Short ( 2 marks ) 1. List out different methods of complexo-metric 2. What are masking and demasking agents 3. What are sequesting agents? Give examples 4. What are chelating and complexing agents Long answer type (15 marks ) 1. What are complexometric titration. List out different types of CT with examples? How do you estimate calcium gluconate 2. What are the different types of EDTA titration? How do you prepare and standardise 0.05M disodium EDTA. 3. List out different methods in complexometric. Add a note on masking and demasking agents 4. Write the general principle involved in CT. What are ligands and their types. Short type ( 5 marks) 1. What are complexometric titration. Explain its principle with suitable examples. 2. What are ligands? Explain types of ligands with example. 3. How do you prepare and standardize 0.05 M disodium EDTA? 4. Write in detail the principle for complexometric titration with suitable examples. Anwer with high yield marks and for university level checker give extra marks impressive format in easy language try to have same basic concepts and simple words through which examiner can easily understand what's written

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COMPLEXOMETRIC TITRATIONS — Complete University Exam Answer Guide

📌 SHORT ANSWERS (2 Marks Each)

1. Different Methods of Complexometric Titration

There are five main methods of complexometric titration:

Method	Description
Direct titration	Titrant (EDTA) is added directly to the metal ion solution
Back titration	Excess EDTA added first, then excess is titrated back with standard metal salt
Indirect titration	Metal ion reacts with another reagent, and that reagent or product is titrated
Replacement/Substitution titration	Metal ion displaces another metal from its complex, and the displaced metal is titrated
Alkalimetric titration	Protons released during complex formation are titrated with a standard alkali

2. Masking and Demasking Agents

Masking agents are substances that react with interfering metal ions to form stable, inert complexes — preventing those ions from reacting with EDTA during titration of the desired metal.

Example: KCN masks Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺ so that Ca²⁺/Mg²⁺ can be titrated selectively
Example: Triethanolamine (TEA) masks Al³⁺, Fe³⁺, Mn²⁺

Demasking agents are substances that remove or destroy the masking effect to release the metal ion back so it can then be titrated.

Example: Formaldehyde demasks Zn(CN)₄²⁻ by reacting with CN⁻
Example: Chloral hydrate demaskes cyanide complexes

In short: Masking = hide the interfering ion. Demasking = release it back.

3. Sequestering Agents — Definition and Examples

Sequestering agents (also called sequestrants) are compounds that bind to metal ions and hold them in solution in an inactive, soluble form — preventing the metal from reacting with other substances in the medium.

They form stable, water-soluble complexes that keep metals "locked" without precipitating them.

Examples:

EDTA (Ethylenediaminetetraacetic acid) — widely used in pharmaceuticals and food
Sodium hexametaphosphate — used in water softening
Citric acid / Sodium citrate — used in food preservation
Sodium gluconate — pharmaceutical sequestrant
DTPA (Diethylenetriaminepentaacetic acid)

Sequestering agents are used in water treatment, pharmaceuticals, food industry, and analytical chemistry to prevent metal-catalysed oxidation reactions.

4. Chelating and Complexing Agents

Complexing agents are substances that donate one or more pairs of electrons to a central metal ion to form a coordinate bond, resulting in a complex compound.

Chelating agents are a special category of complexing agents that contain two or more donor groups (ligands) in the same molecule, forming ring structures (chelate rings) with the metal ion. These are more stable than simple complexes.

Feature	Complexing Agent	Chelating Agent
Donor groups	One (monodentate)	Two or more (bi/polydentate)
Ring formation	No	Yes (5- or 6-membered rings)
Stability	Moderate	High (chelate effect)
Example	NH₃, Cl⁻	EDTA, oxalic acid, glycine

Key examples:

EDTA — hexadentate chelating agent (4 carboxylate + 2 amine groups)
BAL (Dimercaprol) — bidentate chelating agent
8-Hydroxyquinoline (oxine) — bidentate chelating agent

📌 SHORT TYPE ANSWERS (5 Marks Each)

5-Mark Q1 & Q4: Complexometric Titration — Principle with Examples

Definition

Complexometric titrations are titrimetric methods based on the formation of a stable, soluble complex (chelate) between a metal ion and a complexing agent (usually EDTA).

