Measuring Aluminium Ions

There is no Ion Selective Electrode for Aluminium however due to the strong reaction with Fluoride in solution an indirect method is possible using a Fluoride ISE and some prepared reagents. A simpler method is possible  


The EDT directION Fluoride 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:

1.   Model QP459 Ion Meter

2.   3221 Fluoride Combination ISE

3.   0.01 Molar SodiumFluoride Titrant

4.   Acetate Buffer.

5.   Aluminium Standard solution

6.   Gran’s plot paper 10% volume corrected

7.   Glassware. Beakers , Burette 50ml

8.   Deionised water.

9.   Magnetic Stirrer

Reagent Preparation.

2M Acetate buffer:                                          

82gm of CH3COONa and 57ml CH3COOH made up to 1 litre  

0.01M Aluminium standardising solution: 2.41gm AlCl36H2O made up up to 1 litre



To 100ml of distilled water in a beaker. Add 1ml of buffer. Immerse the Fluoride Electrode and select the mV mode  on the DR359TX Ion Meter

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. To 100ml in 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


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

Stock ISA Solution :     5M sodium perchlorate rather than distilled water.