EXPERIMENT 2

TITLE : COMPLEXOMETRIC TITRATION

OBJECTIVES OF THE EXPERIMENT :

After completing the experiment, we are able to:

1. Define a complexometric titration.

2. Describe the reaction between a metal cation and EDTA.

3. Calculate the concentration the unknown solution given the titration data.

INTRODUCTION:

Many metal ions react with electron pair donors to form coordination compounds or

complex ions. The formation of a particular class of coordination compounds, called chelates, are

especially well suited for quantitative methods. A chelate is formed when a metal ion coordinates

with two (or more) donor groups of a single ligand. Tertiary amine compounds such as

ethylenadiaminetetraacetic acid (EDTA) are widely used for the formation of chelates.

EDTA solution is not only stable - it can be stored for months - but it can be also

prepared without a need of standardization. First, EDTA can be obtained in the form pure

enough. Second, after thorough drying its crystallic from has pretty well defined amount of water

of crystallization. Most commonly used solutions are 0.01M (that is 0.01N - regardless of the

fact that EDTA has four protons it always reacts with metal cations on a 1:1 base). However,

depending on the needs (concentration of metal to be determined) it is possible to prepare and

use EDTA solutions of concentrations ranging from 0.1M to 0.001M. Crystallic EDTA - in the

form of either disodium EDTA dihydrate or anhydrous disodium EDTA salt - has to be

thoroughly dried out before solutions preparation. Also note, that solution preparation is time

consuming - EDTA dissolves in water very slowly.

Complexometric titrations with EDTA have been reported for the analysis of nearly all metal

ions. Because EDTA has four acidic protons, the formation of metal-ion/EDTA complexes is

dependent upon the pH. For the titration of Zn2+, one must buffer the solution to a pH of 10 so

that complex formation will be quantitative. The reaction of Zn2+ with EDTA may be expressed

as: Zn2+ + (EDTA)4- → [Zn(EDTA)]2-

This reaction can be used to determine the concentration of zinc ion by using an EDTA

standard solution and an indicator which can form a colored complex with Zn2+. The following

two factors are important for determination of the concentration:

i) The indicator used must form a complex with zinc (much more slowly) compared to the

EDTA. Zn2+ indicator complex should only form after all the EDTA has reacted with the Zn2+

cation.

ii) Zn2+ indicator complex must have a different colour from the indicator. “Eriochrome Black

T” (EBT) indicator, fulfill all the requirements mentioned earlier and can be used for this

purpose. This method of determination is the simplest way to determine the concentration of the

metal because 1 mol of EDTA usually reacts with 1 mol of metal.

METHODOLOGY:

(A) Preparation of EDTA 0.1M solution.

1. 9.5g of disodium EDTA (dehydrate) salt was weighed approximately and it was

transferred into a 250cm3 beaker.

2. It was dissolved with distilled water and diluted until the mark. It was stirred until

homogenous.

(B) Standardization of EDTA solution.

1. The burette was rinsed with small amount of the dilute EDTA solution it was drained,

then the burette was filled to the mark.

2. 25.00cm3 of the calcium carbonate solution given was pipette into 250cm3 Erlenmeyer

flask.

3. It was diluted to about 100cm3 with distilled water.

4. 2cm3 of magnesium complex solution, 5cm3 of buffer solution and 1-2 drops of (EBT)

indicator was added.

5. It was titrated with EDTA solution until the dark colour turned blue.

6. The titration was repeated three times and the average concentration of the EDTA

solution was determined.

(C) The determination of Zn solution concentration.

1. 25cm3 of Zn2+ solution given was pipette into a 250cm3 Erlenmeyer flask.

2. It was diluted with distilled water to approximately 100cm3.

3. 10cm3 of buffer solution and 1 drop of indicator was added.

4. It was titrated slowly and carefully with standard EDTA until the end point (dark red

colour turned blue).

5. The titration was repeated two or three times. The concentration of Zn2+ in g/dm3 was

calculated.

RESULT:

(A)Preparation of EDTA solution

Weight of EDTA salt: 9.5002g

(B) Standardization of EDTA solution

Experiment 1 2 3

Initial burette readings (cm3)

0.00 0.00 0.00

Final burette readings (cm3)

25.30 25.30 25.30

Volume of EDTA used (cm3)

25.30 25.30 25.30

Average Volume of EDTA used: 25.30cm3 = 0.0253dm3

(C) The determination of Zn solution concentration

Experiment 1 2 3

Initial burette readings (cm3)

0.00 0.00 0.00

Final burette readings (cm3)

27.40 26.30 26.30

Volume of EDTA used (cm3)

27.40 26.30 26.30

Average volume of EDTA used: 26.67cm3 = 0.0267dm3

CALCULATION:

