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5- Lab5- Permanganate and Dichromate Titration

5- Lab5- Permanganate and Dichromate Titration - Chem 223...

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Chem 223: Experiment 5 Redox Oxidation with Potassium Permanganate and Dichromate- Determination of Iron in an Ore. References : Harris: Chap. 16. Harvey. 9D Kealy: pp. 90-94 This experiment is partly based on the experiment 7- Chem 1502-Villanova University and Expt. 31- Chem 10 – UWI- Mona University. A. Redox Titration with Potassium Permanganate. I. Introduction: In this experiment, the percentage of iron in an unknown iron oxide sample is determined by titration with a standard solution of potassium permanganate. The overall reaction is: MnO 4 + 8 H + + 5 Fe 2+ Mn 2+ + 5 Fe 3+ + 4 H 2 O To ensure that all of the iron in the unknown is in the Fe 2+ state prior to titration, the unknown is treated with tin(II) chloride, which reduces any small amount of Fe 3+ that might be present: Sn 2+ + 2 Fe 3+ Sn 4+ + 2 Fe 2+ The excess Sn 2+ is then removed by reaction with mercury(II) chloride, which produces insoluble mercury(I) chloride: Sn 2+ + 2 HgCl 2 Sn 4+ + Hg 2 Cl 2 (s) + 2 Cl The potassium permanganate is previously standardized against sodium oxalate in acid solution: 2 MnO 4 + 5 H 2 C 2 O 4 + 6 H + 2 Mn 2+ + 10 CO 2 + 8 H 2 O Potassium permanganate acts as its own indicator both in the standardization and in the iron determination. II. Reagents Provided: * Prepared 0.02 M KMnO 4 solution as follow: Dissolve about 3.2 g of KMnO 4 in 1 L of deionized water using a large beaker. Cover that beaker with a watch-glass and heat to boiling (using a hot plate in the hood first) to destroy reducible substances (traces of organics remaining in deionized water). Keep the solution at a gentle boil for about 1 hr. Let the solution stand overnight. Remove MnO 2 by filtering through a filter crucible. Transfer the solution to a clean amber glass-stoppered bottle; store in the dark when not in use. If the solution is kept in the dark and is not acidified, its concentration will remain stable for several weeks. Important: clean the filter and beakers using a solution of sodium bisulfite (use about 1g in 400 mL of water. Store this solution in an amber bottle ).
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Prepare 1 L of 1 M H 2 SO 4 by slowly adding 60 mL of concentrated acid to about 700 mL of water then diluting to 1 L. Dry about 1.5 g of primary-standard sodium oxylate, Na 2 C 2 O 4 , at about 110 °C for at least 1 hr . Cool in the desiccator. Dry the unknown iron ore at 110 0 C for at least 3 hr . Keep it in the desiccator. A fresh solution of the SnCl 2 reducing agent needs to be prepared each lab session. Dissolve 6 g of SnCl 2 H 2 O in 10 mL of concentrated HCl, using a 100 mL volumetric flask. After the SnCl 2 dissolves, dilute to volume. Keep this solution tightly sealed to prevent air oxidation. Mercury(II) chloride, 5% (w/v) .: Dissolve 50 g of HgCl 2 in 1 L of distilled water . Zimmerman-Reinhard reagent or "preventive" solution: Dissolve 300 g of MnSO 4 . 4 H 2 O in 1 L of water. Cautiously add 400 mL of concentrated H 2 SO 4 , 400 mL of 85% H 3 PO 4 , and dilute to 3 L.
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