In an acid–base titration the volume of titrant needed to reach the equivalence point is proportional to the moles of titrand. Because the pH of the titrand or the titrant is a function of its concentration, however, the change in pH at the equivalence point—and thus the feasibility of an acid–base titration—depends on their respective concentrations. The illustration below, for example, shows a series of titration curves for the titration of several concentrations of HCl, each 25 mL in volume, with equimolar solutions NaOH: (a) 10–1 M HCl, (b) 10–2 M HCl, (c) 10–3 M HCl, (d) 10–4 M HCl, and (e) 10–5 M HCl. For titrand and titrant concentrations smaller than 10–3 M, the change in pH at the end point is too small to provide accurate and precise results.
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