Version 2012Updated on 0510 Copyright © All rights reserved Dong-Sun Lee, Prof., Ph.D. Department of Chemistry, Seoul Women’s University Chapter 18 Applications of Redox Titrations

Linus Pauling ( ) His work in chemical bonding, X-ray crystallography, and related areas had a tremendous impact on chemistry, physics, and biology. He is the only person to receive two unshared Nobel prizes: for chemistry(1954) and for his efforts to ban nuclear weapons, the peace prize (1962). This photo of Pauling tossing an orange into the air is symbolic of his work and importance of being able to determine concentrations of ascorbic acid at all levels in fruits and commercial vitamin preparations. Redox titrations with iodine are widely used to determine ascorbic acid.

Preadjustment of analyte oxidation state It is necessary to adjust the oxidation state of the analyte to one that can be titrated with an auxiliary oxidizing or reducing agent. Ex. Preadjustment by auxiliary reagent Fe(II), Fe(III) Fe(II) 4 – Titration Ce 4+ Preoxidation : Peroxydisulfate ( (NH 4 ) 2 S 2 O 8 ) 2– ) Sodium bismuthate ( NaBiO 3 ) Hydrogen peroxide (H 2 O 2 ) Prereduction : Stannous chloride ( SnCl 2 ) Chromous chloride Jones reductor (zinc coated with zinc amalgam) Walden reductor ( solid Ag and 1M HCl)

Jones reductor : 2Zn (s) + Hg 2+  Zn 2+ + Zn(Hg) (s)

Reagents used in redox titration Reducing agents Ferrous salts : ammonium iron(II) sulfate hexahydrate (Mohr’s salt) FeSO 4 (NH 4 ) 2 SO 4 · 6H 2 O iron(II) ethylene diamine sulfate (Oesper’s salt) FeC 2 H 4 (NH 3 ) 2 (SO 4 ) 2 · 4H 2 O Sodium thiosulfate pentahydrate Na 2 S 2 O 3 ·5H 2 O Arsenic trioxide: arsenious oxide As 2 O 3 Sodium oxalate and oxalic acid dihydarte Na 2 (COO) 2, (COOH) 2 ·2H 2 O Titanium trichloride TiCl 3 Potassium ferrocyanide K 4 Fe(CN) 6 · 3H 2 O

Sodium thiosulfate, Na 2 S 2 O 3 Thiosulfate ion is a moderately strong reducing agent that has been widely used to determine oxidizing agents by an indirect procedure that involves iodine as an intermediate. With iodine, thiosulfate ion is oxidized quantitatively to tetrathionate ion according to the half-reaction: 2S 2 O 3 2–  S 4 O 6 2– + 2e E o = 0.08 Ex. Determination of hypochlorite in bleaches [CaCl(OCl)H 2 O]: OCl – + 2I – + 2H +  Cl – + I 2 + H 2 O (unmeasured excess KI) I S 2 O 3 2–  2I – + S 4 O 6 2– Indicator: soluble starch (  -amylose)

Standardization of thiosulfate solution: Primary standard : potassium iodate (KIO 3 ), K 2 Cr 2 O 7, KBrO 3 Titration reactions: KIO 3 + 5KI + 6HCl  3I 2 + 6KCl + 3 H 2 O I 2 + 2Na 2 S 2 O 3  2NaI + Na 2 S 4 O 6 KIO 3  3I 2  6Na 2 S 2 O 3 ·5H 2 O  6 Equivalent mw g  6 × g g / 6  1 N × 1000 ml g  1 N × 1000 ml a g  x N × V ml x N = ( a g × 1 N × 1000 ml) / (35.67 g × V ml) Stabilizer for sodium thiosulfate solution : Na 2 CO 3 Na 2 S 2 O 3 + H 2 O + CO 2  Na 2 CO 3 + H 2 S 2 O 3 H 2 S 2 O 3  H 2 SO 3 + S

Calculations  Equivalent weight = ( formula weight) / ( e – change) Equivalents = g / eq. wt. meq = mg / eq. Wt. Normality (N) = eq / L = meq / ml Reaction eq. wt of reactant Fe 2+  Fe 3+ + e FW Fe ÷ 1 KMnO 4 + 5e  Mn 2+ FW KMnO 4 ÷ 5 Na 2 S 2 O 3 5H 2 O  ½ S 4 O 6 – + e FW Na 2 S 2 O 3 5H 2 O ÷ 1 Cr 2 O 7 2 – + 6e  2 Cr 3+ FW Cr 2 O 7 2 – ÷ 6

Molecular model of thiosulfate ion.

View down the starch helix, showing iodine, inside the helix Structure of the repeating unit of the sugar amylose. Schematic structure of the starch-iodine complex. The amylose chain forms a helix around I 6 unit.

