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DrX Whiz Niraj
Complexometric Titration: EDTA & Masking
Master the concepts of Chelometry! Complete notes on Metal Ion Indicators, Masking/Demasking Reagents, and Assays of Magnesium Sulphate & Calcium Gluconate.
Complexometric Titration (or Chelometry) is a form of volumetric analysis where the formation of a colored complex is used to indicate the end point of a titration. It is widely used for the determination of metal ions (like Ca²⁺, Mg²⁺, Zn²⁺) in a solution.
The Role of Ligands (EDTA):
A Ligand is an ion or molecule that donates a pair of electrons to a central metal atom to form a complex. The most widely used titrant in complexometry is Disodium EDTA (Ethylenediaminetetraacetic acid).
EDTA is a Hexadentate Ligand (It has 6 binding sites: 4 Oxygen atoms and 2 Nitrogen atoms). It wraps around the metal ion like a claw to form a highly stable, water-soluble complex called a Chelate.
1. Direct Titration
The simplest method. The metal ion solution is directly titrated against standard EDTA using a suitable indicator. (e.g., Assay of Magnesium Sulphate).
2. Back Titration
Used when the metal precipitates in absence of EDTA, or reacts too slowly. Excess EDTA is added, and the unreacted EDTA is back-titrated with a standard metal solution (like ZnSO₄).
3. Replacement Titration
Used for metal ions that do not have a suitable indicator. The metal is treated with a Magnesium-EDTA complex. The metal replaces Mg²⁺, and the freed Mg²⁺ is then titrated with standard EDTA.
4. Indirect Titration
Used for determining anions (like SO₄²⁻). The anion is precipitated with an excess of a metal ion (e.g., Barium). The excess unreacted Barium is then titrated with EDTA.
A Metallochromic Indicator (pM indicator) is an organic dye that forms a colored complex with the metal ion.
Condition: The Metal-Indicator complex must be less stable than the Metal-EDTA complex, so that EDTA can snatch the metal from the indicator at the end point.
| Indicator Name | Color with Metal (Start) | Color without Metal (End Point) | Used for metals |
|---|---|---|---|
| Eriochrome Black T (EBT) / Mordant Black II | Wine Red | Blue | Mg²⁺, Zn²⁺, Pb²⁺ (At pH 10) |
| Murexide (Ammonium purpurate) | Pink | Purple | Ca²⁺, Co²⁺, Ni²⁺ |
| Xylenol Orange | Red | Yellow | Bi³⁺, Th⁴⁺ |
Sometimes a sample contains a mixture of multiple metals (e.g., Zinc and Magnesium). If we titrate it, EDTA will react with both. To determine them individually, we use Masking Agents.
- Masking Agent: A reagent that reacts with an interfering metal ion and forms a stable complex, preventing it from reacting with EDTA. (Example: Potassium Cyanide masks Zinc, Copper, and Cobalt).
- Demasking Agent: A reagent that releases the masked metal ion back into the solution so it can be titrated later. (Example: Formaldehyde/Chloral Hydrate demasks Zinc from the Cyanide complex).
1. Assay of Magnesium Sulphate
It is estimated by Direct Complexometric Titration.
- Titrant: 0.05 M Disodium EDTA.
- Buffer: Ammonia-Ammonium Chloride buffer (to maintain pH 10).
- Indicator: Mordant Black II (Eriochrome Black T).
- End Point: Color changes from Wine Red to Pure Blue.
2. Assay of Calcium Gluconate
It is estimated by Replacement Titration (or direct, depending on pharmacopoeia).
- Titrant: 0.05 M Disodium EDTA.
- Additives: Magnesium sulphate is added. Calcium replaces Magnesium from the Mg-EDTA complex.
- Indicator: Mordant Black II (EBT).
- End Point: Color changes to Blue.
Practical Alert: Why use Ammonia Buffer?
EDTA titrations are highly pH-dependent. At lower pH levels, the H+ ions compete with metal ions for the EDTA binding sites. For effective chelation of Magnesium and Calcium, the solution must be strongly basic (pH 10). The Ammonia-Ammonium chloride buffer ensures the pH stays at exactly 10 throughout the titration.