Fluoride Ion Selective electrode is used in the following titration of aluminium ions with a sodium fluoride titrant. Electrode potentials corresponding to additions of titrant are plotted directly on 10% volume corrected Gran's plot paper. Aluminium ions can be measured with a precision of about ±3 x 10-6 M for concentrations in the range 0 -> 2 x 10-4 M.

Equipment Required:

  • Gran's plot paper 10% volume corrected
  • Digital pH meter
  • 3221 detectION Combination Fluoride electrode
  • Magnetic stirrer
  • Burette

Reagents Required:

  • 0.01M Fluoride titrant
  • 2M Acetate buffer :                                          
  • 82gm of CH3COONa and 57ml CH3COOH made up to 1 litre
  • 0.01M Aluminium standard solution :
  • 2.41gm AlCl36H2O made up up to 1 litre



100ml of distilled water in a beaker. Add 1ml of buffer.

Add 0.5ml of fluoride standard at intervals until 5ml have been added noting readings.

Plot the electrode potentials in the Gran's plot paper, and draw a straight line through them, the intercept to the horizontal axis should be close to the origin.

Standard                           .

Prepare a 10-4 M aluminium solution by 1:100 dilution of the stock aluminium standard, into a beaker and add 1ml of buffer.

Add fluoride titrant to the sample in 0.5ml aliquots. Before the end point has been reached potentials will be too low to record, do not wait for steady state reading initially but proceed until potentials are obtained which can be plotted on the paper.

Continue to take electrode potentials and plot until the readings cease to be on the plot paper. Draw the best straight line through the four or five points highest on the paper, the intercept point should be about 2.7ml (Stoichiometry is 2.7F;Al).     


Put 100ml of sample in a beaker, add 1ml of buffer, add 0.5ml of titrant and plot potentials produced. Obtain the intercept, the unknown level of aluminium is then obtained from

 X    =  Unknown Intercept   _    Blank Intercept

10-4      Standard Intercept   -     Blank Intercept

If the ionic strength of the sample is varying or is high ionic strength adjustment solution should be added to both titrant and sample.

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.