Redox Titrations PDF

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Redox Titrations  Redox titration is a volumetric method of analysis which relies on oxidation or reduction of the analyte.  Redox titrations involve the titration of an oxidizing agent with a reducing agent, and vice versa.  An oxidizing agent gains electrons  A reducing agent loses electrons  A half-reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.  A half-reaction does not occur by itself, at least 2 reactions must be coupled so that the electron released by one electron is accepted by another.  Therefore oxidation and reduction reactions must take place simultaneously. This is called redox reaction.  An oxidizing agent and reducing agent which appear on opposite sides of a halfequation constitute a redox couple.  For every oxidizing agent there is a corresponding reducing agent, and every reducing agent a corresponding oxidizing agent.  Redox couples are analogous to conjugate acid-base pairs and behave in the same way.  The stronger an oxidizing agent, the weaker the corresponding reducing agent.  The stronger the reducing agent, the weaker the corresponding oxidizing agent. The redox titrations based on redox reactions can be expressed by the equation:

ox1 + red2

⇄

ox2 + red1

 Changing property of the solution is its redox potential. There must be a sufficiently large difference between the oxidizing and reducing capabilities of the reactants for the reaction to go to completion and give a sharp end point.  One should be a strong oxidizing agent and the other, a strong reducing agent.

Nernst Equation

E = Eo + 0.059 lg (ox)a (red)b

n

 E = redox potential of the couple  N = redox capacity ( no. of electrons an oxidizing agent will take on or a reductant will supply)  Eo = standard redox potential at 25C  (ox) and (red) = molar concentrations of both the oxidized and reduced forms of the couple  a and b = stoichiometric coefficients

Using the redox capacity, n, the equivalent mass of the oxidizers and reductants can be determined: Eqmass = M(ox or red) n

 The equivalence point is found when the moles of the oxidizing and reducing agent are combined in proper stoichiometric ratio.  Most redox titrations are detected by using redox indicators - highly coloured dyes that are weak reducing or oxidizing agents.  In some redox titrations there is no need for a special indicator. The redox reagents themselves are distinctly coloured.  For example, permanganate has a strong colour and a small excess is immediately visible.  Unlike neutralization reactions, a lot of redox reactions are not sufficiently fast and special care should be taken - heating of a solution, using a catalyst etc.  There should be no side reactions or interfering substances.

 4Redox titrations are important analyses performed in many areas of application, fo example, food and pharmaceutical.

Permanganometry  Potassium permanganate (KMnO4) - One of the strongest oxidizing agents  In strongly acidic solutions permanganate is reduced to colourless Mn2+  In neutral or slightly alkaline solution, the product is a brown solid, MnO2  In very strong alkaline solution, green manganate ion is formed (MnO4)

In neutral medium the reaction proceeds according to the equation:

MnO4- + 2H2O = MnO2 + 4OHEqmass = MKMnO4 and EMnO4/MnO2 = +0.54V 3

 Permanganate titrations do not require use of indicators.  Permanganate itself has a very intense, purple colour and small excess of the titrant is usually enough to colour the solution and to signal end point.  Permanganate reduction is very slow at room temperature, but is enhanced by the presence of an acid and temperature of 70-80C  Titrants are usually performed in sulphuric acid medium because nitric acid is also a strong oxidizing agent.  It can be used to determine the amount of Fe2+, H2O2, As3+ etc.  Potassium permanganate is one of the most commonly used oxidizing agents as it it extremely powerful, inexpensive and readily available  A drawback is because it is such a strong oxidizing agent, it reacts with practically anything that can be oxidized.  It cannot be used as a primary standard due to its reaction with contaminants in distilled water. It must be standardized against a primary standard reducing agent

directly prior to its use as an oxidizing agent.  The more frequently used solutions of potassium permanganate are 0.01 N or 0.005 N.  When the solution is first prepared, small amounts of reducing impurities in the solution reduce a small amount of MnO4, producing MnO2  The MnO2 acts as a catalyst for further decomposition of the permanganate, which produces more MnO2, and so on. This is called autocatalytic decomposition.  Potassium permanganate can be standardized by titrating against primary standard oxalic acid. 

Permanganate is a very strong oxidizer thus extreme care must be taken in the handling, use and disposal of any excess solutions.

 Once permanganate is dissolved it should be boiled for an hour to oxidize organic contaminations  After colling is left to stand for 4-5 days  Then the solution is filtered through a sintered-glass filter, transfered to a dark bottle and diluted with distilled water till 1L  The solution is not stable so should be filtered every 2-3 months and should be standardized after each filtration  Also needed for the standardization is oxalic acid solution which has been accurately prepared by weighing the calculated quantity of pure substance and dissolving it in a volumetric flask with distilled water  With a pipette, transfer 25ml of oxalic acid into a Erlenmeyer flask and add 5ml of 2 N H2SO4  The mixture is heated to 70-80C and titrated with potassium permanganate solution  As soon as the titration is complete, a fraction of a drop excess MnO4 solution imparts a definite pink colour indicating the reaction is complete....