Question

The indicator used in determination of peroxide value of ghee is

• A. Phenolphthalein
• B. Starch
• C. Methylene blue
• D. Methyl red

Hint:

Titration Indicators in Ghee Analysis:
Peroxide Value (PV) Iodine Value = STARCH INDICATOR (Blue to Colorless).
Reichert-Meissl (RM) Polenske Value (PV) Free Fatty Acids (FFA) = PHENOLPHTHALEIN (Colorless to Pink).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Iodometric titration in lipid oxidation analysis: the Peroxide Value (PV) assay oxidizes potassium iodide to elemental iodine ($ ext{I}_2$), which is titrated with standard sodium thiosulfate using 1% starch solution as the color indicator (intense blue triiodide-amylose complex to colorless endpoint).
Key Formula or Approach:
\[ \text{Lipid Hydroperoxide (ROOH)} + 2\text{I}^- + 2\text{H}^+ \longrightarrow \text{ROH} + \mathbf{\text{I}_2} + \text{H}_2\text{O} \]
\[ \mathbf{\text{I}_2 + \text{Starch (Blue-Black)}} + 2\text{Na}_2\text{S}_2\text{O}_3 \longrightarrow \mathbf{\text{Colorless Endpoint}} + 2\text{NaI} + \text{Na}_2\text{S}_4\text{O}_6 \]

Step 2: Detailed Explanation:
In lipid analytical chemistry and oxidative rancidity testing of ghee (AOAC IS: 3508 AOCS Cd 8b-90):
- Peroxide Value (PV) measures primary lipid oxidation products (lipid hydroperoxides):
1. Ghee sample is dissolved in glacial acetic acid-chloroform ($3:2$) and reacted with saturated Potassium Iodide (KI).
2. Lipid hydroperoxides quantitatively oxidize iodide ions into free elemental iodine ($\text{I}_2$).
3. The liberated iodine is titrated against standard $0.01\text{ N}$ Sodium Thiosulfate ($\text{Na}_2\text{S}_2\text{O}_3$).
4. Near the endpoint (pale yellow), Fresh 1% Starch Indicator Solution (B) is added, forming an intense blue-black starch-triiodide complex.
5. Titration continues until the blue color completely disappears, yielding a sharp colorless endpoint.

Step 3: Final Answer:
Therefore, the indicator used in determination of peroxide value is Starch, corresponding to option (B).