Preparation and Standardization of Disodium EDTA

Preparation and Standardization of Disodium EDTA: Disodium EDTA (Disodium ethylenediaminetetraacetate) is an important complexing agent used in complexometric titrations for the quantitative estimation of metal ions such as Ca²⁺, Mg²⁺, Zn²⁺, Cu²⁺ and Pb²⁺.

Its commonly used form is:

Disodium EDTA dihydrate
Formula: Na₂H₂Y·2H₂O
Molecular weight: 372.24 g/mol

2. Preparation of 0.05 M Disodium EDTA Solution

Materials required

• Disodium EDTA dihydrate
• Distilled/deionized water
• Analytical balance
• Beaker
• Volumetric flask
• Glass rod
• Funnel

Calculation

For preparation of 1 L of 0.05 M solution:

Weight = Molarity × Molecular weight × Volume

= 0.05 × 372.24 × 1

= 18.612 g

Therefore, 18.61 g of disodium EDTA dihydrate is required to prepare 1 L of 0.05 M solution.

Procedure

1. Accurately weigh 18.61 g of disodium EDTA dihydrate.
2. Transfer it into a beaker.
3. Add a suitable quantity of distilled water.
4. Stir until the EDTA dissolves completely.
5. Transfer the solution quantitatively into a 1000 mL volumetric flask.
6. Wash the beaker several times with distilled water and transfer the washings into the flask.
7. Make up the volume to the mark with distilled water.
8. Mix thoroughly.
9. Store the prepared solution in a suitable, tightly closed container.

Important: The prepared EDTA solution should be standardized before accurate analytical use.

3. Standardization of Disodium EDTA Solution

Principle

Disodium EDTA is standardized using a standard solution of a suitable metal ion, commonly calcium carbonate/calcium solution.

A known amount of calcium is titrated with the prepared EDTA solution.

The reaction is:

Ca²⁺ + EDTA⁴⁻ → Ca − EDTA

The reaction occurs in a 1:1 molar ratio.

Indicator

Eriochrome Black T (EBT) can be used for calcium/magnesium titrations under suitable conditions. For calcium-specific standardization, murexide is commonly used.

Standardization using Calcium Carbonate

Procedure

1. Accurately weigh a known amount of calcium carbonate (CaCO₃).
2. Dissolve it carefully in dilute hydrochloric acid.
3. Transfer the solution to a volumetric flask and make up to a known volume with distilled water.
4. Pipette a known volume of the calcium solution into a conical flask.
5. Add the appropriate buffer solution to maintain the required pH.
6. Add a suitable metal ion indicator, such as murexide.
7. Titrate with the prepared disodium EDTA solution.
8. Continue titration until the characteristic indicator colour changes, indicating the endpoint.
9. Repeat the titration until concordant readings are obtained.

4. Calculation of Strength

Because EDTA reacts with calcium in a 1:1 ratio:

M_EDTA​ V_EDTA ​= M_Ca​V_Ca​

Therefore,

Where:

• M_EDTA = molarity of EDTA
• V_EDTA = volume of EDTA consumed
• M_Ca = molarity of calcium solution
• V_Ca = volume of calcium solution taken

Strength of EDTA

Strength = Molarity × Molecular weight

For disodium EDTA dihydrate:

Strength = M × 372.24 g/L

5. Important Points for Exams

• Disodium EDTA is a commonly used complexometric titrant.
• Formula of disodium EDTA dihydrate: Na₂H₂Y·2H₂O
• Molecular weight: 372.24 g/mol
• 18.61 g/L gives approximately 0.05 M solution.
• EDTA forms 1:1 complexes with most metal ions.
• EDTA solution is standardized before use.
• Calcium salts are commonly used for standardization.
• Murexide is commonly used for calcium-specific titration.
• EBT is widely used for Ca²⁺/Mg²⁺ complexometric titrations.
• Buffer pH is important because the stability of metal–EDTA complexes depends strongly on pH.

Editorial Note

This article has been carefully researched and written by Deepak Rajput with a focus on accuracy, clarity, and evidence-based healthcare information. Pharmaacademias.com