Introduction
Iodometric titrations involve the use of iodine (I₂) as the analyte and sodium thiosulfate (Na₂S₂O₃) as the titrant. These titrations are used to determine the concentration of oxidizing agents like copper(II) ions or potassium iodate by indirect measurement through iodine liberation.

🔹 Principle of the Titration

Iodine (I₂) reacts with thiosulfate (S₂O₃²⁻), which reduces it to iodide ions (I⁻), while itself is oxidized to tetrathionate (S₄O₆²⁻).

Balanced Equation:

🔹 Common Titration Types

1. Direct I₂ vs S₂O₃²⁻

   • Iodine solution is titrated directly with sodium thiosulfate.

2. Indirect (Back Titration):

   • An oxidizing agent (e.g., KIO₃ or Cu²⁺) liberates iodine from iodide in acidic conditions, then I₂ is titrated with thiosulfate.

Reaction example (KIO₃ in acid):

🔹 Apparatus and Chemicals

• Burette, pipette, conical flask

• Sodium thiosulfate (in burette)

• Liberated iodine (in flask)

• Potassium iodide (KI)

• Dilute H₂SO₄ or HCl

• Starch indicator

🔹 Procedure

1. Prepare a solution that contains liberated iodine (I₂)

2. Fill burette with standard sodium thiosulfate solution

3. Pipette iodine-containing solution into the conical flask

4. Add a few drops of fresh starch indicator when the solution becomes pale yellow

5. Titrate until the blue-black color disappears

6. Record readings and repeat for concordant titres

🧪 Calculation Example

Given:

• Volume of Na₂S₂O₃ used = 22.60 mL

• Molarity = 0.100 M

• Mole ratio: I₂ : S₂O₃²⁻ = 1 : 2

Step 1: Moles of thiosulfate

Step 2: Moles of I₂

✅ Answer: 1.13 mmol of iodine was present in the sample.

🔹 Indicator and Color Change

• Start: Brown/yellow (due to iodine)

• Near endpoint: Pale yellow

• Add starch: Solution turns blue-black

• Final endpoint: Blue-black disappears → solution becomes colorless

🧠 NECTA Tips

• Always prepare starch solution fresh

• Add starch only when the solution turns pale yellow

• Keep away from sunlight – iodine evaporates

• Use a stopper for iodine solutions

• Show all equations in calculations

✅ Summary

• Iodometric titrations measure iodine via redox with thiosulfate

• Titration based on 1 I₂ : 2 S₂O₃²⁻ mole ratio

• Starch is the key indicator → blue-black to colorless

• Useful for analyzing copper(II), KIO₃, or H₂O₂ indirectly

• Important in NECTA for both theory and practical