Determination of Iron Content by Redox Titration

Determination of Iron Content by Redox Titration

Introduction

Compounds containing iron in the +2 oxidation state can be analyzed by redox titration with a strong oxidizing agent. Potassium permanganate is a strong oxidizing agent that will react with iron(II) ion in an acidic solution to quantitatively form the iron(III) and manganese(II) ions. Since the permanganate ion is deep purple and the manganese(II) ion is almost colorless, the permanganate ion acts as an internal indicator showing the titration end point when a faint pink color persists in the solution. Since potassium permanganate is very reactive, solutions containing this ion must be standardized using a known primary standard before they are used. In this experiment, you will standardize a solution of potassium permanganate and then use it to analyze various solids for iron(II) content.

Procedure

1.  Standardization of Permanganate Solution

In the first part of this experiment, you must standardize the permanganate solution (approximately 0.1M) using an acidified solution of ferrous ammonium sulfate hexahydrate, FeSO4.(NH4)2SO4.6H2O (or FAS) Dissolve about 0.1 g of solid FAS in an small Erlenmeyer flask with 5 mL of 3 M sulfuric acid and 10mL of distilled water. Titrate to the end point .Repeat.

2.  Determination of Iron Content in Unknown

Dissolve about 0.1 g of the unknown iron(II) salt in a small Erlenmeyer flask with 5 mL of 3 M H2SO4 and 10mL of water. Titrate to the end point. Repeat.

3.  Determination of iron(II) Content in Iron Pill

Obtain an iron pill, mass it, and grind it up with a mortar and pestle. Transfer 0.15 g of the solid to a small Erlenmeyer flask. Add 5 mL of 3M sulfuric acid and 10 mL of water. Dissolve the pill as much as possible. Titrate to the end point which may be more difficult to observe depending on the color of the pill. Repeat.

Calculations

1.  Write a balanced net ionic equation for the reaction between permanganate and iron(II).

2.  Calculate the concentration of the permanganate ion in the standardized solution.

3.  Determine the mass percent of iron(II) in the unknown salt and the mass in milligrams of iron(II) in the unknown salt.

4.  Determine the mass in mg of iron(II) in the iron pill. Calculate your percent error.

Questions

1.Unknown iron salts are typically dried in an oven before analyzed. Thoroughly explain how the percent mass of iron in the iron salt would be affected if the unknown had not been dried.

2. Predict how the calculated mass of iron would be affected (too high, too low, or not affected) by the following laboratory procedures and explain your answer.

a.  After rinsing the buret with distilled water, the buret was filled with standardized permanganate solution; the unknown iron salt solution was titrated to the end point.

b.  An air bubble passes unnoticed through the tip of the buret during titration.