Experiment # 5 Titrimetry II (Redox titration)

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1 Experiment # 5 Titrimetry II (Redox titration) I. PURPOSE OF THE EXPERIENT In this experiment the mass of Fe (Iron) in 1 liter of unknown solution (Iron II sulphate) will be determined by redox (reduction-oxidation) titrimetry. (A) A standard solution of is prepared and standardized against Potassium permanganate (KnO ) solution. (B) After this standardization the KnO solution becomes a secondary standard. It is then titrated with unknown solution. II. INTRODUCTION Oxidation-Reduction: It can be defined in many ways.. When an atom, either free or in a ecule or ion, loses electrons, it is oxidized; its oxidation state increases. When an atom, either free or in a ecule or ion, gains electrons, it is reduced; its oxidation state decreases. Reactions involving oxidation and reduction are referred to as oxidation-reduction reactions, or redox reactions. Ex: o o + 2 S + O 2 SO 2 Sulfur is oxidized in this reaction, its oxidation state increases from 0 to +, as indicated by the small numbers above the equation. As oxygen is reduced, its oxidation state decreases from 0 to 2. Species that gives up electrons to another reactant in an oxidation-reduction reaction is called reducing agent. It causes the other reactant to be reduced. The oxidizing agent in a redox reaction gains electrons and causes another reactant ( the reducing agent) to be oxidized. The oxidizing agent picks up electrons during a redox reaction, so it is reduced. In the above example: S is reducing agent and O 2 is oxidizing agent. 5

2 Redox titration In some respects, redox reactions are similar to acid-base reaction. For instance, redox reactions involve the transfer of electrons and acid-base reactions involve the transfer of protons. Just as an acid can be titrated with a base, we can titrate an oxidizing agent against a reducing agent, using the procedure as in acid-base titration. The equivalence point is reached when the reducing agent is completely oxidized by the oxidizing agent. Internal indicator In many cases, the oxidizing agent itself can be used as an internal indicator in a redox titration. This is particularly true of permanganate ion (no - ) because a permanganate solution, which is purple when acting as an oxidant in acid solution, is reduced to nearly colorless manganese (II) ion (n 2+ ). For the determination of iron (II) in solution, KnO can be used as the oxidizing agent. The redox reaction is carried out in dilute H 2 SO medium as there is no action of H 2 SO upon permanganate in dilute solution. The redox reaction between iron(ii) and permanganate ions is described below a) (aq) Fe (aq) + SO (aq) +1 - b) KnO (aq) K (aq) + no (aq) Fe (aq) 5 Fe (aq) + 5e - (1) Oxidation no (aq) + 8 H + (aq) + 5e - n (aq) + H 2 O (l) (2) Reduction 5Fe 2+ (aq) + no (aq) + 8H + 5Fe 3+ + n +2 + H 2 O (l) The e ratio is ole of no - : Fe 2+ 1 : 5 6

3 III. EXPERIENTA 3.1 Chemicals: Standard solution, Unknown KnO solution, Unknown solution, Dilute H 2 SO 3.2 Equipment: Titration set 3.3 Procedure Standardization of KnO solution Place the unknown KnO solution in the buret. Transfer10 m of the standard solution in the Erlenmeyer flask (conical flask) with the aid of the pipet. Add -5 m of dil. H 2 SO (use measuring cylinder). Warm the solution on a hot plate and swirl the flask continuously and titrate with the standard solution until 1 drop of the KnO solution produces a permanent pink tinge in the solution. This is the end point of the titration. Repeat the titrations until duplicate determinations agree within 0.05 m of each other Determination of unknown solution Titration between standard KnO solution ( previously determined in the first titration), and the unknown solution is carried out exactly in the same manner as in the first titration. In the second titration the unknown solution is used in place of standard solution ---. Notes: 1. For titrant and titrand volumes must be known exactly; therefore accurate volume measurements are required. Pipettes and burettes are used. 2. For measurement of dilute sulphuric acid (dil. H 2 SO ) accuracy is not required. easuring cylinder is used. 3. The permanganate end point is not permanent because excess permanganate ions react slowly with the relative large concentration of manganese (II) ions present at the end point. - 2nO + 3 n H 2 O 5nO 2(s) + H + Therefore it is possible to titrate warm, acidic solutions of reactants with permanganate without error provided the reagent is added slowly enough so that excesses do not accumulate. 7

4 IV. RESUTS and DISCUSSION (i) olarity of KnO solution (ii) olarity of unknown solution.. (iii) ass content of Fe in 1 iter of solution.(g / ) Discuss how this analysis can be useful in environmental chemistry and in industry. V. CONCUSION Fe content in the given unknown solution was determined. 8

5 1 st Titration. 10 m of Std. FeS0 solution 0.1 A m of KnO solution No. Initial volume / m Final volume / m Difference / m Rough Fine ean A m Calculation The net ionic equation between iron (II) ion and permanganate ion in dilute sulphuric acid medium is: 5Fe no + 8H + 5Fe 3+ + n 2+ + H 2 O (e ratio of : KnO 5:1) esof esofkno 5 1 KnO KnO 5 KnO KnO 1 xv 0.1x10m ( i) KnO " a" 5 V 5xAm KnO 9

6 2 nd Titration. B m of Std.KnO solution a 10 m of unknown solution No. Initial volume / m Final volume / m Difference / m Rough Fine Calculation ean B m esof esofkno 5 1 KnO KnO. V 5 KnO V 5 xv KnO KnO 5(" a" x" B" m) 10m ( ii) " y" " y". V e olarity mass olarity e armassx KnO mass armass ( iii) g of " Fe" " y" x55.85 g ) ( armassoffe g 55.85y 50

PURPOSE: 1. To illustrate an oxidation-reduction titration with potassium permanganate 2. To determine the percent mass of iron in an unknown.

PURPOSE: 1. To illustrate an oxidation-reduction titration with potassium permanganate 2. To determine the percent mass of iron in an unknown. PRINCIPLES: Oxidation and reduction reactions, commonly called