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18.5 - Titration Curves

18.5.1: Draw and explain the general shape of graphs of pH against volume of titrant for titrations involving monoprotic acids and bases. All combinations should be covered: strong acid + strong base, strong acid + weak base, weak acid + strong base, and weak acid + weak base.

Strong acid strong base titrations

Titration curves are the graphs obtained by plotting the pH of the reaction mixture against the volume of base (or acid) added during the titration of either an acid by a base or vice versa.

A typical strong acid - strong base titration curve looks like this:

The blue curve represents the titration of 25 cm3 hydrochloric acid (0.1M) by additions of 0.1M Sodium hydroxide solution. The pH starts off at 1 and increases only very slowly at first. Near the equivalence point there is a rapid and dramatic increase in pH. This means that most indicators are suitable for carrying out ths type of titration.

The red curve represents the titration of a 25cm3 sample of sodium hyroxide (0.1M) by successive additions of 0.1M hydrochloric acid. Again a rapid inflexion is seen near the equivalence point.

Note: The equivalence point is the point at which the number of moles of acid exactly neutralises the equivalent number of moles of base.

Weak acid - strong base titration

The curve shows the titration of 25 cm3 ethanoic acid (0.1M) by additions of 0.1M sodium hydroxide solution. The pH starts off at 2.4 and increases steadily at first. Near the equivalence point there is a rapid increase in pH. Only indicators that change colour in the region pH 6 - 10 are suitable for carrying out this type of titration.

Phenolphthalein is the indicator of choice fopr this type of titration. It registers colourless in the acidic region and changes to red in base. The first hint of pink is taken as the end point.

Take care with phenlphthalein, as the red colour fades fairly rapidly if the solution is left standing.

Weak base strong acid titration

The red curve represents the titration of 25 cm3 ammonia (0.1M) by additions of 0.1M hydrochloric acid solution. The pH starts off near 12 and decreases steadily towards the equivalence point where there is a rapid decrease in pH. This means that only indicators that act in the acidic pH region are suitable for carrying out ths type of titration.

Methyl orange is the indicator of choice. It registers red in acid and yellow in base with the endpoint being orange.

More examples

General Type	Example	Features of Curve
Strong acid & Strong Base	HCl added to NaOH	Curve begins at high pH typical of strong base and ends at low pH typical of strong acid. There is a large rapid change in pH near the equivalence point (pH =7).
Strong base & strong acid	NaOH added to HCl	Curve begins at low pH typical of strong acid, and ends at high pH typical of strong base. There is a large rapid change in pH near the equivalence point (pH=7).
Weak acid & Strong base	NaOH added to ethanoic acid (CH₃COOH)	Curve begins at a higher acidic pH and ends at high basic pH. The pH change at the equivalence point (pH > 7) is not so great.
Strong acid & Weak base	Ammonia (NH₃) added to HCl	Curve begins at low pH and ends at a less high basic pH. The pH change at the equivalence point (pH < 7) is similar to that for Strong base & Weak acid.
Weak acid & Weak base	Ammonia (NH₃) added to ethanoic acid (CH₃COOH)	Curve begins at higher acidic pH and ends at low basic pH. There is not a great pH change at the equivalence point (pH ~ 7) making this a very difficult titration to perform.