BP106T • Unit 4Topic 3 of 3⏱ 5 Min Read

3. Redox Titrations: Permanganometry, Cerimetry, Iodimetry vs. Iodometry

💡 Core Concept & Clinical Context:

Redox (Oxidation-Reduction) titrations involve the transfer of electrons between an oxidizing agent (which gains electrons and is reduced) and a reducing agent (which loses electrons and is oxidized). Permanganometry uses Potassium Permanganate ($KMnO_4$) as a self-indicating oxidizing agent in dilute $H_2SO_4$ medium ($MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$, endpoint = persistent faint pink). Cerimetry uses Ceric Ammonium Sulphate [$Ce(SO_4)_2$] in dilute $H_2SO_4$ with Ferroin indicator (color change: intense red to pale blue). Iodimetry is the direct titration of reducing analytes (e.g., Ascorbic Acid, Sodium Thiosulphate) with standard Iodine solution. Iodometry is the indirect titration where an oxidizing analyte oxidizes excess Potassium Iodide ($KI$) to liberate free Iodine ($I_2$), which is subsequently titrated against standardized Sodium Thiosulphate ($Na_2S_2O_3$) using freshly prepared Starch indicator.

Clinical Scenarios & Pharmacopoeial Examples:

1Comparison of Iodimetry (Direct) vs Iodometry (Indirect)

Principles and chemical equations.

Step-by-Step Breakdown:

•A. IODIMETRY (Direct Titration):

•Analyte (Reducing Agent, e.g. Ascorbic Acid / Vitamin C) titrated directly with standard 0.05M Iodine (I2):

•C6H8O6 (Ascorbic Acid) + I2 -> C6H6O6 (Dehydroascorbic Acid) + 2 I- + 2 H+

•• End Point: First drop of excess I2 forms deep blue complex with starch indicator.

•B. IODOMETRY (Indirect Titration):

•Step 1: Oxidizing Analyte (e.g. Copper Sulphate, Bleaching Powder, Potassium Permanganate) added to excess KI:

•2 CuSO4 + 4 KI -> 2 CuI (Cuprous Iodide precipitate) v + I2 (Liberated free Iodine) + 2 K2SO4

•Step 2: Liberated free I2 titrated with standardized 0.1M Sodium Thiosulphate (Na2S2O3):

•I2 + 2 Na2S2O3 -> 2 NaI + Na2S4O6 (Sodium Tetrathionate)

•• Starch Indicator Timing: Starch must be added NEAR THE END POINT (when dark brown solution fades to straw-yellow); adding starch too early irreversibly traps iodine inside amylose helices, ruining accuracy!

📋Key Takeaways & Exam Highlights

Self-Indicator: $KMnO_4$ acts as its own indicator; the first excess drop imparts a persistent pink color.
Why $HCl$ is avoided in Permanganometry: Permanganate oxidizes $Cl^-$ to toxic Chlorine gas ($Cl_2$), causing massive positive titration error; dilute $H_2SO_4$ is mandatory.
Starch Indicator Mechanism: Linear Amylose chains in starch trap tri-iodide ions ($I_3^-$) inside helical channels, producing an intense deep blue-black coordination complex.

🇮🇳 हिन्दी व्याख्या (3. Redox Titrations: Permanganometry, Cerimetry, Iodimetry vs. Iodometry)

रेडॉक्स टाइट्रेशन में इलेक्ट्रॉन का आदान-प्रदान होता है। KMnO4 खुद ही इंडिकेटर का काम करता है और गुलाबी रंग देता है। आयोडीमेट्री में सीधे आयोडीन से टाइट्रेशन होता है, जबकि आयोडोमेट्री में निकलने वाले आयोडीन को सोडियम थायोसल्फेट और स्टार्च इंडिकेटर से नापा जाता है।