Redox Titration PPT PDF

Oxidation-Reduction Titrations (or) Redox titrations Manipal College of Pharmaceutical Sciences  OXIDATION is the process in which an atom, ion or molecule loses one or more electrons is the process in which addition of oxygen to any su...

Oxidation-Reduction Titrations (or) Redox titrations Manipal College of Pharmaceutical Sciences  OXIDATION is the process in which an atom, ion or molecule loses one or more electrons is the process in which addition of oxygen to any substance is the process in which removal of hydrogen from any substance Reaction in which an atomic or ionic system changes in to more electropositive M A N I PA L or less electronegative state by loss of one or more electrons U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  OXIDATION In oxidation half reactions, electrons are SO2 + O2 SO3 written on the right because electrons are lost H2 S + O S + H 2O Fe Fe 2+ + 2e- Fe 2+ Fe 3+ + e- Zn Zn2+ + 2e- M A N I PA L Sn2+ Sn4+ + 2e- U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  REDUCTION is the process in which an atom, ion or molecule gains one or more electrons is the process in which removal of oxygen to any substance is the process in which addition of hydrogen from any substance Reaction in which an atomic or ionic system changes into less electropositive or more electronegative state by gain of one or more electrons M A N I P A L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  REDUCTION In Reduction half reactions, electrons are written on the left because electrons are gained CuO + 2H Cu + H2O Cl2 + 2e 2Cl- C2H2 + 2H C 2H4 I2 + 2e- 2I- Fe 3+ + e- Fe2+ Ce4+ + e- Ce3+ Sn4+ + 2e- Sn2+ M A N I PA L Sn2+ + 2e- Sn U N I V E R S I T Y Cu2+ + 2e- Cu Manipal College of Pharmaceutical Sciences  Oxidation Reduction Reaction [Redox Reaction] Oxidation and reduction usually occurs simultaneously in a reaction, one substance becoming reduced in the process of oxidizing the other called Redox reaction. In other words Fe2+ Fe3+ + e- Ce4+ + e- Ce3+ Reaction involves the simultaneous loss and M A N I PA L Ce4+ + Fe2+ Fe3+ + Ce3+ U N I V E R S I T Y gain of electrons Manipal College of Pharmaceutical Sciences  OXIDISING AGENT: [OXIDANT] is a substance that gains one or more electrons and is there by reduced in a redox reaction Potassium permanganate (KMnO4) Potassium bromate (KBrO3) Potassium iodate (KIO3) Potassium ferricyanide (C6N6FeK3) Ceric ammonium sulphate ((NH4)4Ce(SO4)4) M A N I PA L Iodine (I2) U N I V E R S I T Y 2,6, di chlorphenol indophenol (C12H7NCl2O2) Manipal College of Pharmaceutical Sciences  REDUCING AGENT:[REDUCTANT] is a substance that loses one or more electrons and is there by oxidized in a redox reaction Ferrous ammonium sulphate Oxalic acid Potassium iodide Sodium thiosulphate Stannous chloride M A N I PA L Arsenic trioxide. U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Redox potential is the potential difference that develops between the electrodes of the cell is a measure of the tendency for the reaction to proceed from a non-equilibrium state to the condition of equilibrium. Relative strength of oxidizing agents or reducing agents can be obtained by comparing their tendencies to accept or give electrons. This tendency is measured by a quantity called Redox Potential and the direction of M A N I PA L oxidation-reduction can be predicted if some quantitative characteristic of oxidizing and U N I V E R S I T Y reducing agent is known, this is known as Redox potential. Manipal College of Pharmaceutical Sciences  M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  THE NERNST EQUATION Usually concentrations of reactants differ from one another and also change during the course of a reaction E = E° + (RT/nF) loge C E = potential under the nonstandard conditions E° = standard electrode potential