mass
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
S2. ACID-BASE TITRATIONS
S2.1 pH CHANGES DURING ACID-BASE TITRATIONS
Consider the neutralization of an acid by an alkali: the pH characteristic of such reactions is found to vary with the:
1. amount of alkali added to the acid;
2. extent of ionization (strength) of the reacting acid and alkali.
Inspection of the above plots shows that, in an ACID-BASE titration, the stoichiometric point does not necessarily correspond to a pH = 7 (@ 25 °C). This is due to hydrolysis in the cases of salts of strong acids/weak bases, e.g., ammonium sulphate, and salts of weak acids/strong bases, e.g., sodium acetate (ethanoate).
weak acid + strong base
strong acid + weak base
strong acid + strong base
weak acid + weak base
S2.2 ACID-BASE INDICATORS & SUITABILITY FOR PARTICULAR REACTIONS
Indicators do not change colour sharply at one particular pH, but rather over a narrow range of pH. Details for a selection of indicators are set out below.
Litmus has a colour change that occurs over an unusually wide pH range but it is useful nonetheless for detecting acids and alkalis in the laboratory because it changes colour around pH 7. It would not be suitable for detecting a sharp end point in an acid-base titration because too large a volume of alkali (or acid) would be required to produce the desired colour change.
It follows, therefore, that the choice of indicator for a particular acid-base titration will depend on the pH range of its colour changes.
Phth has an end point suitable for estimating this stoichiometric point.
(MeOr would be unsuitable despite still giving an end point – this end point is very far from the stoichiometric point).
MeOr has an end point suitable for estimating this stoichiometric point.
(Phth would be unsuitable despite still giving an end point – this end point is very far from the stoichiometric point).