tartaric acid and titrating it with sodium hydroxide. The pH starts low and changes little until the first point of inflexion, marking the first proton donation. There is then a further ‘flat’ region until the second point of inflexion, corresponding to the loss of the second proton.
Diprotic acids have two Ka values; the second of these is always much smaller than the first (second proton less easily lost). This means that the second equivalence point always happens at a higher pH than the first.
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updated: 13 December 2006
