Standardization of EDTA ~ A lab report

Title: Standardization of EDTA (ethylenediaminetetraacetic acid)

Objective: To standardize EDTA (ethylenediaminetetraacetic acid) solution with calcium nitrate.

Introduction: The concentration of a solution of EDTA was determines by standardising against a solution of Ca²⁺ prepared from the primary standard of Ca(NO₃)₂. EDTA is a versatile titrant that can be used for the analysis of virtually all metal ions. Although EDTA is the most commonly employed titrant for the complexation titrations involving metal ions, it cannot be used for direct analysis of anions or neutral ligands. Solutions of EDTA are prepared from the soluble disodium salt, Na₂H₂Y.2H₂O. Concentrations can be determined directly from the known mass of EDTA, however, for more accurate work, standardisation is accomplished by titrating against a solution made from the primary standard Ca(NO₃)₂.

The formation of a metal-EDTA is illustrated as follows;

Ca²⁺ (aq) + Y^4-(aq) ® CaY^2-(aq)

Where Y^4- is the shorthand notation for the chemical form of EDTA.

Apparatus and materials: calcium nitrate, 0.02M EDTA solution at pH 10, primary standard, NH₃-NH₄Cl buffer solution

Procedure:

1.        25cm³ of calcium nitrate solution and 2cm³ NH₃-NH₄Cl buffer solution in a conical flask was titrated against EDTA solution.
    2.   The volume of EDTA solution required to turn the red solution to blue was recorded.
    3.   The experiment was repeated.

Results:

Titration Number	Rough	1	2	3
Initial volume of burette (cm3)	3.0	9.0	14.0	9.0
Final volume of burette (cm3)	7.6	13.6	18.6	13.6
Titre volume (cm3)	4.6	4.6	4.6	4.6

 Discussion 
Calcium metal ion reacts with electron-pair donors to form coordination compounds.  The electron-pair donor is more generally called the ligand.  The ligand must have at least one pair of unshared electrons available for bond formation.  The type of bond formed is termed a covalent bond.  The number of covalent bonds a metal ion forms with the ligand is called its coordination number.  The resulting compound, or complex, formed between the calcium ion and EDTA may be electrically positive, negative or neutral.  Particularly stable complexes are formed when the ligand can form more than one covalent bond with the metal ion.  A class of such ligands are the so called chelating agents.  A complex is produced when a metal ion coordinates with two (or more) donor groups of a single ligand to form a five or six membered hetereocyclic ring with the metal ion.  A ligand that has a single electron-donor group is called a unidentate ligand, bidentate (2 electron-donor groups), tridentate (3 electron-donor groups) etc., chelating agent.

Aminopolycarboxylic acids are a group of compounds which form very stable chelates with many metal ions. Ethylenediaminetetra-acetic acid (EDTA) is one such aminopoly- carboxylic acid and is widely used to determine metal ions.  The structure of EDTA is shown in Figure 1 below:

A number of indicators are available for use in EDTA titration the most common is Eriochrome Black T.  Eriochrome Black T undergoes the following color transition at the equivalence point.

wine-red ® pure blue

Conclusion
EDTA (ethylenediaminetetraacetic acid) solution can be standardized with calcium   nitrate.

References: 
http://faculty.ccri.edu/aahughes/GenChemII/Lab%20Experiments/Calcium_Analysis_EDTA_Titration.pdf
Silberberg, M.S., Chemistry: The molecular nature of Matter and Change (5th edition, McGraw-Hill 2009)

Questions:

         Calculate the molarity of EDTA solution.

Molarity of Ca(NO₃)₂ = 0.4/164.08  ÷  500/1000
                                     = 4.876 × 10^-3

M₁V₁=M₂V₂

(4.876 ×10^-3×25)/1000=  (M2 ×4.6)/1000

M₂ = 0.0265 mol dm^-3

     Draw the complex structure of Ca²⁺-EDTA.