In complexometric titrations, initially the indicator forms a metal indicator complex with metal ions which is less stable compared to the metal-EDTA complex. Hexadentate ligand like EDTA acts as a chelating agent that complex with the metal ions in the solution. When all the metal ions form a complex with a chelating agent, the original colour of the indicator in the buffer is observed which reflects the endpoint of the titration

1. SHREE MALLIKARJUN COLLEGE CLASS:SYBSC
SAFETY: Refer to MSDS of Ethylenediaminetetraacetic acid (EDTA), ammonium hydroxide, ammonium chloride,
Eriochrom Black T (EBT), zinc sulphate
-Dr. Mithil S. FalDesai
Aim: Estimation of Zn by complexometric titrations using EDTA.
Chemicals: Ethylenediaminetetraacetic acid (EDTA), ammonium hydroxide, ammonium
chloride, Eriochrome Black T (EBT), zinc sulphate.
Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass
rod.
Theory: In complexometric titrations, initially the indicator forms a metal indicator complex
with metal ions which is less stable compared to the metal-EDTA complex. Hexadentate ligand
like EDTA acts as a chelating agent that complex with the metal ions in the solution. When all
the metal ions form a complex with a chelating agent, the original colour of the indicator in the
buffer is observed which reflects the endpoint of the titration.
Zn2+ + Na2C10H14N2O8  [ZnC10H12N2O8]2- + 2Na+ + 2H+
[molecular formula of EDTA is C10H16N2O8]
Procedure:
i) Standardization of EDTA (~0.01M)
Rinse and fill the burette with EDTA solution. Pipette out 10 mL of 0.01M of ZnSO4 solution
and add 10 mL of the buffer of pH 10. Add 2-3 drops of EBT indicator solution to the flask.
Titrate the solution from the flask against EDTA solution until the red colour of the solution
changes to blue. Repeat the titration to get constant readings.
ii) Estimation of Zn2+ ions.
Rinse and fill the burette with EDTA solution. Pipette out 10 mL of Zn2+ ion solution in a
conical flask and add 10 mL of the buffer of pH 10. Add 2-3 drops of EBT indicator solution to
the flask. Titrate the solution from the flask against EDTA solution until the red colour of the
solution changes to blue. Repeat the titration to get constant readings.
Results: i) Strength of EDTA solution is =____M
ii) Amount of Zn2+ in given water sample = ____g/L.
Further reading (available in the library)
1) Experiments in Applied Chemistry, S. Rattan, S. K Kataria and Sons.
2) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns,
M. Thomas, B. Sivasankar, Pearson India Education, 6th Edition.
Questions: 1) What is the theoretical endpoint of a complexometric titration?
2) How complexation stabilize the metal ions?
3) What is the importance of pH in complexometric titration?
4) List any two applications of EDTA titration.
5) How a mixture of Ca and Mg can be determined volumetrically?