Reduction-Oxidation

• Reduction-Oxidation

  • Involve a change in valence of the reacting substances
  • Also called redox
  • Occurs simultaneously in a chemical reaction
  • One cannot take place without the other
• Oxidation number

  • Also called oxidation state
  • Represents # of electrons lost or gained or unequally shared by an atom
  • Can be zero, + or –
• Oxidation number


  A number assigned to an element in a compound that represents the degree of oxidation of the element
• Rules for assigning oxidation numbers

  • All elements in their free state(uncombined w/ other elements) have an oxidation # of zero (Na, Ca, Cl2 etc)
  • H is +1, except in metal hydrides, where it is -1 (NaH, CaH2)
  • O is -2, except in peroxides, where it is -1 and in OF2 where it is +2
  • The metallic element in an ionic compound has a positive oxidation #
  • In covalent compounds, the negative oxidation # is assigned to the most electronegative atom
  • The algebraic sum of the oxidation # of the elements in a compound is zero
  • The algebraic sum of the oxidation # of the elements in a polyatomic ion is equal to the charge of the ion
• OXIDATION

  • converted to a higher state of oxidation
  • the reactant which loses electrons in an oxidation-reduction reaction is the REDUCING AGENT
  • Fe 2+→Fe 3+ + e
• REDUCTION

  • converted to a lower state of oxidation
  • the reactant which gains electrons is the OXIDIZING AGENT
  • Ce 4++ e →Ce 3+ (Cerium)
  • Fe 3++ e →Fe 2+
• Reducing Agents – for analysis of oxidizing agents:

  • Ferrous Ammonium Sulfate
  • Sodium thiosulfate
  • Oxalic acid
  • Potassium arsenite
  • Titanium chloride
  • Ascorbic acid
• Oxidizing Agents – for analysis ofreducing agents

  • Potassium permanganate
  • Potassium dichromate (K2Cr2O7 )
  • Potassium bromate
  • Potassium iodate
  • Potassium ferricyanide
  • Ceric sulfate (Ce(SO)4
  • Ferric ammonium sulfate
  • Iodine
  • Bromine
• The equivalent weights of OXIDIZING AGENTS:

  • Permanganate ion → manganous ion
  • MnO4¯ + 5 e → Mn 2+

  = reduced

  = gain of 5 e
• The equivalent weights of OXIDIZING AGENTS:

  • Dichromate ion to chromous ion
  • Cr2O7 2- + 6e →2Cr3+
  • = reduced
  • = gain of 6e
  • Iodine to iodide ion
  • Ceric to cerous ion
• The equivalent weights of OXIDIZING AGENTS:

  • Bromate ion to bromide ion
  • BrO3 -+ 6 e-→Br -
• Reducing agents:

  • Titanous ion → Titanic ion
  • Ti3+ → Ti 4+ + 1e
  • Ferrous ion → Ferric ion
  • Fe2+ → Fe3++ 1e
  • Arsenite ion → Arsenate ion
  • AsO 2 -+ H2O → AsO3 -+ 2H+ + 2e
• • Oxalate ion to carbon dioxide
  • C2O4 2-→2CO2 + 2 e
  • Thiosulfate ion to tetrathionate ion
  • 2S2O3 2 - →S4O6 -2 + 2e
• • Direct Titration Method: Assay of H2O2
  • Indirect Titration Method: Assay of Cherry Juice for Malic Acid –malic acid is converted to calcium salt, which is converted to insoluble calcium oxalate, the oxalate is then liberated and oxidized with permanganate; Assay of Manganese Dioxide, Titanium Dioxide
  • Residual Titration Methods: Assay of sodium nitrite
• 2 Types of Residual Titration w/ KMnO4:

  • 1.Titration in w/c an excess of standard KMnO4 soln. is employed to oxidize a substance and the amount in excess is determined by reduction w/:
  • a.excess standard oxalic acid
  • b.excess ferrous ammonium sulfate and back titration w/ more standard KMnO42.
  • 2. Titration in w/c an excess of standard oxalic acid soln. is added to the substance and the excess oxalic acid is titrated w/standard KMnO4
• CERIC SULFATE TITRATION:

  • A soln. of ceric sulfate in diluted sulfuric acid is a strong oxidizing agent and more stable than std. permanganate soln., provided sufficient sulfuric acid is present to prevent hydrolysis and precipitation of basic salts
  • Ferrous phenanthroline (ferroin) is the indicator,
  • Form complex rxn which is red in color
• Orthophenanthroline Test Solution:

