Titration of a strong acid by a weak base

The applet below draws the graph of pH against volume of titrant added for the titration of strong acid by a weak base. Specifically:

 
  • a solution of 0.1 mol/L base (aq) is the titrant added from a burette. You can select from a range of bases. Ammonia solution is an approproate example to start with.

  • 25 cm3 of a solution of 0.1 mol/L HCl (aq) is the analyte in a flask below the burette.

  • titrant is added beyond the equivalence point when 25ml of titrant has been added.

  • turn on the data table feature to see the pH value of the equivalence point.

 

Select a weak base (note: default is strong acid/strong base) and then press the run button to draw the pH curve for the strong acid/weak base titration.

Message: Java Applet not loaded !

If you were using a Java-enabled browser ,you would see pH curve plotter program here instead of this paragraph.

Check your browser options list to enable Java programs to run in this browser.

Note that for the titration of a strong acid with a weak base:

 
  • the equivalence point is less than pH 7.0
 

The shape of the graph is explained as follows:

Before the equivalence point:

For a titration between ammonia solution and hydrochloric acid, added NH3(aq) titratant is neutralised by the strong acid in the analyte solution to produce a salt and water. In the case of ammonia, a solution of the salt ammonium chloride is produced.

The ammonium chloride solution has little effect on the hydronium ion solution in the hydrochloric acid and so the curve follows the shape for a strong acid/strong base and for the same reason: acid has simply been consumed by added base.

At the equivalence point:

At the equivalence point the hydrochloric acid has just been neutralised by the base to give a solution of ammonium chloride in water. However, the ammonium cation, NH4+(aq), is the conjugate acid of ammonia, it combines with hydroxide ions in solution, OH-(aq), reducing their concentration and lowering the pH.

After the equivalence point:

After the equivalence point is reached the the continued addition of ammonia solution does not raise the pH value sharply as ammonia is only a weak base and does not greatly increase the OH-(aq) concentration.

 
  • After the equivalence point the pH value does not rise as quickly as in the strong acid/strong base titration.