Fundamentals of Redox Titration and Analytical Chemistry

ANALYTICAL CHEMISTRY LABORATORY • VOLUMETRIC ANALYSIS: REDOX TITRATION OXIDIZING AND REDUCING AGENTS INTRODUCTION • • • • • What are the 3 subatomic particles? Proton, Neutron, Electron The charge of an electron is negative A positively charged ion. Cation Cations are formed due to _____ of electrons, becoming _____ charged. lose; positively The charge of an atom is generally neutral/zero THE ELECTRON THEORY OF MATTER • • • An atom is composed of; o Proton (+) o Neutron o Electron (-) Oxidation o Valence/charge (before) = 0 o Loses eo Valence/charge (after) = +1 o 0 → +1 Valence INCREASE due to LOSS of electron o Valence Increase Loss of Electrons Reduction o Valence/charge (before) = 0 o Gains eo Valence/charge (after) = -1 o 0 → -1 Valence DECREASE due to Gain of electron o Valence Decrease Gain of Electrons Sample Problems o Identify the oxidizing and reducing agentsin the reactants of the reaction: 2Ag+ + Cu(s) ⇌ 2Ag(s) + Cu2+ OA RA ▪ Cu(s) ⇌ Cu2+ (0) (+2) Atoms in elemental form have zero valence Valence Increase Loss of Electron (2e-) feq depends on the no. of e- lost or gained feq = 2 Oxidation Cu is a Reducing Agent ▪ Ag+ is an oxidizing agent while Cu (s) is a reducing agent o Identify if elemental zinc is an O.A. or R.A., and determine the number of e- gain or lost. Zn(s) ⇌ Zn2+ (0) (+2) Valence Increase 0 → +2 Loss of Electron 0 → +2 + 2e- (feq = 2) Oxidation Zn(s) ⇌ Zn2+ + 2eZn is a Reducing agent; elemental Zn is a reducing agent, w/ 2e- lost upon rxn OXIDATION-REDUCTION ANALYSIS OXIDATION-REDUCTION ANALYSIS • • • Analysis that involves a change in valence of the reacting substances Commonly seen between electrolytes in aqueous solutions It is always accompanied by a transfer of electrons OXIDATION-REDUCTION (REDOX) REACTIONS • Electrons are transferred from one reactant to another. Reactants Product Ce4+ + Fe2+ ⇌ Ce3+ + Fe3+ OA RA RA OA Ce4+ ⇌ Ce3+ +4 → +3 Valence Decrease dure o gain of electron How many electrons gained? One Where will you place this one e- in the equation? Left or right? Try to figure out where to place -1 to balance +4 -1 = +3. Left Side Ce4+ + e- ⇌ Ce3+ Valence Decrease Gain of Electrons Reduction Oxidizing Agent o Fe2+ ⇌ Fe3+ +2 → +3 Valence Increase due to loss of electrons How many electron/s is/are lost? One Where will you place this one e- in the equation? Left or right? Try to figure out where to place -1 to balance +2 = +3 -1. right side Fe2+ ⇌ Fe3+ + eValence Increase Loss of Electrons Oxidation Reducing Agent Involves a change in valence of the reactants o • • Oxidation-Reduction o VILEORA ▪ Valence Increase due to Loss of Electrons is known as Oxidation reaction, the substance is said to be a Reducing Agent o VDGEROA ▪ Valence Decrease due to Gain of Electrons is known as Reduction reaction, the substance is said to be a Oxidizing Agent OXIDATION • There are three definitions you can use for oxidation: o The loss of electrons ▪ Na(S) → Na+ + eo The gain of oxygen ▪ Burning of Coal - C(s) + O2(g) → CO2 ▪ Rusting of Iron - 2Fe(s) + 3 O2(g) → 2Fe2O3(s) o The loss of hydrogen ▪ CH3OH(l) → CH2O(l) + H2(g) REDUCTION • Like oxidation, there are three definitions you can use to describe reduction: o The gain of electrons ▪ Ag+ + e- → Ag o The loss of oxygen ▪ Fe2O3(s) + 3CO(g) → 2Fe(s) + 3 CO2 (g) o The gain of hydrogen ▪ CO(g) + 2H2(g) → CH3OH(l) OXIDIZING AGENTS • • • • • Potassium permanganate Potassium dichromate Ceric sulfate Iodine solution Potassium bromate - 5e 6e 1e 2e 6e - 