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.