Introduction
Complexometric titration is a type of volumetric analysis in which a complex compound forms between the analyte and the titrant. The formation of this complex results in a measurable change, such as a color shift, allowing the endpoint to be detected visually or instrumentally.
Complexometric titrations are particularly used to determine concentrations of metal ions in solutions by forming coordination complexes with chelating agents such as EDTA.
Principle
Complexometric titration is based on the formation of a stable complex between metal ions and a chelating agent (ligand). The endpoint is indicated by a change in color due to complex formation or displacement of metal ions from a colored metal-indicator complex.
Instruments such as spectrophotometers, potentiometers, and amperometers can also be used for endpoint detection when visual indicators are not suitable.
Common Complexing Agents
1. EDTA (Ethylenediamine Tetraacetic Acid)
EDTA is a hexadentate ligand containing four carboxylic acid groups and two amine groups that donate six pairs of electrons to form coordinate covalent bonds with metal ions.
The disodium salt of EDTA (Na₂H₂Y·2H₂O) is commonly used in titrations and has a molecular weight of 372.24. It forms stable chelate complexes with most cationic metals.
2. DTPA (Diethylenetriamine Pentaacetic Acid)
DTPA is an extended version of EDTA, containing an additional carboxylic acid and amine group. It acts as an octadentate ligand and forms highly stable complexes with metal ions.
3. EGTA (Ethyleneglycol-bis(aminoethyl ether) Tetraacetic Acid)
EGTA selectively binds calcium ions over magnesium, making it useful in biological systems where calcium regulation is critical. It is hexadentate in nature with a molecular weight of 380.35.
4. Ethylenediamine (NH₂CH₂CH₂NH₂)
A bidentate ligand forming chelate complexes, especially with copper ions. It has a molecular weight of 60.10.
5. Ammonia (NH₃)
A monodentate ligand that forms complexes with transition metals such as copper. Its lone pair on nitrogen donates to metal ions to form coordinate bonds.
Masking and Demasking Agents
Since EDTA can form complexes with multiple metal ions, masking agents are used to block specific ions, while demasking agents are used to release them for subsequent titration.
Example:
If a solution contains three metal ions A, B, and C, all capable of reacting with EDTA, masking agents can be added to selectively block certain ions (e.g., mask B and C) so that only A is titrated. Then a demasking agent can free B for a second titration, and so on, allowing determination of individual metal ion concentrations.
Indicators
Indicators used in complexometric titrations are known as metal ion indicators or metallochromic indicators. These indicators form colored complexes with metal ions. When EDTA binds the metal ion, the indicator is displaced and changes color, signaling the endpoint.
Common Indicators:
1. Eriochrome Black T (EBT)
Also known as Solochrome Black T, it is a triprotic acid used in the form H₃In. It changes from wine red (metal complex) to blue (free indicator) at the endpoint.
2. Calmagite
Chemically similar to EBT, calmagite is more stable in aqueous solution and can serve as a replacement indicator. It changes color from red to blue in the presence of EDTA.
3. Murexide
The ammonium salt of purpuric acid. It forms colored complexes with metals like calcium and copper, showing different hues at varying pH levels.
4. Xylenol Orange
Used for titrating metals like calcium, magnesium, and lead. It changes from red to yellow as the endpoint is reached.
5. Calcon
Also known as Solochrome Dark Blue, it is useful at high pH (around 12). It changes from blue to red with increasing pH or complexation.
6. Catechol Violet
Also known as Pyrocatechol Violet, a tetraprotic acid that changes from yellow to blue during complex formation.
Note: The metal-indicator complex must be less stable than the metal-EDTA complex for a sharp and clear endpoint.
Instrumental Methods for Endpoint Detection
1. Spectrophotometric Detection
Based on measuring absorbance changes as the metal-ion complex forms or breaks. Suitable for colored complexes or when high precision is needed.
2. Amperometric Titration
Measures diffusion current changes as complexation proceeds. The endpoint is identified when all free metal ions are complexed with EDTA.
3. Potentiometric Titration
Monitors potential change due to complex formation. Commonly applied to Fe³⁺ and Cu²⁺ ions where redox potential shifts upon EDTA addition.
4. High-Frequency Titrator
Used for very dilute solutions (down to 0.0002 M). Measures changes in dielectric properties as metal ions are complexed or released.
Types of Complexometric Titrations
1. Direct Titration
EDTA solution is directly added to the metal ion solution until the color change indicates the endpoint.
Example: Determination of calcium in calcium lactate tablets.
2. Back Titration
An excess of EDTA is added to the metal ion solution, and the remaining unreacted EDTA is titrated with a standard metal solution.
Example: Determination of Mn²⁺ where direct titration is not feasible due to Mn(OH)₂ precipitation.
3. Replacement Titration
The analyte metal displaces another metal from its EDTA complex. The displaced metal is then titrated with EDTA.
Example: Mn²⁺ displacing Mg²⁺ from Mg-EDTA complex.
4. Indirect (Alkalimetric) Titration
Used for ions that do not react directly with EDTA. The analyte forms a secondary complex with a metal ion, which is then titrated with EDTA.
Example: Determination of barbiturates through Hg-barbiturate complex titration.
Preparation and Standardization of 0.05 M Disodium Edetate Solution
Requirements:
- Disodium edetate (Na₂H₂Y·2H₂O)
- Mordant Black II indicator (Eriochrome Black T)
- Distilled water
- Volumetric flask, burette, pipette, conical flask
Preparation:
- Weigh accurately 18.612 g of disodium edetate dihydrate.
- Dissolve in 200 ml of distilled water.
- Transfer to a 1000 ml volumetric flask and make up to volume with water.
Preparation of Supporting Reagents:
- Bromine Water: Dissolve 3 ml of bromine in 100 ml of distilled water.
- 2 N NaOH: Dissolve 8 g of NaOH in 100 ml of water.
- Ammonia Buffer (pH 10): Dissolve 5.4 g of NH₄Cl in 70 ml of 5N ammonia and dilute to 100 ml.
Experimental Method:
- Dissolve ~800 mg of zinc granules in 12 ml of dilute HCl and warm gently.
- Add a few drops of bromine water, boil, and then cool the solution.
- Dilute the solution to 200 ml with distilled water.
- Take 20 ml of this solution, neutralize with 2N NaOH, and adjust pH using pH paper.
- Add 5 ml ammonia buffer (pH 10) and 3–4 drops of Mordant Black II indicator.
- Titrate with 0.05 M disodium edetate until the color changes from blue-violet to light green.
Result:
The disodium edetate solution was standardized using standard zinc solution. Molarity of the prepared EDTA solution was found to be x M.
Detailed Notes:
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