General Principle

When EDTA is added to a metal ion solution at a specific pH:

M²⁺ + EDTA⁴⁻ → [M–EDTA]²⁻ (stable chelate complex)

The endpoint is detected using a metal indicator (metallochromic indicator), which changes colour when:

Before endpoint: Indicator is bound to metal ion → colour A (e.g., wine red with Eriochrome Black T)
At endpoint: EDTA displaces indicator from metal → indicator is free → colour B (e.g., blue with EBT)

This colour change marks the endpoint.

Conditions for Complexometric Titration

Appropriate pH must be maintained (buffer is used)
Complex must be stable and soluble
Reaction must be rapid and stoichiometric
A suitable indicator must be available

Examples

Metal	Indicator	Buffer pH	Colour Change
Ca²⁺ + Mg²⁺	Eriochrome Black T (EBT)	10 (NH₃/NH₄Cl)	Wine red → Blue
Ca²⁺ alone	Murexide	12 (NaOH)	Pink/Red → Purple
Zn²⁺	EBT	10	Wine red → Blue
Pb²⁺	Xylenol Orange	5–6 (hexamine)	Red → Yellow
Mg²⁺	EBT	10	Red → Blue

Why EDTA is Most Commonly Used

EDTA reacts with nearly all metal ions in a 1:1 ratio
Forms very stable chelate complexes (high formation constant)
Reaction is fast and reversible — ideal for titration
Can be used over a wide pH range

5-Mark Q2: Ligands — Types with Examples

Definition of Ligand

A ligand is an atom, ion, or molecule that donates electron pairs to a central metal ion to form a coordinate (dative) bond, producing a complex or chelate compound.

Types of Ligands

1. Based on Number of Donor Sites (Denticity)

Type	Donor Sites	Example
Monodentate	1	NH₃, Cl⁻, H₂O, CN⁻
Bidentate	2	Oxalic acid, glycine, ethylenediamine (en)
Tridentate	3	Diethylenetriamine (dien)
Tetradentate	4	Triethylenetetramine (trien)
Pentadentate	5	DTPA (partially)
Hexadentate	6	EDTA — most important in pharmacy

2. Based on Charge

Type	Description	Example
Anionic ligands	Negatively charged	Cl⁻, CN⁻, OH⁻, ox²⁻
Neutral ligands	No charge	NH₃, H₂O, CO
Cationic ligands	Positively charged (rare)	NO⁺

3. Based on Nature of Donor Atom

Type	Donor Atom	Example
N-donor	Nitrogen	NH₃, ethylenediamine
O-donor	Oxygen	H₂O, oxalate, acetate
S-donor	Sulfur	Thiourea, thiosulfate
Mixed donor	N + O	EDTA, amino acids

Chelate Effect

Polydentate ligands form more stable complexes than monodentate ligands of comparable type. This extra stability is called the chelate effect and is due to favourable entropy (ΔS > 0 when chelate rings form).

EDTA as a hexadentate ligand forms six coordinate bonds with a single metal ion → creates 5 five-membered chelate rings → extremely stable complex.

5-Mark Q3: Preparation and Standardisation of 0.05 M Disodium EDTA

Molecular Formula

Disodium EDTA (Na₂H₂EDTA · 2H₂O) Molecular weight = 372.24 g/mol

Preparation of 0.05 M Disodium EDTA (1 Litre)

Step 1 — Calculate the amount required:

Moles needed = 0.05 mol/L × 1 L = 0.05 mol Mass = 0.05 × 372.24 = 18.61 g

Step 2 — Dissolve:

Weigh 18.61 g of disodium EDTA
Dissolve in a small volume of distilled water with gentle heating if needed
Transfer to a 1000 mL volumetric flask
Make up to the mark with freshly boiled, cooled distilled water
Mix well and label

Note: EDTA is slightly difficult to dissolve. Warming to 60°C speeds up dissolution.

