Acid-Base Titrations
Titrations are analytical procedures that can determine an unknown
concentration. The equivalence point of a titration is reached when the
moles of acid equal the moles of base. This idea can be expressed as: na = nb
or MaVa = MbVb. The concept is to accurately measure three of the four
quantities and solve for the unknown.
A titration is performed where a strong base (1 M) is added to 20 mL
of a strong acid (1 M). The pH is plotted against volume of base and the
following plot is obtained.
14
pH
7
0
0
10
20
Volume of base (mL)
30
40
If a strong base is added to a weak acid the titration curve will alter in
several ways.
1) the original pH of the weak acid will be higher than a strong acid
2) the first half of the curve will have a buffering region – this is a
region where a large quantity of base will only change the pH a
relatively small amount – buffers are created by the presence of an
equilibrium established between a weak acid (eg. CH3COOH) and its
conjugate base (CH3COO1-). The buffer capacity is exceeded when
[CH3COOH] = 0. The curve will then resemble the strong acid/base
graph.
3) the pH at the equivalence point will be higher than 7.0