Determination of Iron in Dietary Supplement through
Redox Titrations
Gezahegn Chaka, Ph.D.
Collin College
Department of Chemistry
Introduction:
Dietary supplements contain a wide variety of vitamins and mineral nutrients. Among
those, iron plays a critical role in the human circulatory system. For example, the uptake
of oxygen and the removal of carbon dioxide from our blood depend on the hemoglobin
molecule, at the center of which is an atom of iron. It is well known that a deficiency of
iron in the blood can lead to a loss of hemoglobin and the onset of certain types of
anemia. Those cases may require the intake of iron supplements in the diet.
Oxidation-reduction (or redox) reactions are one type of the many chemical reactions.
In this experiment, a redox reaction will be used in the analysis of iron present in a
common dietary supplement sample.
In inorganic oxidation-reduction reactions, electrons are transferred from one species to
another. Consider the following simple reaction:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
(1)
In this reaction, electrons are transferred from zinc metal to aqueous copper(II) ions.
This can be seen more easily if the overall reaction is written in terms of two halfreactions.
Oxidation:
Zn(s) → Zn2+(aq) + 2e-
Reduction:
Cu2+(aq) + 2e- → Cu(s)
Addition of these two half-reactions provide the overall balanced equation for the redox
reaction. The number of electrons lost in the oxidation reaction is equal to the number
of electrons gained by the reduction reaction.
In this experiment, one of the most commonly used strong oxidizing agents known,
potassium permanganate (KMnO4), will be used to oxidize iron (II) to iron(III) while
itself being reduced to manganese(II) in acidic solution.
MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)
Copyright©2010 Gezahegn T. Chaka
(2)
Potassium permanganate has a unique advantage among titrants since it does not
require an indicator to signal the end-point of the titration. Even a very dilute solution
of potassium permanganate has an intense purple color and during a titration using
KMnO4, an excess drop of potassium permanganate turns the color of the solution pink
marking the end point.
However, permanganate solutions tend to decompose with time, which causes a change
in concentration. This limitation is circumvented by standardizing a KMnO4 solution
using a known primary standard sample. One method of standardizing KMnO4 solution
is using the primary standard salt iron(II) ammonium sulfate hexahydrate,
FeSO4∙(NH4)2SO4∙6H2O. A fresh, standardized KMnO4 solution will be provided for use
in this experiment.
Objective:
You will be determining the mass percentage of iron(II) in a common iron supplement
sample.
Brief Procedure:
Determination of Fe2+
Accurately weigh a sample of a dietary iron supplement. Place the sample in a clean 250
mL Erlenmeyer flask. Fill a clean buret with the standard KMnO4 solution (make sure to
rinse the buret three times with a few milliliters of the KMnO4 solution and disposing of
the used KMnO4 in the appropriate waste container). Add 50 mL of 1.0 M H2SO4
solution to the iron sample and dissolve it completely. Immediately titrate the iron
solution with the standard KMnO4 solution. As the permanganate solution is added to
the sample, a light yellow color develops in the iron solution. Add 3 mL of 85% H3PO4 to
the sample solution and continue the titration, swirling the flask constantly until the
first appearance of a pale pink color that persists for 30 seconds. Repeat the titration
twice with other iron samples of the same unknown.
Pre-lab Questions:
1.
2.
3.
4.
Write the two half-reactions for the redox reaction shown by equation (2).
How many moles of electrons are transferred per mole of reaction in question 1?
To what does the term strong oxidizing agent refer? Provide examples.
What is one disadvantage of using the very strong oxidizing agent potassium
permanganate as a titrant?
5. What is the purpose of adding the sulfuric acid? (Hint: consider the products of the
reaction and their properties)
6. What is the purpose of adding the phosphoric acid at some point of the titration?
7. Provide a step by step procedure for the standardization of a KMnO4 solution.
Copyright©2010 Gezahegn T. Chaka
8. If 45.52 mL of a potassium permanganate solution is needed to titrate 2.145 g of
iron(II) ammonium sulfate hexahydrate, calculate the molarity of the KMnO4
solution.
9. A 1.545 g solid sample of an unknown containing iron(II) in the dissolved sample
requires 32.85 mL of a 0.025 M KMnO4 solution to reach the pink end point of the
titration. Calculate the mass of iron in the sample (in grams) and the percentage of
iron in the sample.
Critical Data and Discussion to Include in the Lab Report:

Data table that includes:
Molarity of standard KMnO4 solution
Mass of iron sample
Final buret reading
Initial buret reading
Volume of KMnO4 used
Moles of KMnO4
Moles of Fe2+
Mass of Fe2+ in the sample
Percentage of Fe2+ in sample
Average percent of Fe2+ in sample
Relative average deviation

Calculations from the data

Discussion of the results and possible sources of error

Work cited
Reference:
Harris, D.C; Quantitative Chemical Analysis, 7th Ed., W.H. Freeman and Company, NY,
2007, pp 336-337.
Copyright©2010 Gezahegn T. Chaka