1e 2e 4e 1e 2e • • Indicator: Starch TS End-point: Blue color IODOMETRY • • • • • • Method of titration: Indirect Analyte: Oxidizing agents Titrant: 0.1N Sodium thiosulfate Indicator: Starch TS End-point: Disappearance of blue color Starch Test Solution o Freshly prepared prior to use o It is prepared by dissolving 1 gram starch in 10 mL cold water, adding it to 200 mL boiling water, stir constantly o Components ▪ Alpha-amylose (amylopectin)– insoluble starch ▪ Beta-amylose – soluble starch + iodine = blue REDUCING AGENTS • • • • • BROMINE TITRATIONS Ferrous ammonium sulfate Sodium oxalate Arsenic trioxide Titanium chloride Sodium thiosulfate PERMANGANOMETRY • • Advantages o Standardized easily o Retain its concentration over long periods of time o Rapid reactions o Does not need an indicator Preparation and standardization o Sodium oxalate – primary standard o End-point – permanent pink color • • • 0.1N Bromine solution which is also known as “Koppeschaar’s solution” The solution contains an equivalent amount of potassium bromate and excess potassium bromide, bromine is liberated when the solution is acidified It is an alternative for iodine solution for the assay of compounds like aniline, phenol and resorcinol POTASSIUM IODATE TITRATIONS • • • 0.05M Potassium iodate solution It is an oxidizing agent used for the titration of iodides, arsenites and other reducing agents The method depends upon the formation of iodine monochloride in strong hydrochloric acid solution DIAZOTIZATION ASSAYS WITH NITRITE PERMANGANATE TITRATIONS • • • Direct PT o Assay of H2O2 Solution Indirect PT o Assay of Cherry Juice for Malic Acid Residual PT o Assay of Sodium Nitrite CERIMETRY • Advantages o Cerous ion is colorless so it does not obscure the indicator end-point o React quantitatively with oxalate or arsenite ion o Stable even on boiling o No intermediate products are formed in the reduction of ceric ion o High concentrations of chloride ions are not oxidized by ceric salts so that ferrous ion can be determined in the presence of chlorides o Ferrous phenanthroline ion (ferroin) is a very satisfactory indicator • • 0.1M Sodium nitrite Useful for the assay of sulfa and arylamino group other compounds containing an by diazotization with nitrite PREPARATION AND STANDARDIZATION OF 0.1 N POTASSIUM PERMANGANATE OXIDATION-REDUCTION (REDOX) REACTIONS • Electrons are transferred from one reactant to another Ce4+ + Fe2+ ⇌ Ce3+ + Fe3+ Ce4+ ⇌ Ce3+ Valence Decrease - Gains e- (-1) Valence Decrease Gain of Electrons Reduction Oxidizing Agent o Fe2+ ⇌ Fe3+ Valence Increase - Loses e- (-1) Valence Increase Loss of Electrons Oxidation Reducing Agent Involves a change in valence of the reactants o • PERMANGANOMETRY IODIMETRY • • • Method of titration: Direct Analyte: Reducing agents Titrant: 0.1N Iodine solution • • Permanganate Titrations Uses KMnO4 standard solutions o Standardized easily o Retains concentration over long periods o • • Sodium oxalate as suitable primary standard. ▪ Sodium oxalate + H2SO4 → Oxalic acid Reactions process becomes more rapidly over time (AUTOCATALYSIS) 2MnO4- + 5H2C2O42- + 6H+ ⇌ 2Mn2+ + 10CO2 + 8H2O Self-indicator: o Very slight excess of KMnO4 imparts distinct pink color STANDARDIZATION OF 0.1 N KMnO4 • • • • • • • Type and Methods: Titrant: Stock Solution: Primary Standard: Indicator: Endpoint: Results: Direct permanganometry 0.1 N KMnO4 0.1 N KMnO4 Sodium Oxalate (Na2C2O4) none pink color • ASSAY OF HYDROGEN PEROXIDE CONTENT IN AGUA OXIGENADA SOLUTION • • • • • • • • • • Working