Arsenious oxide, As 4 O 6 As 4 O 6 + 6H 2 O = 4H 3 AsO 3 H 3 AsO 3 + I 3 – + H 2 O = H 3 AsO 4 + 3I – + 2H + The As 4 O 6 molecule consists of an As 4 tetrahedron with a bridging oxygen atom on each edge

Reagents used in redox titration Oxidizing agents Potassium permanganate KMnO 4 : Permanganometry Ceric sulfate / Ceric ammonium sulfate Ce(SO 4 ) 2 ·2(NH 4 ) 2 SO 4 · 4H 2 O : Cerimetry Potassium dichromate K 2 Cr 2 O 7 : Dichrometry Iodine I 2 : Iodimetry, Iodometry Potassium iodate KIO 3 : Iodatimetry Potassium bromate KBrO 3 : Bromatimetry Sodium nitrite NaNO 2 : Calcium hypochlorite Ca(ClO) 2 :

Permanganate titration Oxidation with permanganate : Reduction of permanaganate KMnO 4 Powerful oxidant that the most widely used. In strongly acidic solutions (1M H 2 SO 4 or HCl, pH  1) MnO 4 – + 8H + + 5e = Mn H 2 O E o = 1.51 V violet color colorless manganous KMnO 4 is a self-indicator. In feebly acidic, neutral, or alkaline solutions MnO 4 – + 4H + + 3e = MnO 2 (s) + 2H 2 O E o = V brown manganese dioxide solid In very strongly alkaline solution (2M NaOH) MnO 4 – + e = MnO 4 2 – E o = V green manganate 

Standardization of KMnO 4 solution Potassium permanganate is not primary standard, because traces of MnO 2 are invariably present. Standardization by titration of sodium oxalate (primary standard) : 2KMnO Na 2 (COO) 2 + 8H 2 SO 4 = 2MnSO 4 + K 2 SO 4 + 5Na 2 SO CO 2 + 8H 2 O 2KMnO 4  5 Na 2 (COO) 2  10 Equivalent mw mw g / 5  g / 2  1 Eq g  g 1N × 1000 ml  g x N × V ml a g x N = ( a g × 1N × 1000 ml) / ( g × V ml)

Preparation of 0.1 N potassium permanganate solution KMnO 4 is not pure. Distilled water contains traces of organic reducing substances which react slowly with permanganate to form hydrous managnese dioxide. Manganesse dioxide promotes the autodecomposition of permanganate. 1) Dissolve about 3.2 g of KMnO 4 (mw=158.04) in 1000ml of water, heat the solution to boiling, and keep slightly below the boiling point for 1 hr. Alternatively, allow the solution to stand at room temperature for 2 or 3 days. 2)Filter the liquid through a sintered-glass filter crucible to remove solid MnO 2. 3)Transfer the filtrate to a clean stoppered bottle freed from grease with cleaning mixture. 4)Protect the solution from evaporation, dust, and reducing vapors, and keep it in the dark or in diffuse light. 5)If in time managanese dioxide settles out, refilter the solution and restandardize it.

Applications of permanganometry (1)H 2 O 2 2KMnO H 2 O 2 + 3H 2 SO 4 = 2MnSO 4 + K 2 SO 4 + 5O 2 + 8H 2 O (2) NaNO 2 2NaNO 2 + H 2 SO 4 = Na 2 SO 4 + HNO 2 2KMnO HNO 2 + 3H 2 SO 4 = 2MnSO 4 + K 2 SO 4 + 5HNO 3 + 3H 2 O (3) FeSO 4 2KMnO FeSO 4 + 8H 2 SO 4 = 2MnSO 4 + K 2 SO 4 + 5Fe 2 (SO 4 ) 3 + 8H 2 O (4) CaO CaO + 2HCl = CaCl 2 + H 2 O CaCl 2 + H 2 C 2 O 4 = CaC 2 O 4 + 2HCl (excess oxalic acid) 2KMnO H 2 C 2 O 4 + 3H 2 SO 4 = 2MnSO 4 + K 2 SO CO 2 + 8H 2 O (back tit) (5) Calcium gluconate [CH 2 OH(CHOH) 4 COO] 2 Ca + 2HCl = CaCl + 2CH 2 OH9CHOH) 4 COOH (NH 4 ) 2 C 2 O 4 + CaCl 2 = CaC 2 O NH 4 Cl CaCl 2 + H 2 SO 4 = H 2 C 2 O 4 + CaSO 4 2KMnO H 2 C 2 O 4 + 3H 2 SO 4 = 2MnSO 4 + K 2 SO CO 2 + 8H 2 O

Oxidation with Ce 4+ Ce 4+ + e = Ce V in 1 N HClO 4 yellow colorless 1.61 V in 1N HNO V in 1N HCl 1.44 V in 1M HSO 4 Indicator : ferroin, diphenylamine Preparation and standardization: Ammonium hexanitratocerate, (NH 4 ) 2 Ce(NO 3 ) 6, (primary standard grade) Ce(HSO 4 ) 4, (NH 4 ) 4 Ce(SO 4 ) 4 ·2H 2 O Standardized with Sodium oxalate.

