Oxidation – Reduction Titration: Determination of Iron

6 June 2016

Abstract: The purpose of this experiment was to determine the percentage purity of the iron(II) salt in an unknown sample. We performed titrations using Potassium permanganate which is a strong oxidizing agent, with unknown sample dissolved in deionized water. The result of the experiment was a 99.5% purity for the anhydrous iron (II) ammonium sulfate.

Introduction: In this experiment, oxidation/reduction (or redox) will be used in the titration analysis of an iron compound. We will use potassium permanganate, KMnO4, as the titrant in the analysis of an unknown sample containing iron to determine the percent iron by mass in the sample. In acidic solution, potassium permanganate rapidly and quantitively oxidizes iron(II) to iron (III), while itself being reduced to manganese(II). (2) The half reactions for the process are:

MnO4+8H + 5e→Mn2++4H2O reduction Fe2+ → Fe3+ + e− Oxidation When these half reaction are combined to give the overall balanced chemical reaction equation, a factor of five must be used with the iron half-reaction so that the number of electrons lost in the overall oxidation will equal the number of electrons gained in the reduction:

MnO4+8H + Fe2+ →Mn2++4H2O + 5Fe3

Potassium permanganate is one of the most commonly used oxidizing agents because it is extremely powerful, inexpensive and ready available. (1) it does have some drawbacks however. Because KMnO4 is a strong oxidizing agent, it reacts with practically anything that can be oxidized. This tends to make solutions of KMnO4, difficult to store without it decomposing or changing in concentration. Because of this limitation, it is common to prepare a standardized, (3) and then use KMnO4 solutions for analysis all on the same day. It is not possible to prepare a KMnO4 standard solution based on a mass: Solid potassium permanganate cannot be obtained in a completely pure state due to the high reactivity mentioned above.

Thather,potassium permanganate solutions are prepared to be an approximate concentration, and are then standardized against a known primary standard sample of the same substance which is to be analyzed in the unknown sample. The KMnO4 provided for this experiment has been standardized for us.

Potassium permanganate is particularly useful among titrants since it requires no indicator to signal the endpoint of a titration. Potassium permanganate solutions-even at fairly dilute concentrations-are intensely coloured purple. The product of the permanganate reduction half-reaction, manganese (II), in dilute solution shows almost no colour. Therefore, during a titration using KMnO4, when one drop excess of potassium permanganate has been added to the sample, the sample will take on a pale red/pink colour.

Materials Required

•Potassium permanganate solution •Burette •Deionized water •Clamp •Beakers •Magnetic stirrer •Weighing paper •Sulfuric acid •250 mL Erlenmeyer flask •Graduated cylinder •Analytical balance •Pipette •Analytical balance

Methods: Standardization fo potassium Permanganate 1 Obtain two 0.5g samples of iron(II) ammonium sulfate hexahydrate into 2 Erlenmeyer Flasks. Record the weight. 2 Add deionized water and 25mL 3M of H2SO4 to each flask. 3 Fill the burette with potassium permanganate stock solution, and titrate it with the 4Repeat steps above with the 2nd sample. The volume per gram should agree within 0.5mL/g, otherwise a thrird titration is needed. The percentage purity of the iron(II) Salt

1Obtain the unknown sample and weigh out to 1.0g 2Dissolve the solid in deionized water and transfer the solution to a 250.0mL volumetric flask. 3Pipette 50.00mL of the iron solution into an Erlenmeyer flask and add 25mL H2SO4. Then titrate the solution with potassium permanganate until a faint pink colour appears. 4Repeat the titration and the two volumes must agree to within 0.3mL. If not, do a 3rd


Data: Table of standardization of Potassium Permanganate

Mass (g) Mass (g)

Mass (g) Mass of FeSO4(NH4)2SO4 and weighing boat0.4494 g0.4871g0.4800g Mass of the weighing boat 0.02640.02180.0244 Mass of FeSO4(NH4)2SO40.4494g0.4871g0.4800g Volume (mL) Volume (mL) Volume (mL)

Final Burette reading23.85 31.9032.10 Initial burette reading00.30 7.207.80 Volume of Potassium permanganate23.55 24.7024.30 Ratio (mL/g)Ratio (mL/g) Ratio (mL/g)

Titration volume per gram of Fe.A.S.49.12350.70850.625

Mass of impure Iron(II) salt = 1.322g Table of standardization of the unknown sample Volume (mL)Volume (mL)Volume (mL)

0.6119.3228.21 Initial Burette reading10.4110.4119.32 Volume of KMnO49.808.918.89

Discussion: In part 1: standardization of the potassium permanganate, as expected, a faint pink colour indicating the endpoint appeared after titrating with 24.50mL of titrant. This shows that the half reactions:

MnO4- + 4H+ +3e-  MnO2 + 2H2O Fe2+ → Fe3+ + e− The Permanganate was reduced to form MnO2 indicated by a change from purple colour, while the Iron was oxidized to form an iron (III) ion. In the 2nd part: The percentage purity of iron (II) Salt , an unknown sample of Impure Iron (II) salt was weighed to 1.322g. This sample was then diluted in a 250mL of deionized water and titrated with the potassium permanganate until a faint colour was observed. A total of 8.90mL of titrant was used until a faint pink colour was observed. Conclusion:

The percentage of anhydrous iron (II) ammonium sulfate in the unknown sample was found to be 99.5%.

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