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Teachers demand that texts include what they have always taught, but this is not a criterion for what they ought to teach. I have sympathized elsewhere (1) with the need of textbook authors to meet the demands of the market. However, we should persuade teachers to demand the curriculum that students need rather than what our profession wants to teach. If teaching something that is otherwise valueless provides an opportunity to emphasize something that is taught in another part of the curriculum, this is an advantage; but the teacher may question whether the advantage outweighs the time spent learning that otherwise valueless subject.
Calculating a titration curve is an unnecessarily complicated method of explaining how a buffer works. Buffers consist of a conjugate acid–base pair in equilibrium with hydronium ion. The equation for this equilibrium makes it obvious that addition of a small quantity of an acid or base will not alter the hydronium concentration significantly. Even if the added acid or base is a strong one that is neutralized by the buffer components, which in practice it usually is not, it will cause only slight alteration in the concentrations of the acid or base in the pair, and hence small alteration in the concentration of hydronium ion and hence of pH. Students, especially sub-average students, should not be confused by relating buffer action to titrimetry.
Buffer capacity is the change of pH with respect to the number of moles of strong acid or base added per liter of buffer, rather than “with respect to the volume of titrant”. This is more directly related to the usefulness of a buffer than the slope of a titration curve. Buffer capacity is given by δ(mole acid or base added per liter)/δ pH = (ln10)/(1/[salt]+1/[base]).
Choosing an indicator for an acid–base titration requires only calculation of the pH of the salt solution that is formed at the end point. Calculating the whole titration curve merely complicates the issue. In analytical practice, the correct indicator is usually already known. If it is not, the acid dissociation constant is often also unknown, so the calculation is impossible. In the rare case that the pH at the end point is not already known, in practice it is found experimentally, so that the ability to calculate it is merely an academic exercise. Similar considerations apply to other kinds of titration.
Equilibrium calculations are grossly over-emphasized in chemistry teaching, as has been discussed elsewhere (2, 3).
Literature Cited
- Hawkes, S. J. J. Chem. Educ. 2005, 82, 1615.
- Hawkes, S. J. J. Chem. Educ. 2003, 80, 1381.
- Lewis, D. L. J. Chem. Educ. 2004, 81, 1265.
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