Acid-base equilibria
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I am doing my A2 coursework and am confused about calculating the Ka of a diprotic acid (tartaric acid). I have calculated the Kas of monoprotic acids using the principle that at half neutralisation point, Kais equal to the concentration of hydrogen ions. Can I use the same calculation for diprotic acids?
Ulex replies ... Nice question!
As you know, if you titrate a monoprotic acid against a strong alkali you get a titration curve with an extended point of inflection (steeply rising part) which occurs at the end-point. In calculating Ka by the method you describe, it is necessary to identify this end point, either by using an indicator or by direct measurement of pH and actually plotting the curve.
If you titrate a diprotic weak acid with a strong alkali, the resulting titration curve has two points of inflection corresponding to the two protons which are reacting (first one, then the other). If you can plot this curve you can evaluate the first Ka value from the first of the two points of inflection (exactly as for a monprotic acid), and the second Ka value from the midway between the two en-points, where the pH = pKa for the ionisation of the second proton.
In some favourable cases it is even possible to identify the two end points by using first one indicator and then, in the same solution, a second indicator, but indicator choice is quite tricky and almost involves knowing the expected answers before you start. A good pH meter is the best way.
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updated: 25 February 2004