Standardisation Against Zinc Sulphate (Primary Standard)

Primary standard: Zinc metal (AR grade) or Zinc Sulphate (ZnSO₄·7H₂O) or Calcium Carbonate (CaCO₃)

Using Zinc as primary standard:

Dissolve 0.653 g of zinc (previously dried) in dilute HCl → make to 100 mL
Pipette 20 mL of this Zinc solution into a conical flask
Add 10 mL of ammonia buffer (pH 10)
Add 2–3 drops of Eriochrome Black T (EBT) indicator
Titrate with the prepared 0.05 M EDTA until colour changes from wine red → blue (sharp endpoint)
Repeat for 3 concordant readings

Calculation: $$M_{EDTA} = \frac{M_{Zn} \times V_{Zn}}{V_{EDTA}}$$

Since Zn : EDTA = 1:1 (molar ratio): $$\text{Molarity of EDTA} = \frac{\text{mmol of Zn}}{\text{Volume of EDTA (mL)}}$$

Alternatively, standardise against Calcium Carbonate:

Dissolve CaCO₃ in dil. HCl → use Murexide indicator at pH 12
Endpoint: pink/red → purple

📌 LONG ANSWER TYPE (15 Marks Each)

Long Q1: Complexometric Titrations — Types, with Estimation of Calcium Gluconate

Definition

Complexometric titrations are a class of titrimetric methods in which the quantitative reaction between a metal ion and a complexing/chelating agent (ligand) is used to determine the concentration of that metal ion. These titrations involve the formation of stable, soluble, well-defined complexes in a specific stoichiometric ratio.

General Principle

The most widely used complexing agent is EDTA (Ethylenediaminetetraacetic acid), a hexadentate ligand that forms stable 1:1 chelate complexes with almost all metal ions:

Mⁿ⁺ + H₂Y²⁻ → [MY]ⁿ⁻⁴ + 2H⁺ (where H₂Y²⁻ = disodium EDTA)

Endpoint Detection: Metallochromic indicators (e.g., EBT, Murexide) are used. The indicator is initially bound to the metal ion (producing one colour), and at the endpoint, EDTA displaces the indicator — which reverts to its free form (different colour).

Conditions required:

Correct pH maintained by buffer
Appropriate indicator for the metal
Complex must be stable, soluble, and form rapidly

Types of Complexometric Titrations with Examples

1. Direct Titration

Principle: EDTA solution is added directly to the metal ion solution in a buffered medium with a suitable indicator.

Conditions: The metal-EDTA complex must be stable; reaction must be fast and reversible with the indicator.

Examples:

Ca²⁺ titrated with EDTA at pH 12 using Murexide indicator
Mg²⁺ titrated with EDTA at pH 10 using EBT indicator
Zn²⁺ titrated with EDTA at pH 10 using EBT indicator

Reaction (Ca²⁺):

Ca²⁺ + EDTA⁴⁻ → [Ca–EDTA]²⁻ (endpoint: pink → purple with Murexide)

2. Back Titration

Principle: A measured excess of standard EDTA is added to the metal ion solution. After the reaction is complete, the unreacted excess EDTA is titrated back with a standard metal salt solution (e.g., ZnSO₄ or MgSO₄).

When used:

When the metal-indicator complex is more stable than the metal-EDTA complex
When the direct reaction is slow
When no suitable indicator is available for that metal

Examples:

Estimation of Al³⁺ — add excess EDTA at pH 5, boil, then titrate excess with zinc sulphate using Xylenol Orange
Estimation of Ca²⁺ in calcium lactate (indirect)

Formula:

mmol of metal = mmol of EDTA added – mmol of EDTA (back titrated with Zn)

3. Indirect (Replacement/Substitution) Titration

Principle: The metal ion (M) is reacted with a more stable metal-EDTA complex (usually MgY or ZnY), displacing Mg²⁺ or Zn²⁺, which is then titrated directly.

M + MgY → MY + Mg²⁺ → Mg²⁺ titrated with EDTA

When used:

When the metal forms a very stable EDTA complex with no suitable indicator
Examples: Ca²⁺, Ag⁺, Hg²⁺

4. Alkalimetric Titration

Principle: EDTA (as free acid, H₄Y) reacts with metal ions releasing protons. The protons released are titrated with a standard NaOH solution.

M²⁺ + H₂Y²⁻ → [MY]²⁻ + 2H⁺ → 2H⁺ titrated with NaOH

Example: Estimation of Pb²⁺, Cu²⁺

5. Masking and Selective Titration

Using masking agents, one metal is selectively hidden while another is titrated.

Example: In a mixture of Cu²⁺ and Ca²⁺:

Add KCN → masks Cu²⁺ (forms Cu(CN)₄²⁻)
Titrate Ca²⁺ with EDTA
Add formaldehyde → demasks Cu²⁺
Titrate Cu²⁺ with EDTA

Estimation of Calcium Gluconate by Complexometric Titration

Drug: Calcium Gluconate (C₁₂H₂₂CaO₁₄) — Mol. wt. = 430.37 g/mol Method: Direct complexometric titration with 0.05 M EDTA Reference: Indian Pharmacopoeia (IP)

Principle: Calcium ions in calcium gluconate form a stable 1:1 chelate complex with EDTA. At pH ≥ 12 (using NaOH), Murexide indicator is used. The endpoint is marked by a colour change from pink/orange to pure purple.