(A)Preparation of EDTA solution

1. Formula of EDTA: C10H16N2O8

Molar mass = (12.0107×10) + (1.00794×16) + (14.0067×2) + ( 15.9994×8) = 292.24g/mol

2. No. of mole of EDTA: mass

molar mass =

9.5002 g292.24 g/mol

= 0.0325 mol

3. Molarity of EDTA: nv

= 0.0325 mol0.25 dm 3

= 0.130 M

(B) Standardization of EDTA solution

1. Concentration of Ca2+ solution: Let a=EDTA, b=Ca2+

MaVa=MbVb

(0.130M)(0.0253dm3) = Mb(0.10dm3)

Mb = 0.0329 M

2. Amount of Ca2+ ion present in the Erlenmeyer flask:

n = MV/1000

= 0.0329 M ×100 cm3

1000 = 0.00329mol

3. Concentration of the EDTA solution:

MEDTA = nv

= 0.0325mol0.0253dm3

= 1.285 M

(C) The determination of Zn solution concentration:

1. Equation of the reaction: Zn2+ + (EDTA)4- → [Zn(EDTA)]2-

1 mol Zn2+ reacts with 1 mol of EDTA

2. Concentration of Zn2+:

Let a=EDTA b=Zn2+, MaVa=MbVb

Mb=(1.285M)(0.0267dm3) / 0.1dm3

= 0.343M

Concentration of Zn2+ in g/dm3= 0.343 M × 65.409 g/mol = 22.44 g/dm3

3. No. of mole of Zn2+ = 0.343M × 0.1 dm3= 0.0343mol

1 mol of Zn2+ reacted with 1 mol of EDTA

0.0343mol of Zn2+ reacted with 0.0343 mol of EDTA

4. Concentration of EDTA that used: 0.0343 mol0.0267 dm3

= 1.285M

DISCUSSION:

This complexometric titration is an example of classic titrimetry analysis. Classical

methods of analysis such as titrimetric and gravimetric analyses are usually capable of very high

precision and accuracy – typically on the order of +0.1% or even better if done properly.

However, there is always a tradeoff. Usually classical methods are slower and much less

sensitive than modern instrumental methods of analysis such as atomic absorption spectroscopy,

gas and liquid chromatography, and mass spectrometry.

In this experiment, we first have to find the unknown concentration of the calcium

solution from the standardization by EDTA. We used at first 9.5002g of EDTA salt to give

0.13M of EDTA solution before diluted. After Part B is conducted, the concentration of calcium

solution obtained is 0.0329 M and its amount calculated is 0.00329mol.

After the addition of magnesium complex solution, buffer solution and 1-2 drops of

(EBT)indicator into the standard solution, the colour changed to pink. A little of magnesium

complex solution added to the calcium solution before the titration to initiate the rate of reaction

between EDTA and the metal solution.

After all, the average EDTA solution used to turn the dark colour to blue (end-point) in

Part B is 25.30cm3 with 0.000 deviation between its three consecutive repetition of the titration.

The concentration of EDTA that determined in Part B is 1.285M.

Meanwhile, in Part C, we want to determine the concentration of zinc solution using the

concentration of EDTA obtained previously. In this part, the average volume of EDTA used is

26.67cm3 with its deviation less than 3 parts per thousand which meant it is high accuracy

titration. The concentration of zinc solution is 0.343M in mol/dm3 and 22.44 g/dm3 in g/dm3

unit. After that, we used the concentration of zinc to calculate again the concentration of EDTA

used in this experiment, and we got the same value as before, which is 1.285M.

PRECAUTIONARY STEPS:

1. Usually an air bubble is present in the nozzle of the burette,it must be removed before

taking the initial reading.

2. Always read lower meniscus in case of colourless solution and upper meniscus in case of

coloured solutions.

3. Immediately add the buffer solution before titrating a sample to prevent the pH from

changing.

4. Rinse all the apparatus that want to be used with the distilled water followed by

respective chemicals that to be filled in.

CONCLUSION:

1. Complexometric titration is the titration that involved it titrant, a ligand, to

reacts with the analyte, a metal ion, to form a complex, more specifically a chelate in this case.

A chelate is a ligand that has two or more sites that bind to the central ion.

2. EDTA react with almost all metal cation. For this experiment, it shows that EDTA react with

calcium and zinc metal.

3. From the calculation, the concentration of the unknown calcium solution is 0.0329M and the

concentration of the EDTA used is 1.285 M.

QUESTIONS:

1. Buffer solution must be used in titrations involving EDTA because we need to constant the pH

value of the metal solution around pH10 so that the complex formation will be quantitative.

2. Other indicator that can be used in EDTA titration are EGTA and CDTA.

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. Vogel's Textbook of Qualitative Inorganic Analysis by Vogel, A.I., 3rd, Ed., Longman (1961)

444, 445. 

3.Gary D.Christian: Analytical Chemistry Sixth Edition, by John Wiley and Sons Inc.