R = gas constant, 8.314 J/mol.K T = absolute temperature n = number of moles of electrons transferred (Valency of metal ions) M A N I PA L U N I V E R S I T Y F = Faraday constant, 96,485 coulombs/g-eq C= Concentration of metal ions Manipal College of Pharmaceutical Sciences At 25 oC (298K)Substituting the values for the constants and changing the base from e to 10, the equation will be 0.0591 E= Eo+ log10 C n 0.0591 [Oxidised state] Ece𝑙𝑙 = Eocell+ 𝑙𝑜𝑔 n [Reduced state] M A N I PA L is very useful in analytical measurements, since it allows the analyst to measure U N I V E R S I T Y variations in concentration during reactions or experiments. Manipal College of Pharmaceutical Sciences Oxidation-Reduction Titration Curves M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  The titration curve is a plot of how the concentration of a reactant varies with titrant Redox titration curve obtained by plotting potential as ordinates against the percentage of titrant added as abscissae A titration curve has three distinct regions: ✓ Before the equivalence point, ✓ At the equivalence point, and M A N I PA L U N I V E R S I T Y ✓ After the equivalence point Manipal College of Pharmaceutical Sciences Before the equivalence point At the equivalence point After the equivalence point M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Eg: The titration of 100ml 0.1M iron(II) with 0.1M cerium (IV) in the presence of dilute sulphuric acid Ce4+ + Fe2+ Ce3+ + Fe3+ 0.0592 [Fe3+] 0.0592 [Ce4+] E1 = 0.77 + 𝑙𝑜𝑔 E1 = 1.45 + 𝑙𝑜𝑔 1 [Fe2+] 1 [Ce3+] When 10 ml of the oxidizing agent have been added [Fe 3+] / [Fe2+] = 10/90 (approx) & 0.0592 = 0.77+ 0.0592 x log 0.111 M A N I P A L E1 = 0.77 + 𝑙𝑜𝑔 = 0.77+ 0.0592 x -0.9542 U N I V E R S I T Y 1 = 0.77-0.056 = 0.69 volt Manipal College of Pharmaceutical Sciences With 50 ml oxidising agent E1 =E0 = 0.77V With 90 ml oxidising agent E1 =0.77 + 0.0592 log 90/10 = 0.81V With 99 ml oxidising agent E1 =0.77 + 0.0592 log 99/1 = 0.87V With 99.1ml oxidising agent E1 =0.77 + 0.0592 log 99.9/0.1 = 0.94V M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences Before the equivalence point At the equivalence point After the equivalence point M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Detection of Equivalence point/Endpoint in Redox titrations Indicator method: Indicators helps to know whether the reaction is complete or not. Redox indicators mark the sudden change in the reduction potential in the neighborhood of the equivalence point in redox titration. Potentiometric methods: EMF of the cell is measured M A N I PA L Used when suitable indicator not available and for colored and very dilute U N I V E R S I T Y solutions Manipal College of Pharmaceutical Sciences  There are Three Types of indicators are generally used to detect the end point in a redox titration. Self indicator : KMnO4, K2Cr2O7, I2 , Ce4+ External indicator : Potassium Ferricyanide, Starch Iodide paper/paste/solution : General Redox indicators: Ferroin, Nitro ferroin, Diphenyl Internal indicator amine, Methylene Blue M A N I PA L U N I V E R S I T Y : Specific indicators: Starch, Potassium thiocyanate Manipal College of Pharmaceutical Sciences  Self Indicators ✓ Self indicators are the colored titrants which impart color to the solution after completion of the reaction. Potassium permanganate (Pink) Cerium(IV) sulphate (Yellow) Iodine (Brown) M A N I PA L ✓ Sufficiently deeply colored to provide good visual end points U N I V E R S I T Y ✓ Only objection is that the visual end point represents a slight over titration. Manipal College of Pharmaceutical Sciences  External Indicator These are rarely used at present. The best known example is the potassium ferricyanide solution. Use in titration between ferrous sulphate V/s Potassium dichromate Near the equivalence point, drops of