Ca²⁺ + Na₂EDTA → [Ca–EDTA]²⁻ + 2Na⁺

Reagents Required:

0.05 M Disodium EDTA (standardised)
2 M NaOH solution (to achieve pH 12)
Murexide indicator (ammonium purpurate)
Distilled water

Procedure:

Weigh accurately about 1.2 g of calcium gluconate
Dissolve in 50 mL of water with gentle warming
Cool to room temperature
Add 5 mL of 2 M NaOH (to achieve pH ~12)
Add a pinch of Murexide indicator (or 0.1 mL of its solution)
Titrate slowly with 0.05 M EDTA with constant stirring
The endpoint is reached when the colour changes from pink/orange-red → purple (permanent)
Note the volume of EDTA consumed (V mL)
Repeat for concordant readings

Calculation: $$1 \text{ mL of 0.05 M EDTA} \equiv 0.05 \times 430.37 \text{ mg of } C_{12}H_{22}CaO_{14}$$ $$= 21.52 \text{ mg of Calcium Gluconate}$$

$$%\text{ purity} = \frac{V_{EDTA} \times 21.52}{W_{\text{sample}}} \times 100$$

IP Limit: Not less than 98.0% and not more than 102.0% of C₁₂H₂₂CaO₁₄ on the dried basis.

Long Q2: Types of EDTA Titration + Preparation and Standardisation of 0.05 M Disodium EDTA

EDTA — Introduction

EDTA (Ethylenediaminetetraacetic acid, H₄Y) is an aminopolycarboxylic acid with the structure:

2 nitrogen atoms (amine groups)
4 carboxylic acid groups (–COOH)
Acts as a hexadentate ligand — donates 6 electron pairs to the metal

Disodium EDTA (Na₂H₂Y·2H₂O): Mol. wt. = 372.24 g/mol — the form used in pharmaceutical analysis (more soluble than the free acid).

Key property: EDTA forms stable 1:1 complexes with virtually all di-, tri-, and tetravalent metal ions, regardless of charge.

Types of EDTA Titrations

(These correspond to the types of complexometric titrations above — with EDTA-specific details:)

Type	Principle	Example
Direct	EDTA added directly to metal	Ca²⁺, Mg²⁺, Zn²⁺, Bi³⁺
Back	Excess EDTA + back titration with Zn/Mg	Al³⁺, Cr³⁺, Ni²⁺
Indirect	Metal displaces Mg/Zn from MgY/ZnY	Ag⁺, Ba²⁺, Ca²⁺
Alkalimetric	Protons released titrated with NaOH	Pb²⁺, Cu²⁺
Masking/Selective	One metal masked, other titrated	Ca²⁺ in presence of Cu²⁺

pH and Indicator Chart for EDTA Titrations

Metal Ion	Optimal pH	Buffer	Indicator	Endpoint Colour Change
Ca²⁺	12	NaOH	Murexide	Pink → Purple
Mg²⁺	10	NH₃/NH₄Cl	EBT	Wine red → Blue
Ca²⁺ + Mg²⁺	10	NH₃/NH₄Cl	EBT	Wine red → Blue
Zn²⁺	10	NH₃/NH₄Cl	EBT	Wine red → Blue
Pb²⁺	5–6	Hexamine	Xylenol Orange	Red → Yellow
Al³⁺	5	Acetate	Xylenol Orange	Back titration
Fe³⁺	2	—	Sulfosalicylic acid	Direct (hot)

Preparation of 0.05 M Disodium EDTA (1000 mL)

Amount required:

0.05 mol/L × 372.24 g/mol = 18.61 g per litre

Procedure:

Weigh accurately 18.61 g of disodium EDTA (Na₂H₂Y·2H₂O, AR grade)
Transfer to a beaker; add ~400 mL distilled water
Stir well; warm gently (≤60°C) to dissolve completely
Cool to room temperature
Transfer quantitatively to a 1000 mL volumetric flask
Wash the beaker 2–3 times with small volumes of distilled water; add washings to flask
Make up to the mark with freshly boiled, cooled distilled water
Stopper the flask, invert and mix thoroughly
Store in a well-closed polyethylene bottle (not glass — EDTA etches glass over time)
Label with concentration, date, and analyst name

