How can We Determine the Actual Percentage of H2O2
in a Drugstore Bottle of Hydrogen Peroxide?
Background
Redox (reduction-oxidation) reactions include all chemical reactions in which atoms have
their oxidation state changed. Redox reactions are concerned with the transfer of electrons
between species. The term comes from the two concepts of reduction and oxidation. It can be
explained in simple terms:

Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or
ion.

Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or
ion
The oxidation alone and the reduction alone are each called a half-reaction, because two halfreactions always occur together to form a whole reaction. In redox processes, reducing
agents lose electrons and are oxidized, and the oxidizing agents gains electrons and are reduced.
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and
products in chemical reactions. It is founded on the law of conservation of mass: the mass of the
reactants equals the mass of the products. In a balanced chemical reaction, the relationship
between quantities of reactants and products typically form a ratio of positive integers.
Therefore, stoichiometry represents quantitative relationships among substances as they
participate in chemical reactions. Stoichiometry can be used to determine quantities such as the
amount of products (in mass, moles, volume, etc.) that can be produced with given reactants and
percent yield
Titration is a common laboratory method of quantitative chemical analysis that is used to
determine the unknown concentration of an identified analyte. Because volume measurements
play a key role in titration, it is also known as volumetric analysis. A reagent, called the titrant, is
prepared as a standard solution. A known concentration and volume of titrant reacts with a
solution of analyte or to determine concentration. In this lab, KMnO4 is the titrant (known
concentration) and ferrous ammonium sulfate is the analyte (unknown concentration)
Solutions used in lab
Hydrogen Peroxide (H2O2) is a very strong oxidizer. In normal conditions, the chemical exists in a
clear liquid state that is slightly more viscous than water. Due to its oxidizing properties, hydrogen
peroxide is often used as bleach, cleaning agent or disinfectant.
Hydrogen peroxide spontaneously decomposes into water and oxygen. The reaction is shown below
and is thermodynamically favorable.
2 H2O2 → 2 H2O + O2
The rate of decomposition is dependent on the temperature, pH and the solution’s concentration.
A lower temperature in the environment results in a slower rate of decomposition. It is important
to know the actual concentration of a solution of H2O2 as its effectiveness can decrease with
smaller concentrations.
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt
consisting of K+ and MnO4- ions. It dissolves in water to give intensely pink or purple solutions, the
evaporation of which leaves prismatic purplish-black glistening crystals.
Ferrous ammonium sulfate is the inorganic compound with the formula (NH4)2Fe(SO4)2·6H2O.
Containing two different cations, Fe2+ and NH4+, it is classified as a double salt of ferrous sulfate
and ammonium sulfate. The iron element in this compound has an ionic charge of 2+
Reaction between Ferrous ammonium sulfate and Potassium permanganate
When the two compound comes into contact with each other in an acidic environment (containing
an excess amount of H+ ions), a complex reaction happens. This reaction involves two essential
steps. First, the Iron (II) in ferrous ammonium sulfate will be oxidized into Iron (III). Next, the
MnO42- ion, which creates the purple color of the solution, will be reduced into virtually colorless
Mn2+ ions.
Reaction between Potassium permanganate and Hydrogen Peroxide
In the reaction being studied in this lab, solutions of hydrogen peroxide, H2O2, and potassium
permanganate, KMnO4, will be combined in an acidic solution. Deep purple in solution, the Mn in
KMnO4 undergoes reduction very easily. In acidic solution, permanganate ions (MnO4-) from KMnO4
reduce to nearly colorless Mn2+ ions. In the presence of permanganate ions in acid solution, an
aqueous solution of H2O2 will undergo oxidation to make oxygen gas and hydrogen ions.
The sulfuric acid and potassium permanganate solutions require careful handling. Solid KMnO4 and
H2SO4 should not be mixed as an explosion could result.
Pre-lab Questions
1. Write a balanced half-reaction for the reduction of permanganate ion by iron (II) ions. How
many electrons are transferred in this reaction?
2. Besides iron (II) ions and hydrogen peroxide, what are one or two other species that could
be used to reduce permanganate ions?
3. Using the stoichiometry in your balance equation from #1, calculate the moles of Fe2+ ions
that can be oxidized by 0.073moles of MnO4- ions.
4. A sample of oxalic acid, H2C2O4, is titrated with a standardized solution of KMnO4. A
25.00mL sample of oxalic acid required 12.70 mL of 0.0206M KMnO4 to achieve a pinkcolored solution. The balance equation for this reaction is shown below.
6H++2MnO4-+5H2C2O4 → 10CO2+8H2O+2Mn2+
a. What does the pink color signify in this reaction?
b. What is the mole ratio of H2C2O4 molecules to MnO4- ions?
c. How many moles of MnO4- ions reacted with the given amount of H2C2O4?
d. How many moles of H2C2O4 were present in the sample?
e. What is the molarity of the H2C2O4 solution?
f. If the density of the oxalic acid solution is approximately 1.00g/mL, what is the
percentage by mass of oxalic acid in the solution?
Part I:
Read the procedure for the standardization of KMnO4. This step in the lab is essential to the completion
of Part II. We will be using a pre-designed procedure to determine the concentration of the potassium
permanganate. After reading through the Part I procedure you must design a procedure to determine
the concentration of H2O2 samples. Answer the questions from the Guided-Inquiry Design Section and
work out a Part II procedure with your group (3-4 per design group; 2-3 in actual lab group). Part I of the
lab will be completed on November 1. We will discuss the procedures you developed and time allowing
may complete Part II as well. Otherwise, Part II will be completed on Wednesday, November 6.
Peroxide Post-Lab Questions
1. Calculate the moles of KMnO4 solution needed to react with the Fe(NH4)2(SO4)2 solution for
each trial.
2.
Calculate the molarity of the KMnO4 solution for each trial.
3. Calculate the average molarity of the KMnO4.
4. Calculate the moles and mass of H2O2 tritrated for each trial.
5.
If the density of the H2O2 solution tritrated was 1.00g/ml, calculate the percentage of H2O2 in
solution in each trial.
6.
Calculate the average percentage of the H2O2 solution.
7.
Are your average values higher or lower than the reported values of the H2O2 you tested? What
are the likely causes of errors? Justify you explanations. Be sure to discuss any data values that
are outliers.
8.
New dietary supplements do not undergo the same rigorous approval process as new
medications. You performed a redox titration to determine the percentage of a component in a
dietary supplement and in a medication. When you performed repeated trials, your
percentages varied widely for the supplement but were very consistent for the medication.
Why do you think this happened?
9.
What is a titrant? What is an analyte?
10. In tis lab, did a substance serve as both a titrant and an analyte? Is so, what was this substance,
Support your answer.
11. What might have been the product(s) in the original solution if it has remained neutral (the
solution was not acidified with H2SO4)? How could you determine this?
12. What might have been the product(s) in the original solution if it was alkaline? How could you
determine this?
13. How would the concentrations of KMnO4 and the H2O2 solutions have been affected if the
following observations were made about the Erlenmeyer flask? Justify your answer.
a. When the titration was completed the flask was colorless.
b. When the titration was completed the flask was dark red or purple.