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Complexometry Titration

Objectives:
1. To determine the definition of complexometry titration.
2. To determine the concentration of standard EDTA solution Zinc in an unknown sample.
3. To describe the relation between metal cation and EDTA.

Introduction:
Ethylenediaminetetraacetic Acid, also known as EDTA, is a combination of four carboxyl groups and two amine groups that can act as electron pair donors. The ability of EDTA to donate six lone pairs of electron to forms metal cations makes it a hexadentate ligand.

Carrying out the experiment of EDTA with a buffer solution removes H+, and a complex (EDTA) 4+ ion will formed. This reaction also favors the formation of the EDTA-metal cation complex reaction product. As an example, it can react with a zinc ion and forms a complex [Zn(EDTA)]2- as shown below:

Zn2+ + (EDTA) 4- --> [Zn (EDTA)] 2-

The concentration of zinc can also be determined by using an EDTA solution and a correct indicator which can form a coloured complex when reacted. These were two important factors in order to obtain the concentration:
i. The indicator must forms a complex with zinc after all of the EDTA has been completely reacted with zinc cations.
ii. The colour of the indicator must different with the colour of Zn2--indicator complex.

Apparatus and Materials:

250ml Volumetric Flask
25ml Pipette
250ml Conical Flask
Burette
Hydrated EDTA Disodium Salt

Magnesium Complex Solution
Buffer Solution
Zn2+ Solution
Calcium Carbonate Solution
Indicator

Procedure:

A) Preparation of 0.1M EDTA Solution
1. Weigh approximately 0.95g of hydrated EDTA disodium salt.
2. Place it in 250ml volumetric flask and dilute with distilled water. Shake until homogenous.

B) Diluting and standardize EDTA solution
1. 25ml of calcium carbonate solution is pipetted into a 250ml conical flask.
2. The solution is diluted to 100ml. 2ml of magnesium complex, 5ml of buffer solution and 3 drops of indicator is added to the solution.
3. The solution is titrated with EDTA solution until the brick red colour turns to blue.
4. The titration is repeated another two times.

C) Determination of concentration of Zinc solution
1. 25ml of Zn2+ is pipetted into 250ml conical flask.
2. The solution is diluted to 100ml. 10ml of buffer solution and 3 drops of indicator is added to the solution.
3. The solution is titrated with EDTA solution until the brick red colour turns to blue.
4. The titration is repeated another two times.

Data and Calculation:
A) The mass of hydrated EDTA disodium salt:
0.9502g

B) Diluting and standardize EDTA solution

EDTA Volume (ml) Test 1 Test 2 Test 3
Initial Reading 01.50 26.00 14.60
Final Reading 26.00 50.00 38.70
Volume of EDTA used 24.50 24.00 24.10
Average Volume of EDTA used 24.20

C) Determination of concentration of Zinc solution

EDTA Volume (ml) Test 1 Test 2 Test 3
Initial Reading 08.00 14.00 22.00
Final Reading 32.00 37.80 46.20
Volume of EDTA used 24.00 23.80 24.20
Average Volume of EDTA used 24.00

Analysis and Question:
1. What is the function of buffer solution when titrating with EDTA?
The buffer solution can resist any change in pH by prevent any change in amount of concentration of hydronium ion.

2. Calculate the exact concentration of EDTA solution used in the experiment in molar unit.
(All of the calculating must be shown)

3. Calculate the concentration of zinc solution in gdm-3 unit.
(All of the calculating must be shown)

4. Give other examples of indicator that can be used in titration with EDTA.
Murexide, Fast Sulphon Black, Eriochrome Red B.

5. Why magnesium complex must be added to the calcium solution before titrating with EDTA is made? Explain.
The magnesium complex, Mg2+ is added in order to obtain a sharper colour change at the end point. The indicator used, Eriochrome Black T exists as a brick-red complex when Mg2+ is added in the solution. When the EDTA has titrated with all the Mg2+ present in solution, the colour will changes to blue.

Discussion:
1. Titration must be done slowly when reach the endpoint to avoid getting a wrong final reading of EDTA.
2. The indicator used, Eriochrome Black T, is easy to be oxidized. Ensure the container containing the indicator must be secure to avoid a very indistinct endpoint.
3. Always discard a solution that wrongly prepared to avoid error in reading.
4. Ensure the entire safety label must be read and all safety measure must be taken before doing experiment.

Conclusion:
The concentration zinc solution can be determined by titrating the Zn2+ solution with a standard EDTA solution.

References:
1. D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical Chemistry: An Introduction, 7th ed. Chapter 15, pp. 345-381.
2. http://www.chem.uky.edu/courses/che226/labs/complexometric_zinc.html
3. http://www.newworldencyclopedia.org/entry/Buffer_solution
4. http://wiki.answers.com/Q/Why_buffer_solution_must_be_used_for_titration_with_EDTA#ixzz1ZDXjP0KJ
5. http://web.pdx.edu/~atkinsdb/teach/321/EDTA.htm
6. http://www.scribd.com/doc/47084424/Experiment-Complexometric-Titration

Labels: chemistry, experiment