the solution from conical flask is removed and brought in to the contact with external indicator kept in spot plate. M A N I PA L The end point is shown by change in colour of the of the external indicator kept U N I V E R S I T Y in spot plate Manipal College of Pharmaceutical Sciences  INTERNAL INDICATOR : General Redox indicators: Ferroin, Nitro ferroin, Diphenyl Internal indicator amine, Methylene Blue : Specific indicators: Starch, Potassium thiocyanate Most of the redox indicators are dye Since the dyes are intensely colored, the indicator can be used at M Asuch N I Pa A Llow U N I V E R S I T Y concentration that there is no interference with system under examination. Manipal College of Pharmaceutical Sciences  Specific indicators Starch: which forms a dark blue complex with triiodide ion. This complex signals the end point in titrations in which iodine is either produced or consumed Potassium thiocyanate: Used in the titration of Iron III with solution of titanium III sulphate. M A N I PA L The end point involves the disappearance of the red colour of the Iron III U N I V E R S I T Y thiocyanate Manipal College of Pharmaceutical Sciences  TITRATION INVOLVING CERIC AMMONIUM SULPHATE Ce4+ + e- Ce3+ E0 = 1.44 V (1 M H2SO4) Advantage particularly as ceric sulphate solution is very stable and have a high oxidation potential. In the presence of reducing agent it undergoes reduction to form ceric to cerous state. Ce(SO4)2.2(NH4)2SO4, 2H2O Mol wt.: 632.57 M A N I PA L Ceric ammonium sulphate being more soluble than ceric sulphate and it is used in U N I V E R S I T Y preparing the solution. Manipal College of Pharmaceutical Sciences  Ceric ammonium sulphate Standardized using ferrous ammonium sulphate arsenic trioxide or anhydrous potassium ferrocyanide. M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Advantages Ceric over Potassium permanganate Ceric solutions are remarkably stable for over prolonged period. They need not be protected from light & can boiled for a short time without appreciable change in concentration. The stability of acid solution of cerium is better than that of permanganate solution. Ceric can be used even in presence of high concentration of hydrochloric acid. Ceric is not intensely colored to obstruct the visualization of meniscus in volumetric apparatus. M A N I PA L The reaction of ceric is simple and single unlike permanganate. U N I V E R S I T Y Ce+4 + e Ce+3 Manipal College of Pharmaceutical Sciences  Ferroin is used as an indicator and the color change is from red to blue. O-phenanthroline-ferrous iron (Ferroin) is a bright red complex formed by the combination of the base o-Phenanthroline with ferrous ions. This complex is readily oxidized reversibly to the corresponding ferric complex, which is pale blue in color. + Ce4+ + Ce3+ N N N N M A N I PA L Fe3+ U N I V E R S I T Y Fe2+ Red in colour Blue in colour Manipal College of Pharmaceutical Sciences  STANDARDIZATION OF 0.1M Ce(SO4)2 1. Preparation of 0.1M Ce(SO4)2.2(NH4)2. SO4. 2H2O: Dissolve 64 gm of Ceric Ammonium Sulphate with the aid of gentle heat in a mixture of 30 ml of H2SO4 and 500ml of water. cool, filter the solution if turbid and dilute to 1000ml with water. M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences Procedure: Weigh accurately about 0.2gm of As2O3 (Arsenic Trioxide), previously dried at 105˚ for 1 hour, and transfer to a 500ml conical flask. Wash down the inner walls of the flask with 25ml of 8% w/v NaOH solution, swirl to dissolve and add 100ml of water and mix. Add 30ml of dilute H2SO4 , 0.15 ml osmic acid, 0.1 ml ferroin sulfate solution and titration with Ce(SO4)2.2(NH4)2. SO4. 2H2O solution (0.1M) until the pink color is changed to a very pale blue color. M A N I PA L IP/ EQUIVALENT FACTOR: U N I V E R S I T Y Each ml of 0.1M Ce(SO4)2.2(NH4)2. SO4. 