To determine the Ferrous content in the supplied sample of iron ore by titrimetric analysis against standard K2Cr2O7 solution using K3Fe(CN)6 as external indicator

Introduction

Potassium dichromate acts as an oxidizing agent (E0red= +1.33V) in presence of an acid and each molecule of it gives up three atom of oxygen available for oxidation of the reducing agent. Potassium dichromate is a primary standard and it is less powerful oxidizing than KMnO4 (E0red= +1.33V). Oxidizing character of these species depends on pH of the solution, KMnO4 as an oxidizing agent in all acidic, basic and neutral medium but having most oxidizing character in acidic medium. Unlike KMnO4, K2Cr2O7 behaves as oxidizing agent in acidic medium only according to the following reaction

$$\text{K}_2\text{Cr}_2\text{O}_7 + 4\text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + \text{Cr}(SO_4)_3 + 4\text{H}_2\text{O} + 3[O]$$

When acidic K2Cr2O7 is added an iron ore sample containing Fe2+ ions, Fe2+ get oxidized to Fe3+ ions according to the following reaction

$$\text{K}_2\text{Cr}_2\text{O}_7 + 4\text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + \text{Cr}(SO_4)_3 + 4\text{H}_2\text{O} + 3[O]$$

In this titration K3Fe(CN)6 is used as an external indicator it gives blue color on reaction. If ferrous ions (Fe2+) are present, a strong blue color is developed due to

$$2\text{K}_3[Fe(CN)_6] + \text{FeSO}_4 \rightarrow \text{Fe}_3[Fe(CN)_6]_2 + 3\text{K}_2\text{SO}_4$$

At the end point no Fe2+ ions are present in the solution, so indicator fails to produce a blue color.

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Fig. 1 Titration of FAS solution against K2Cr2O7 solution

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Fig. 2 K3Fe(CN)6 as an External Indicator

Applications of Determination of Ferrous Content


1. Quality Control in Iron and Steel Production

2. Metallurgical Process Optimization

3. Environmental Monitoring and Compliance

4. Research in Ore Characterization

5. Educational Purposes in Chemistry Labs