Determination of Aluminium ; Back Titration

: Experiment : Determination of Aluminium: Back titration . Apparatus: Burette, pipette, conical flask, Beakers.

Theory:

Back titration :

Many metals for various reasons cannot be titrated directly; they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or  a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of  EDTA is back titrated with a standard metal ion solution; a solution of Zinc Chloride or  sulphate or Magnesium chloride or sulphate is often used for this purpose .The end point is deleted with the aid of the metal indicator which responds to the Zinc or Magnesium ions introduced in the back titration. Procedure:

A. Preparation of about 0.01 M EDTA solution. 0.9 gm of pure dry EDTA was weighted out roughly and dissolved in 250 ml volumetric flask with deionised water up to the mark.

B).Preparation of standard 0.01 M Zinc ion solution Weigh out accurately 0.163 gm of Zinc and dissolve it in 5ml conc.HCl and 5 ml water in a 500ml beaker and allow it to react. Neutralise it by adding NaOH solution drop by drop until it gives turbidit turbidity y .Stir it properly properly .When turbidity turbidity appears appears add a drop of dil. HCl to neutralize excess NaOH. Make the solution 250ml.

C. Standardization of EDTA solution with Zinc ion solution. Pipette out 25ml of Zinc ion solution in 250 ml conical flask. Add 2ml buffer solution (NH4OH / NH4Cl of pH 10) and 10-12 drops of Erichrome Erichrome black -T indicator indicator (about 30-40 mg).Titrate it with EDTA solution until wine red color changes to deep blue color. Repeat the process to get the concurrent readings.

D. Pipette 25 ml of aluminium ion solution (approximately 0.01M) into a conical flask  and from a burette add a slight excess of 0.01M EDTA solution. Adjust the pH  between 7 to 8 by the addition of ammonia solution. Boil the solution for a few minutes to ensure complete complexation of the aluminium, cool to room temperature. Again adjust the pH to 7-8 .Add 50 mg of solochrome black (1% in  potassium nitrate mixture) and titrate rapidly with standard 0.01 M Zinc ion solution until the color changes from blue to wine red.

Observation:

Table 1: Titration of standard Zinc sulphate solution vs, EDTA.  No. of obs.

Vol.of Zn2+solution Initial

Burette reading Final Concurrent

Table 2: Titration of standard Zinc sulphate solution vs remaining EDTA.

 No. of obs.

Vol.of Zn2+ solution

Calculation :

Strength of Zn2+ solution = 0.01M (Use blank space) Result:

Initial

Burette reading Final

Concurrent