2H2O solution = 0.004946gm of As2O3. Manipal College of Pharmaceutical Sciences  CALCULATION Wt. taken x Expected 𝑀 Molarity of Ce(SO4)2.2(NH4)2. SO4. 2H2O solution = Titration vol x IP factor M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Applications of Ceric titrations Estimation of Ferrous sulphate Assay of Ferrous sulphate tablets Ferrous fumarate tablet Ferrous Gluconate and tablets Ferrous Succinate and tablet Estimation of Ascorbic acid tablet Assay of Iron-Dextran Injection M A N I PA L Assay of Acetaminophen U N I V E R S I T Y Assay of Acetaminophen tablets Manipal College of Pharmaceutical Sciences  Iodine Titrations Oxidizing agent Classified as ✓ Iodometry ✓ Iodimetry Direct iodine titration method termed as Iodimetry and titrations with a standard solution of iodine. Indirect iodine titration method termed as Iodometry deals with the titration of iodine M A N I PA L liberated in chemical reaction from iodide. U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Iodimetry Standard Iodine is the titrant. Preparation of 0.05M Iodine solution: Dissolve about 14g of iodine in a solution of 36g of potassium iodide in 100ml of water, add 3 drops of hydrochloric acid and dilute with water to 1000ml. Elemental iodine is slightly soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C. Potassium iodide may be added to increase solubility via formation of triiodide ions. Store in amber colored glass stoppered bottles. M A N I PA L Iodine solutions can be standardized against Arsenic trioxide or standard sodium U N I V E R S I T Y thiosulfate or barium thiosulfate monohydrate. Manipal College of Pharmaceutical Sciences  STANDARDIZATION OF IODINE USING ARSENIC TRIOXIDE Solution of iodine is standardized using arsenic trioxide as primary standard. The Arsenic trioxide is converted to the sodium arsinite by the treatment with sodium hydroxide. Sodium arsinite on treatment with dilute hydrochloric acid is converted to arsenious acid. Iodine oxidises arsenious acid to pentavalent state (arsenious acid) and iodine itself gets reduced to hydrogeniodide. The liberated hydrogen iodide being a strong reducing agent makes the reaction reversible. M A N I PA L U N I V E R S I T Y The reaction is made to go with right by removal of hydroiodic acid with the help of sodium bicarbonate. Manipal College of Pharmaceutical Sciences  As2O3 + 6NaOH 2Na3AsO3 + 3H2O Arsenic trioxide Sodium Arsinite 2Na3AsO3 + 6HCl 2H3AsO3 + 6NaCl Arsenious acid H3AsO3 + I2 + H2O H3AsO4 + 2HI Arsenious acid (pentavalent ) HI + NaHCO3 NaI + H2O + CO2 M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Iodometry Titrations Is the titration of iodine liberated in a chemical reaction with a standard solution of a reducing agent Oxidizing analyte added to I- to produce I2 which is then titrated with standard thiosulfate, starch is added only near the endpoint. M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Conditions for Iodometric Determinations 1. The potential of I2/2I- system is not high, and therefore many iodometric reactions are reversible and do not go to completion; only if suitable conditions are provided do they proceed practically to the end 2. Since iodine is volatile, the titration is conducted in the cold. This is also necessary because the sensitivity of starch as indicator diminishes with rise of temperature. If a starch solution turned blue by a single drop of iodine is heated, the blue color M A N I PA L disappears when the solution is cooled the color returns. U N I V E R S I T Y Manipal College of Pharmaceutical Sciences 3. Iodometric titrations cannot be performed in strongly alkaline solutions, because iodine reacts with alkaline in accordance with the equation I2 + 2NaOH NaIO + NaI + H2O The presence of hypoiodide (IO- ions) is inadmissible, because it is stronger oxidant than I2 and partially oxidizes thiosulphate to sulphate M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences 4. The higher the OH- concentration in solution, the more thiosulphate is converted into sulphate. 