Applications of cerimetry (1) Menadione (2-methylnaphthoquinon: vitamin K 3 ) O O CH 3 OH CH 3 2 Ce(SO 4 ) 2 HCl, Zn Reduction (2)Iron 2FeSO (NH 4 ) 4 Ce(SO 4 ) 4 = Fe 2 (SO 4 ) 3 + Ce 2 (SO 4 ) (NH 4 ) 2 SO 4

Oxidation with potassium dichromate Cr 2 O 7 2– + 14H + + 6e = 2Cr H 2 O E o = 1.36 V K 2 Cr 2 O 7 is a primary standard. Indicator : diphenylamine sulphonic acid

Ex. Redox titration ( hydroquinone vs dichromate standard solution ) HOOH  OO+ 2H + + 2e E o = Cr 2 O 7 2– + 14H + + 6e  2 Cr H 2 O E o = HO OH + Cr 2 O 7 2– + 8H +  3 O O + 2 Cr H 2 O E o = E o cathode – E o anode = 1.33 – = 0.63 V K = 10 nEo/ = 10 6(0.63) / = redox indicator : diphenylamine colorless to violet Very large : quantitative : complete reaction

Iodimetry and iodometry Iodimetry : a reducing analyte is titrated directly with iodine. Iodometry : an oxidizing analyte is added to excess iodide to produce iodine, which is then titrated with standard thiosulfate solution. Its solubility is enhanced by complexation with iodide. I 2 + I – = I 3 – K = 7  10 2 

Bromatimetry KBrO 3 BrO 3 – + 5Br – + 6H +  3Br 2 + H 2 O 2I – + Br 2  I 2 + 2Br – I S 2 O 3 2–  2I – + 2S 4 O 6 2– Substitution reactions BrO 3 – + 5Br – + 6H +  3Br 2 + H 2 O 2I – + Br 2  I 2 + 2Br – I S 2 O 3 2–  2I – + S 4 O 6 2–

pH 4-9 Al HOC 9 H 6 N  Al(OC 9 H 6 N) 3 (s) + 3H + hot 4M HCl Al(OC 9 H 6 N) 3 (s)  3HOC 9 H 6 N + Al 3+ 3HOC 9 H 6 N + 6 Br 2  3HOC 9 H 4 NBr 2 + 6HBr 1 mol Al 3+  3 mol HOC 9 H 6 N  6 mol Br 2  2 mol KBrO 3

Addition reactions

Determining water with the Karl Fisher Reagent The Karl Fisher reaction : I 2 + SO 2 + 2H 2 O  2HI + H 2 SO 4 For the determination of small amount of water, Karl Fischer(1935) proposed a reagent prepared as an anhydrous methanolic solution containing iodine, sulfur dioxide and anhydrous pyridine in the mole ratio 1:3:10. The reaction with water involves the following reactions : C 5 H 5 NI 2 + C 5 H 5 NSO 2 + C 5 H 5 N + H 2 O  2 C 5 H 5 NHI + C 5 H 5 NSO 3 C 5 H 5 N + SO 3 – + CH 3 OH  C 5 H 5 N(H)SO 4 CH 3 Pyridinium sulfite can also consume water. C 5 H 5 N + SO 3 – + H 2 O  C 5 H 5 NH + SO 4 H – It is always advisable to use fresh reagent because of the presence of various side reactions involving iodine. The reagent is stored in a desiccant-protected container. The end point can be detected either by visual( at the end point, the color changes from dark brown to yellow) or electrometric, or photometric (absorbance at 700nm) titration methods. The detection of water by the coulometric technique with Karl Fischer reagent is popular.

Pyridine free Karl Fisher reagent In recent years, pyridine, and its objectionable odor, have been replaced in the Karl Fisher reagent by other amines, particularly imidazole. (1) Solvolysis 2ROH + SO 2  RSO 3 – + ROH 2 + (2) Buffering B + RSO 3 – + ROH 2 +  BH + SO 3 R – + ROH (3) Redox BI 2 + BH + SO 3 R – + B + H 2 O  BH + SO 4 R – + 2 BH + I –

Summary Preoxidation Oxidizing agent Reducing agent Redox titration Permanganometry Cerimetry Dichrometry Iodimetry Iodometry Iodatimetry Bromatimetry Redox indicator Iodine starch indicator Self indicator Karl Fisher titration Karl Fisher reagent

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