In the complexometric titration of Al3+, excess of EDTA is reacted with Al3+ to form Al-EDTA complex. The unreacted EDTA can be determined by titrating it with a standard solution of Zn2+ using EBT indicator. The pH of the solution is maintained at around 10 using ammonia buffer. The indicator color in the buffer is blue, while the Zn-indicator complex appears wine red. The exact concentration of EDTA salt solution is determined by titrating it with a standard solution of Zn2+ at pH 10, using EBT indicator

1. SHREE MALLIKARJUN COLLEGE CLASS:SYBSC
SAFETY: Refer to MSDS of ethylenediaminetetraacetic acid disodium salt (EDTA), aluminum sulphate, ammonium
hydroxide, ammonium chloride, Eriochrome Black-T (EBT), potassiumnitrate, zinc sulphate. -Dr. Mithil S. Fal Desai
Aim: To estimate aluminum by back titration using zinc sulphate.
Chemicals: Ethylenediaminetetraacetic acid disodium salt (EDTA), aluminum sulphate,
ammonium hydroxide, ammonium chloride, Eriochrome Black-T (EBT), potassium nitrate, zinc
sulphate.
Apparatus: Volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod.
Theory: In the complexometric titration of Al3+, excess of EDTA is reacted with Al3+ to form Al-
EDTA complex. The unreacted EDTA can be determined by titrating it with a standard solution
of Zn2+ using EBT indicator. The pH of the solution is maintained at around 10 using ammonia
buffer. The indicator color in the buffer is blue, while the Zn-indicator complex appears wine red.
The exact concentration of EDTA salt solution is determined by titrating it with a standard solution
of Zn2+ at pH 10, using EBT indicator.
Al3+ + Na2C10H14N2O8  [AlC10H12N2O8]- + 2Na+ + 2H+
Zn2+ + Na2C10H14N2O8  [ZnC10H12N2O8]2- + 2Na+ + 2H+
Procedure:
A) Preparation of 0.01M EDTA (100 mL) calculate
B) Preparation of 0.01M ZnSO4.H2O (100 mL) calculate
C) Preparation of buffer of pH 10
Dissolve 32 g of NH4Cl in 300 mL of 28-30% ammonia solution. Slowly and carefully add 200
mL distilled water.
D) Standardization of EDTA salt solution (~0.01M)
Rinse and fill the burette with EDTA salt solution. Pipette out 10 mL of 0.01M of ZnSO4 solution
and add 10 mL of a buffer of pH 10. Add 2-3 drops of EBT indicator solution to the flask. Titrate
the solution from the flask against EDTA until the wine red color of the solution changes to blue.
Repeat the titration to get constant readings.
E) Estimation of Al3+ ions.
Rinse and fill the burette with standard Zn2+ solution. Pipette out 10 mL of Al3+ ion solution in a
conical flask and adjust the pH to 7-8 using ammonia solution. Pipette out 20 mL of 0.01M EDTA
solution and boil it for 1 min. Cool the solution and adjust the pH to 10 by the addition of 10 mL
of the buffer. Add about 10-20 mg of 1% (w/w) EBT indicator in potassium nitrate. Titrate the
unreacted EDTA until the blue color of the solution changes to red. Repeat the titration to get
constant readings.
Results: i) Strength of EDTA solution =____M
ii) Strength of given Al3+ ion solution =____M
iii) Amount of Al3+ in given sample = ____g/L.
iv) Amount of Al2SO4.18H2O in given sample = ____g/L.
1) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns, M.
Thomas, B. Sivasankar, Pearson India Education, 6th Edition.