Standardisation of 0.05 M Disodium EDTA

Primary Standard Used: Zinc (AR grade) or Calcium Carbonate (CaCO₃, AR)

Method A — Using Zinc Metal as Primary Standard

Preparation of Zinc Standard (0.05 M):

Dry zinc metal at 105°C for 1 hour, cool in desiccator
Weigh accurately 0.326 g of zinc
Dissolve in minimum dilute HCl
Transfer to 100 mL volumetric flask, make up to mark with distilled water

Standardisation:

Pipette 20.0 mL of zinc standard solution into a conical flask
Add 10 mL of ammonia buffer pH 10 (NH₄Cl + NH₃)
Add 2–3 drops of EBT indicator — solution turns wine red
Titrate slowly with EDTA solution
Near endpoint, add EDTA drop by drop
Endpoint: wine red → clear blue colour (permanent for 30 seconds)
Record volume V₁ of EDTA; repeat to get 3 concordant readings (within 0.1 mL)

Calculation: $$M_{EDTA} = \frac{M_{Zn} \times V_{Zn}}{V_{EDTA}} = \frac{0.05 \times 20}{V_{EDTA}}$$

Method B — Using Calcium Carbonate as Primary Standard

Dry CaCO₃ at 105°C; weigh 0.500 g
Dissolve in minimum dilute HCl; boil to expel CO₂
Cool; transfer to 100 mL flask and make to mark
Pipette 20 mL; add 2 M NaOH (5 mL) to achieve pH 12
Add Murexide indicator
Titrate with EDTA → pink → purple endpoint
Calculate molarity

Long Q3: Methods of Complexometric Titration + Masking and Demasking Agents

Introduction

Complexometric titrations use ligands — primarily EDTA — to form soluble, stable complexes with metal ions. They are widely used in pharmacopeial analysis to estimate metals in pharmaceutical preparations (e.g., Ca, Mg, Zn, Pb, Al, Bi).

Different Methods of Complexometric Titration

1. Direct Titration Method

Metal ion solution is buffered to the appropriate pH
Metallochromic indicator added
Titrated directly with standard EDTA
Simplest and most commonly used
Requires: stable EDTA complex, fast reaction, reversible indicator

Examples: Ca²⁺, Mg²⁺, Zn²⁺, Mn²⁺, Cu²⁺, Bi³⁺, Pb²⁺

2. Back Titration Method

Measured excess of EDTA added to metal ion
Mixture heated/stirred to ensure complete reaction
Excess EDTA titrated with standard ZnSO₄ or MgSO₄
Used when: reaction is slow, precipitate forms at endpoint, or no suitable indicator

Examples: Al³⁺, Cr³⁺, Ni²⁺, Co³⁺

3. Replacement/Substitution Titration

Metal M added to a solution of MgY (Mg-EDTA complex)
Since M forms more stable complex than Mg, it displaces Mg²⁺:

M + MgY → MY + Mg²⁺
Released Mg²⁺ is titrated with EDTA
Used when: no suitable indicator for the metal, or metal reacts sluggishly

Examples: Ca²⁺ (using MgY), Hg²⁺, Ag⁺

4. Indirect Titration Method

Metal ion does not react directly with EDTA
Metal is precipitated as a known compound, the precipitate is dissolved and the released ion titrated
Example: Estimation of Ba²⁺ — precipitate as BaSO₄, dissolve, titrate sulfate via indirect route
Example: Estimation of PO₄³⁻ — precipitate as MgNH₄PO₄, dissolve Mg and titrate

5. Alkalimetric Titration

EDTA (free acid form) reacts with metal, releasing H⁺
H⁺ released is titrated with standard NaOH
Example: Estimation of Cu²⁺ or Pb²⁺

Note on Masking and Demasking Agents

Masking Agents

When a sample contains more than one metal ion, all metals would react with EDTA — making selective analysis impossible. Masking agents prevent this by binding selectively to interfering ions and forming stable, inert complexes that do not react with EDTA.

