ANALYTICAL cHEMISTRY - V

Experiment 3

Principle

• Aluminium (Al) and magnesium (Mg) are present in almost all the antacid, which are prescribed in case of acidity and indigestion. They are present in hydroxide form and hence behave as base. Their analysis is thus important.

• Determination of the aluminium and magnesium content is mainly done by titration with EDTA solution. Control of pH plays important role in determining these elements as they form a stable metal-EDTA complex in different pH ranges.

• By the use of masking agents, some of the cations in the mixture can be masked, and thus, they no longer react with EDTA. Masking may be defined as the process, in which a substance without its physical separation is so transformed that it does not enter into a particular reaction.

Apparatus required

Conical flask, Pipette, Burette, Funnel, etc.

Chemicals required

Antacid tablet, Eriochrome black T (as indicator), Xylenol orange (as indicator), Buffer pH 10, Buffer pH 5, 0.02 M EDTA solution, 0.02 M Pb(NO₃)₂ solution, etc.

Preparation of reagents

1. 0.02 M EDTA solution:

1000 mL 1 M EDTA ≡ 372.24 g

250 mL 0.02 M EDTA ≡ _______ g.

2. 0.02 M Pb(NO₃)₂ solution:

1000 mL 1 M Pb(NO₃)₂ ≡ 331.2 g

250 mL 0.02 M Pb(NO₃)₂ ≡ ________ g.

3. Buffer pH 10 solution:

Take 17.5 g of NH₄Cl and dissolve it in 142 mL liquor ammonia. Make the volume up to 250 mL in a 250 mL volumetric flask.

4. Buffer pH 5 solution:

Take 10.21 g potassium hydrogen phthalate and dissolve it in 226 mL of 0.1 M NaOH and make up the volume up to the mark in a 250 mL volumetric flask.

5. Antacid tablet solution:

Dissolve a tablet using 2-4 mL concentrated HCl, if required. Add distilled water till tablet dissolves completely. Make the volume up to the mark in a 100 mL volumetric flask.

Procedure

1. For blank titration:

Pipette out 10 mL 0.02 M EDTA in a conical flask. Add 10 mL buffer pH 10 and titrate this solution against 0.02 M Pb(NO₃)₂ solution from the burette using EBT as an indicator. Color change is from blue to pinkish red.

2. For total aluminium and magnesium content:

Pipette out 1 mL of sample solution in a conical flask and add 10 mL 0.02 M EDTA to it and 10 mL buffer pH 10. Titrate it against 0.02 M Pb(NO₃)₂ solution from burette using EBT as an indicator. Color change is from blue to pinkish red.

3. For aluminium content:

Pipette out 1 mL of sample solution in a conical flask and add 10 mL 0.02 M EDTA and 10 mL buffer pH 5. Titrate it against 0.02 M Pb(NO₃)₂ solution from the burette using xylenol orange as an indicator. Color change is from yellow to red.

Result

1. Total content of aluminium = __________ mg.

2. Total content of magnesium = ___________ mg.

Reference Material

1. G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition

2. S. Suzanne Nielsen, Food analysis, 4th Edition

Questions

1. Is EDTA a primary standard? If no, how we can standardize EDTA solution?

2. What is the role of Xylenol orange indicator in this experiment?

3. Give the structure of EBT indicator.

4. Draw the structure of EDTA-Mg and EDTA-Ca complexes.

5. What is the role of lead nitrate in this titration?