During the titration of a weak monoprotic acid with a strong base
CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases
Problem
During the titration of a weak monoprotic acid with a strong base, the indicator used changes color at pH 9.3, slightly after the stoichiometric completion of the reaction. Which of the following best explains why the end point does not exactly coincide with the equivalence point in this titration?
(a) A small excess of strong base is required to overcome the buffering action of the conjugate base before the indicator’s colour change becomes visible.
(b) The indicator undergoes its colour change over a finite pH range that may not match the pH at equivalence.
(c) The equivalence point for a weak acid–strong base titration is at pH 7, whereas the indicator changes at a higher pH, so they cannot coincide.
(d) Some of the added base first deprotonates the indicator itself, so additional base must be delivered before a colour change is observed.