  • Dissolve 150 mg of orthophenanthroline in 10 mL of soln. of ferrous sulfate prepared by dissolving 1.48 g of ferrous sulfate in 100 mL of water
  • Form complex ions known as ferroin (red color), strong oxidizing agent convert the ferrous to ferric complex, w/c has slight blue color
• Assay of Ascorbic Acid:

  • A solution of dichlorophenol-indophenol, w/c has been standardized using USP ascorbic acid reference standard, is used as a standard oxidizing agent in determination of ascorbic acid
  • No indicator is necessary since the oxidized form of diclorophenol-indophenol is blue in alkali and red in acid while reduced form is colorless
  • A soln. of ascorbic acid is titrated in the presence of metaphosphoric acid and acetic acid w/ dichlorophenol-indophenol standard soln. until a rose-pink color persisting for at least 5 s results
• IODIMETRY :

  • direct procedure
  • analysis of reducing agents (thiosulfates and arsenites)
  • titrant used: iodine
  • endpoint: colorless to blue
  • Ex. Assay of antimony potassium tartrate, ascorbic acid
  • Reducing Agents: Ferrous Ammonium Sulfate, Sodium thiosulfate, Oxalic acid, Potassium arsenite, Titanium chloride, Ascorbic acid
• IODOMETRY:

  • indirect procedure
  • analysis of oxidizing agents (ferric, cupric salts, available chlorine)
  • titrant used: sodium thiosulfate
  • endpoint: blue to colorless
  • Ex: assay of cupric sulfate, methionine, sodium hypochlorite
  • Oxidizing Agents – for analysis of reducing agents: Potassium permanganate, Potassium dichromate (K2Cr2O7 ), Potassium bromate, Potassium iodate, Potassium ferricyanide, Ceric sulfate (Ce(SO)4, Ferric ammonium sulfate, Iodine, Bromine
• Standard Solutions in Iodometric Method:

  • 0.1 N Iodine
  • 0.1 N sodium thiosulfate
  • 0.1 N potassium arsenite
• • Std. Iodine soln.: soln. of iodine in KI soln.
  • Starch Indicator soln.- Contains B-amylose
  • Koppeschaar’s soln.- 0.1 N Bromine solution
• Na2S2O3 (sodium thiosulfate)

  1. dissolved in water which was boiled for sterilization
  2. bacteria labile solution
• Indicators For REDOX:

  • Dichlorophenolindophenol
  • Dicyanobis – rxn is like orthophenanthroline
  • Diphenylamine
  • Iodine
  • Methyl orange
  • Nitrophenanthroline
  • Ortho-phenanthroline
  • Oxalic acid VS – this std soln is used w/out indicator; depends on decolorization of KMnO4
• Other Solutions Used in REDOX:

  • Potassium permanganate (KMnO4) VS –this colored soln is decolorized on being reduced, so a separate indicator is not required
  • Potassium thiocyanate – used in conjunction w/ ferric chloride VS, a red compound is produced at the endpoint
  • Starch Iodide Paste TS – 5% suspension of potato starch in 0.75% potassium iodide with zinc chloride preservative
  • Starch – Potassium Iodide TS – 0.5% KI in starch TS
  • Starch TS – a 0.5% suspension of arrowroot starch in water; a blue color is produced by starch in the presence of free iodine
• OXIDATION-REDUCTION METHODS W/0.1N BROMINE(Koppeschaar’s soln.):

  • In the assay of aniline, phenol and resorcinol, bromine is employed as an oxidizing agent in place of iodine
  • The standard solution employed does not contain bromine as such but rather an equivalent amount of potassium bromate and excess potassium bromide
  • Bromine is liberated when the solution is acidified
  • Ex. Assay of Phenol, assay of Ethacrynic Acid
• OXIDATION-REDUCTION METHODS WITH POTASSIUM IODATE:

  • KIO3 maybe used as the oxidizing agent in the assay of iodides, arsenites and other reducing agents
  • The method depends upon the formation of iodine monochloride in strong HCl soln.
  • Ex. Assay of KI
• DIAZOTIZATION ASSAYS WITH NITRITE:

  • The assays of sulfa drugs and other compounds containing an arylamino group by diazotization with sodium nitrite are not strictly iodometric methods
  • They maybe included at this point because the endpoint is determined by the liberation of iodine from iodide