Page 96 - Instant notes

C4
ACID-BASE TITRATIONS

Key Notes
An acid-base titration involves the addition of a titrant solution to
an analyte solution. One of these solutions contains an acid,
whilst the other contains a base; the titrant is usually a strong acid
or strong base. A pH titration curve of pH versus volume of
titrant is produced. The end point of the reaction is when the
reaction between the acid and the base has gone to completion
and each has neutralized the other, which corresponds to a sharp
change in pH. The end point is at pH=7 for a strong acid-strong
base titration, at pH>7 for a weak acid-strong base titration and at
pH<7 for a weak base-strong acid titration.
A buffer solution contains large and equal concentrations of an
acid and its conjugate base The pH of this solution is
approximately equal to the pK a of the acid. Addition of small
amounts of acid or base results in the mopping up or the release
of protons by the conjugate base or the acid as necessary, which
keeps the solution pH constant.
An acid-base indicator is a molecule that is differently colored in
its acid and its conjugate base forms. The indicator changes color
when the pH of the solution changes between pK a −1 (the acid
form) and pK a +1 (the base form). The color change of an
indicator can be used to determine the end point of an acid-base
titration providing its pK a coincides with the pH at the end point.

Related topics Fundamentals of equilibria (C1) Further acids and bases
(C3)

Titration curves

An acid-base titration is an extremely useful experimental method for determining the
pK a and also the amount of an unknown acid or base in solution. In an acid-base titration,
the pH of the solution is measured, generally with a glass electrode (see Topic E5), as
reaction between an acid and a base occurs. This is achieved by using an acid solution
and a base solution and adding one to the other. The solution that is added is called the
titrant solution; this is added to the analyte solution, and the whole process is called
titration. Generally the titrant is a strong acid or a strong base as appropriate. The results
are plotted as the pH of the solution versus the volume of titrant added, called a pH
titration curve.