1601 Redox Titration

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1 1601 Redox Titration Redox Titration in Analytical Chemistry Dr. Fred Omega Garces Analytical Chemistry 251 Miramar College, SDCCD Recommended: 1 Redox Titration

Introduction Oxidation and reduction titrations may be used to measure many species especially metals in high or low valency states, iodine and iodides and easily oxidized organic compounds.

2 Introduction Oxidation and reduction titrations may be used to measure many species especially metals in high or low valency states, iodine and iodides and easily oxidized organic compounds. In redox titration the concentration of oxidizing agent and reducing agent will change simultaneously. Consider the following redox reaction: SCE a (Ox), a (Red) Pt The cell emf is: E = E + (RT/F) ln [a (Ox) / a (Red)] - E SCE As the reaction proceeds during titration, the ratio of the concentration will change as well as the emf. 2 Redox Titration

3 Titration Preparation For standard solutions, sodium oxalate, iron(ii) ammonium sulfate or potassium iodate can be used. Potassium permanganate and iodine solution must be standardized before using because of decomposition. An indicator for redox reactions are reagents whose oxidized and reduced form differ in color: ln (Ox) [color 1] + ne- = ln (Red) [color 2] Consider the example of 1,10-phenanthroline iron(ii): [Fe(C 12 H 8 N 2 ) 3 ] 3+ (pale blue) + ne- = [Fe(C 12 H 8 N 2 ) 3 ] 2+ (deep red) In some reactions the H 3 O + results in a change in ph. 3 Redox Titration

4 Below are common redox indictors Redox Indicators Redox Oxidizing Reduced Eln (V) Solution 1,10-phen iron(iii) Pale blue Red M H 2 SO 4 Diphenylamine Violet Colorless 0.76 Dilute acid Methylene blue Blue Colorless M acid Phenosafranine Red Colorless M acid Another indicator involving iodine is starch solution (or synthetically similar material). The starch forms a blue-black complex with iodine which but is colorless when there no iodine is present, i.e., iodine is converted to triiodide (I 3- ). For a redox indicator at 25 C, the color change takes place at the potential range of E = Eln Redox Titration

5 Starch as an Indicators Starch is often used in chemistry as an indicator for redox titration where triiodide is present. Starch forms a very dark blue-black complex with triiodide which can be made by mixing iodine with iodide (often from potassium iodide). However, the complex is not formed if only iodine (I 2 ) or only iodide (I - ) is present. The color of the starch complex is so deep, that it can be detected visually when the concentration of the iodine is as low as M at 20 C. During iodine titrations, concentrated iodine solutions must be reacted with some titrant, often thiosulfate, in order to remove most of the iodine before the starch is added. This is due to the insolubility of the starch-iodine complex which may prevent some of the iodine reacting with the titrant. Close to the end-point, the starch is added, and the titration process is resumed taking into account the amount of thiosulfate added before adding the starch 5 Redox Titration

6 Tri iodide is formed by: Experiment 4 KIO 3 + KI g I 3 - I 2 (aq) + I I 3 C 6 H 8 O 6 + I 3 VitaminC C 6 H 6 O 6 + I dehydroa scorbic acid 6 Redox Titration

7 Redox Titration Balance redox chem eqn: Solve problem using stoichiometric strategy. Q: g Fe ore requires ml of M KMnO 4. How pure is the ore sample? When iron ore is titrated with KMnO 4. The equivalent point results when: KMnO 4 (purple) g Mn (+7) Mn 2+ (pink) Mn(+2) 7 Redox Titration

8 Redox Titration Balance redox chem eqn: Solve problem using stoichiometric strategy. Q: g Fe (Fe +2 ) ore requires ml of M KMnO 4. How pure is the ore sample? 8 Redox Titration

Redox Titration Balance redox chem eqn: Solve problem using stoichiometric strategy. Q: 1.225 g Fe (Fe +2 ) ore requires 45.30 ml of 0.0180 M KMnO 4. How pure is the ore sample?

9 Redox Titration Balance redox chem eqn: Solve problem using stoichiometric strategy. Q: g Fe (Fe +2 ) ore requires ml of M KMnO 4. How pure is the ore sample? When iron ore is titrated with KMnO 4. The equivalent point results when: KMnO 4 (purple) g Mn (+7) Mn 2+ (pink) Mn(+2) Rxn: Fe +2 + MnO 4 - g Fe +3 + Mn 2+ Bal. rxn: 5 Fe 2+ + MnO H + g 5 Fe 3+ + Mn H 2 O Note Fe 2+ g 5 Fe 3+ : Oxidized Lose e- : Reducing Agent Mol of MnO 4 - = ml 0.180(mol/L) = mmol MnO 4 - Amt of Fe: = mmol 5 mol Fe g = g 1 mol MnO 4-1 mol Fe 2+ % Fe = ( g / g) 100 = 18.6 % 9 Redox Titration

10 Redox Titration: Example Titration of g of pure iron wire was dissolve in acid, reduced to +2 state and titrated with ml of cerium (IV). Calculate the molar concentration of Ce 4+ solution. Reaction : Ce Fe 2+ Ce 3+ + Fe Redox Titration

11 Redox Titration: Example Titration of g of pure iron wire was dissolve in acid, reduced to +2 state and titrated with ml of cerium (IV). Calculate the molar concentration of Ce 4+ solution. Reaction : Ce Fe 2+ Ce 3+ + Fe 3+ [Ce 4 + ] = g Fe 1000 mmol Fe ml Ce g 1 mmol Fe2+ mmol Fe 1 mmol Ce4 + mmol Fe 2+ [Ce 4 + ] = M Ce Redox Titration

12 5 Fe +2 + MnO 4 M KMnO4 = g M KMnO4 = M - + 8H + Mn+2 + 5Fe+3 + 4H 2 O 1 mol Fe 55.8 g 1 mol MnO 4-5 C 2 O g Na 2 C 2 O 4 mol Na C O mol KMnO g + 10 e + 16H e 2Mn H 2 O n=10 1 Redox Titration: Example Petrucci 7th Ed. p A piece of iron wire weighting g is converted to Fe 2+ (aq) and requires ml of a KMnO 4 (aq) solution for its titration. What is the molarity of the KMNO 4 (aq)? 2. Another substance that may be used to standardized KMNO 4 (aq) is sodium oxalate, Na 2 C 2 O 4. If g of Na 2 C 2 O 4 is dissolved in water and titrated with ml KMnO 4, what is the molarity of the KMnO 4 (aq)? 10 CO 2 2 MnO 4 - = M 5 mol Fe 12 Redox Titration 1 5 mol Na 2 C 2 O L KMnO L =

LAD B3 (pg! 1 of 6! ) Analysis by Redox Titration Name Per

LAD B3 (pg! 1 of 6! ) Analysis by Redox Titration Name Per LAD B3 (pg! 1 of 6! ) Name Per Introduction As you know, one common type of reaction in chemistry is oxidation-reduction. It involves the transfer of electrons from one species to another. Atoms undergo