5. This side reaction makes exact calculation of the analytical result impossible. 6. Care must therefore be taken that the solution pH does not exceed 9 7. Since the solubility of iodine in water is low, a considerable excess of KI must be used in Iodometric determinations of oxidizing agents. The iodine liberated by the reaction then dissolves by forming the unstable complex salt K[I3] with MA KIN I P A L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences 8. Despite the use of large amounts of KI and acid, the rate the reaction between the oxidant and I- ions is usually too low. Therefore, some time is generally allowed to elapse after addition of the oxidant before the liberated iodine is titrated. 9. When the reaction mixture is left to stand before the start of the titration it is kept in a dark place, because light accelerates the side reaction in which I- ions are oxidized to I2 M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences 11. Sodium thiosulphate is the titrant for the liberated iodine in Iodometry. 12. Molecular weight of Sodium thiosulphate (Na2S2O3, 5H2O) is 248.19. 13. It is a secondary standard and standardized using potassium dichromate or potassium iodate. K2Cr2O7 + 6KI + 7H2SO4 Cr2(SO4) 3 + 3I2 + 4K2SO4 + 7H2O I2 + 2Na2S2O3 2NaI + Na2S4O6 M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences  Applications of Iodine Titrations Assay of ascorbic acid/Vitamin C by Iodimetry M A N I PA L U N I V E R S I T Y Ascorbic acid Dehydroascorbic acid Manipal College of Pharmaceutical Sciences  Assay of Copper sulphate: (By iodometry) The determination of copper compounds depends on the instability of cupric iodide formed by the reaction between the copper salt and potassium iodide. In the presence of acetic acid, copper sulphate undergoes oxidation with potassium iodide to form unstable cupric iodide. The unstable cupric iodide decomposes quantitatively to form cuprous iodide and equivalent amount of iodine. The liberated iodine is then determined against M A N I PA L standard sodium thiosulfate, by using starch solution as an indicator towards the U N I V E R S I T Y end point. Manipal College of Pharmaceutical Sciences  Assay of Copper sulphate: (By iodometry) The decomposition of cupric iodide is reversible reaction, which can be prevented by converting it to stable cuprous thiocyanate by the addition of potassium thiocyanate. Reactions of copper sulphate Assay: Acetic acid CuSO4 + 4KI 2CuI2 + 2K2SO4 Cupric iodide 2 CuI2 Cu2I2 + I2 Cuprous iodide M A N I PA L Cu2I2 + 2KSCN 2CuSCN + 2KI U N I V E R S I T Y Cuprous thiocyanate I2 + 2Na2S2O3 2NaI + Na2S4O6 Manipal College of Pharmaceutical Sciences  ASSAY OF CHLORINATED LIME/ BLEACHING POWDER/ CALCIUM HYPOCHLORIDE/ DETERMINATION OF AVAILABLE CHLORINE IN HYPOCHLORITES (By Iodometry) An aqueous suspension of chlorinated lime is treated with acetic acid; in the presence of excess of potassium iodide acetic acid liberates chlorine from chlorinated lime. The liberated chlorine displaces an equivalent amount of iodine from potassium/H iodide. The liberated iodine is treated with standard sodium thiosulphate solution using starch as indicator. Standardize the sodium thiosulphate using potassium iodate as primary standard. M A N I PA L U N I V E R S I T Y Hypochlorous acid Manipal College of Pharmaceutical Sciences  Determination of Arsenic Assay of phenol Assay of Isoniazid Assay of sodium thiosulphate Assay of sodium thiosulphate injection Assay of Thyroid Assay of Thyroid tablets Assay of Povidone- iodide solution. M A N I PA L U N I V E R S I T Y Manipal College of Pharmaceutical Sciences