Equations: o MW Na2C2O4 = 134.00 g/mol o feq = 2 o MEQ wt = MW / feq x 1000 o N = g / MEQ wt. x mL ASSAY OF HYDROGEN PEROXIDE CONTENT IN AGUA OXIGENADA SOLUTION • • Titrant: OXIDIZING agent Titrand: REDUCING agent * and vice versa Example: o Titrant: KMnO4 → OXIDIZING agent o Titrand: Na2C2O4 → REDUCING agent STANDARD SOLUTIONS USED IN OFFICIAL ASSAYS OXIDIZING AGENTS Ferric ammonium sulfate Potassium permanganate Potassium dichromate Potassium bromate Potassium iodate Potassium ferricyanide Ceric sulfate Iodine Bromine REDUCING AGENTS Ferrous ammonium sulfate Oxalic acid Potassium arsenite Titanium (III) chloride Sodium thiosulfate “AGUA OXIGENADA” SOLUTIONS • Active ingredient: o Hydrogen peroxide (H2O2) Type and Methods: Direct permanganometry Titrant: 0.1 N KMnO4 Standard Solution: 0.1 N KMnO4 Analyte: Hydrogen peroxide (H2O2) Indicator: none Endpoint: pink color USP Limit: 2.5 to 3.5% 2MnO4- + 5H2O2- + 6H+ ⇌ 2Mn2+ + 5O2 + 8H2O o Potassium permanganate → Oxidizing agent o Hydrogen Peroxide → Reducing agent Results: Working Equations: o MW H2O2 = 34.01 g/mol o feq = 2 o MEQ wt = MW / feq x 1000 o %A = mL x N x MEQ wt. x 100 mL PREPARATION AND STANDARDIZATION OF 0.1 N IODINE DIRECT REDOX TITRATIONS • • Available in o 3% (10 volumes) ▪ Red bottle ▪ Used as antiseptic o 6% (20 volumes) ▪ Blue bottle ▪ Used as bleaching agent IODINE SOLUTION • • • • A weak oxidizing agent o Primary used for determination of strong reducing agents Iodine crystals are NOT very water soluble (1g:2950 mL) o Potassium iodide (KI) is added as solubilizing agent. I2(s) + I- ⇌ I3Iodimetry o Involves direct titration using iodine as a titrant (standard solution) STANDARDIZATION OF 0.1 N IODINE • • • • • • Type and Methods: Titrant: Stock Solution: Primary Standard: Indicator: Endpoint: Iodimetry 0.1 N Iodine 0.1 N Iodine Arsenic Trioxide (As2O3) Starch Blue- black color *β-amylose – soluble starch ** α-amylose (amylopectin)insoluble • SAMPLE PROBLEMS REDOX Results REDOX SAMPLE PROBLEM 1 • • If a 0.2500 g sample of sodium oxalate required 32.4 mL of the permanganate solution, what will be the normality of potassium permanganate? Na2C2O4 = 2(23) + 2(12) + 4(16) MEQ = 134/2000 = 0.067 N = 0.2500 g = 0.1152 N 32.4(0.067) 0.1264N; 0.2561g; v=? V= 0.2561 = 30.2mL 0.1264(0.067) Working Equations o MW As2O3 = 197.84 g/mol o feq = 4 o MEQ wt = MW / feq x 1000 o N = g / MEQ wt. x mL 0.1186N; 29.9mL; wt. ? wt. = 0.1186(29.9)(0.067) = 0.2376 g ASSAY OF ASCORBIC ACID REDOX SAMPLE PROBLEM 2 ASCORBIC ACID (C6H8O6) • • • a white to very pale yellow crystalline powder with a pleasant sharp acidic taste, almost odorless A fairly strong reducing agent ASSAY OF ASCORBIC ACID • • • • • • • • Type and Method: Titrant: Standard Solution: Analyte: Indicator: Endpoint: USP Limit: Results • Assay of Ascorbic acid by Iodimetry o Titrant/ Standard solution: ▪ Iodine ▪ OXIDIZING agent o Titrand/ Analyte: ▪ Ascorbic acid ▪ REDUCING agent Working Equations: o MW C6H8O6 = 176.12 g/mol o feq = 2 o MEQ wt = MW / feq x 1000 o %A = mL x N x MEQ wt. x 100 mL • Direct Iodometry 0.1 N Iodine 0.1 N Iodine (Ex. 13) Ascorbic Acid (C6H8O6) Starch Blue-black color 99.0 to 100.5% A 0.2295 g sample of sodium oxalate, 98% pure, required 35.1 mL of potassium permanganate solution in a titration. What is the normality of the permanganate solution? N= 0.2295g (98%) = 0.0956 N 35.1mL (0.067) (100%) 0.0956N; 36.3mL; 99%; wt.=? wt. = 0.0956N (36.3mL) (0.067) (100%) = 0.2349g 99% 0.0956N; 0.2418g; 98.3%; v=? V= 0.2418g (98.3%) = 37.1 mL 0.0956N (0.067) (100%)