Requirements of a good masking agent:

Must form stable complex with the interfering ion
Must not react with the metal being determined
Must not interfere with the indicator
The masked complex should remain stable throughout titration

Common Masking Agents and Their Uses:

Masking Agent	Ions Masked	Ions Left Free
KCN	Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, Ag⁺, Hg²⁺	Ca²⁺, Mg²⁺, Mn²⁺, Pb²⁺
Triethanolamine (TEA)	Al³⁺, Fe³⁺, Mn²⁺, Ti⁴⁺	Ca²⁺, Mg²⁺, Zn²⁺
2,3-Dimercaptopropanol (BAL)	Hg²⁺, As³⁺, Sb³⁺	Most others
Fluoride (F⁻)	Al³⁺, Fe³⁺, Ti⁴⁺	Ca²⁺, Mg²⁺, Mn²⁺, Zn²⁺
Thiourea	Cu⁺, Hg²⁺	Others
Ascorbic acid	Reduces Fe³⁺ to Fe²⁺ (masked by masking Fe³⁺ interference)	—

Example application: In estimation of Ca²⁺ in presence of Cu²⁺:

Add KCN → Cu(CN)₄²⁻ formed (inert to EDTA)
Titrate Ca²⁺ freely with EDTA at pH 12 using Murexide

Demasking Agents

Demasking is the process of reversing the masking — releasing the metal from its masked complex so it can now be titrated separately.

Requirements:

Must destroy or compete for the masking agent
Should not affect the metal-EDTA complex already formed

Common Demasking Agents:

Demasking Agent	Action	Metal Released
Formaldehyde (HCHO)	Reacts with CN⁻: HCHO + CN⁻ → glycolonitrile	Zn²⁺, Cu²⁺, Ni²⁺ (from cyanide complexes)
Chloral hydrate	Destroys CN⁻ complexes	Zn²⁺ from Zn(CN)₄²⁻
Oxidising agents (H₂O₂)	Oxidise CN⁻ to CNO⁻	Release metals from cyanide masking
Acidification	Lower pH destabilises some complexes	Metals from fluoride complexes
Mercury(II) salts	Displace metal from cyanide complex	Cu²⁺, Ni²⁺ (by competitive reaction)

Example of sequential titration using masking/demasking: In a solution containing Zn²⁺ and Mg²⁺:

Titrate both with EDTA at pH 10 → V₁ mL (total)
Add KCN → masks Zn²⁺
Titrate Mg²⁺ with EDTA → V₂ mL
Add formaldehyde → demasks Zn²⁺
Titrate Zn²⁺ with EDTA → V₃ mL = V₁ – V₂
Calculate each metal separately

Long Q4: General Principle of Complexometric Titration + Ligands and Their Types

General Principle of Complexometric Titration

1. Formation of Metal-Ligand Complex

When a ligand (L) is added to a metal ion (M), a coordinate (dative) bond is formed:

M + L → ML (complex)

The metal acts as a Lewis acid (electron acceptor) and the ligand acts as a Lewis base (electron donor).

2. Equilibrium and Stability Constant

The stability of the complex is expressed by the formation constant (Kf):

$$K_f = \frac{[ML]}{[M][L]}$$

Higher Kf → more stable complex → more complete reaction → sharper endpoint.

For EDTA complexes, Kf values range from 10⁸ to 10²⁵, indicating very high stability.

3. Effect of pH

EDTA exists as H₄Y at low pH and Y⁴⁻ at high pH
Y⁴⁻ is the fully deprotonated form that reacts with metals
At low pH, EDTA is protonated and less available → complex less stable
At high pH, competing hydroxide precipitation may occur
Therefore, optimal pH must be maintained for each metal

Conditional formation constant: $$K_f' = K_f \times \alpha_{Y^{4-}}$$ where α depends on pH (fraction of EDTA as Y⁴⁻)

4. Indicator Theory (Metallochromic Indicators)

The indicator (Ind) also forms a complex with the metal (M-Ind), but weaker than M-EDTA:

Before endpoint: M-Ind (metal bound to indicator → one colour)
EDTA added → gradually displaces indicator from metal
At endpoint: all M-Ind → M-EDTA; Ind is free → different colour

Key condition: K(M-EDTA) >> K(M-Ind) so displacement is complete at endpoint

5. Titration Curve

A complexometric titration curve plots pM (= –log[M]) against volume of EDTA:

Initially pM is low (high [M])
As EDTA added, [M] decreases slowly, then dramatically at equivalence point
Sharp inflection = equivalence point
Indicator changes colour within this steep section

Ligands — Definition and Classification

Definition

A ligand is an atom, ion, or molecule that has at least one lone pair of electrons which it donates to a central metal ion/atom to form a coordinate bond. The resulting product is called a complex or coordination compound.

