9.5 - Acid - base titrations

9.5.1: Draw and explain a graph showing pH against volume of titrant for titrations involving strong acids and bases.

Titration Curves

General Type	Example	Typical Titration Curve	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 acetic acid (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.

Try it yourself

The following applet (Courtesy of the RSC) allows you to plot a titration curve using the acids and bases of your choice. You can then select a suitable indicator for the titration.

The Y-axis shows the pH of the mixture during the titration and the X-axis shows the amount of base added.

The equivalence point shows the pH at the point when all the acid has been neutralised by the base added. Notice that this is not necessarily at pH=7, it depends on the acids and bases used. For further explanation see section 18.4 "salt hydrolysis"