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18.4 – Acid-Base Titrations

18.4.1 - Sketch the general shapes of graphs of pH against volume for titrations involving

strong and weak acids and bases, and explain their important features

When we perform a titration, we can sketch a titration curve to show how the pH changes

as more acid or base is added. The starting point will be the pH of the acid or base being

analysed. Note that the graphs below all show an acid being added to a base. If a base is

added to an acid, then graphs are simply reversed.

It is also important to recognise that the near-horizontal region on the graph is known as the

buffer region.

There are four combinations of titrations that can be done:

Strong Acid and Strong Base

An example of this is:

At the equivalence point, the pH is neutral, so there is no excess of H+ or OH- ions. The pH

changes from being very high to very low. The graph will show a long vertical rise or drop,

depending on whether the acid or base is being added. There is a point of inflection at pH 7,

which means that the equivalence point is at pH 7. The equivalence point is where the acid

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and base react in equal stoichiometric amounts and neutralise each other so that the

solution only contains salt and water. There is very little change in pH before the

equivalence point.

The most significant feature is that the pH undergoes a large, rapid change at pH 7.

Strong Acid and Weak Base

An example of this is:

The pH starts moderately high and becomes very low (or vice versa). The equivalence point

is below pH 7, and the vertical section is short.

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Weak Acid and Strong Base

An example of this is:

The pH begins very high and finishes moderately low (or vice versa), with an equivalence

point above pH 7. The vertical section is short.

Weak Acid and Weak Base

An example of this is:

Since the vertical drop or rise is so small, it is difficult to determine the equivalence point,

and a back titration often has to be used instead of an indicator.