Classification of Ligands

Type 1: Based on Denticity (Number of Donor Atoms)

Monodentate Ligands (1 donor atom)

Attach to metal through only one atom
Form simple complexes — less stable
Examples: Cl⁻, Br⁻, F⁻, OH⁻, CN⁻, NH₃, H₂O, NO₂⁻, SCN⁻

Bidentate Ligands (2 donor atoms)

Attach to metal through 2 atoms simultaneously
Form 5- or 6-membered chelate rings
Examples:
- Ethylenediamine (en) — N,N donor
- Oxalic acid / Oxalate (ox²⁻) — O,O donor
- Glycinate (gly) — N,O donor
- 8-Hydroxyquinoline (oxine) — N,O donor
- Acetylacetonate (acac) — O,O donor

Tridentate Ligands (3 donor atoms)

Examples: Diethylenetriamine (dien), 2,2',2''-terpyridine (terpy)

Tetradentate Ligands (4 donor atoms)

Examples: Triethylenetetramine (trien), porphyrins (haemoglobin!)

Pentadentate Ligands (5 donor atoms)

Example: DTPA (partially), some Schiff bases

Hexadentate Ligands (6 donor atoms) — most important in pharmacy!

Example: EDTA (Ethylenediaminetetraacetic acid)
- 2 nitrogen donors + 4 oxygen donors = 6 donor atoms
- Forms 5 five-membered chelate rings with the metal
- Most stable complexes (log Kf up to 25)

Type 2: Based on Bridging Ability

Type	Feature	Example
Bridging ligands (μ)	Connect two metal centres	μ-OH, μ-Cl
Terminal/non-bridging ligands	Attach to only one metal	Most common ligands
Ambidentate ligands	Two possible donor atoms, can bind either way	SCN⁻ (via S or N), NO₂⁻ (via N or O)

Type 3: Based on Electronic Character

Type	Description	Example
σ-donor ligands	Donate electrons via σ bond	NH₃, H₂O, Cl⁻
π-donor ligands	Donate via π bond	Alkenes, CO (in part)
π-acceptor ligands	Accept electrons from metal into π*	CO, NO, PPh₃

Chelate Effect — Key Concept

The chelate effect refers to the extra stability of polydentate complexes compared to analogous monodentate complexes:

[Cu(en)₂]²⁺ (bidentate ethylenediamine) is much more stable than [Cu(NH₃)₄]²⁺ (four monodentate NH₃)
Reason: entropy — forming one chelate ring with a bidentate ligand releases fewer solvent molecules than two monodentate ligands, resulting in ΔS > 0 and more negative ΔG

This is why EDTA, with its 6 donor groups, forms the most stable metal complexes and is the ideal titrant in complexometric analysis.

Summary Table of Key Indicators in Complexometric Titration

Indicator	Chemical Name	Metal Determined	pH	Colour Change
EBT (Eriochrome Black T)	Mordant Black 11	Ca²⁺+Mg²⁺, Zn²⁺, Mn²⁺	10	Wine red → Blue
Murexide	Ammonium Purpurate	Ca²⁺, Cu²⁺, Ni²⁺	12	Pink/Red → Purple
Xylenol Orange	—	Pb²⁺, Zn²⁺, Bi³⁺	5–6	Red-violet → Yellow
Calcon (Chrome Black SE)	—	Ca²⁺	12	Red → Blue
PAR	4-(2-Pyridylazo)resorcinol	Zn²⁺, Cu²⁺, Pb²⁺	variable	Yellow → Red

Quick Tip for the Examiner: The format of complexometric titrations in pharmacopoeia always involves — (1) buffer for pH control, (2) metallochromic indicator, (3) EDTA titrant, (4) colour change endpoint. Every IP/BP assay of metal salts (calcium gluconate, zinc sulphate, magnesium hydroxide, etc.) uses this principle.

All content based on pharmaceutical analysis principles as described in standard references including Connors' Textbook of Pharmaceutical Analysis, Indian Pharmacopoeia, and Beckett & Stenlake's Practical Pharmaceutical Chemistry.

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