DETERMINATION OF HYDROGEN PEROXIDE: TITRATION
BASED ON OXIDATION AND REDUCTION
BACKGROUND
Many chemical reactions involving oxidation-reduction are widely used in titrimetric
analysis. The ions of many transition elements (e.g., Mn, Cr, and V) can exist in different
oxidation states, resulting in the possibility of a large number of redox reactions. In the
following experiments you will determine hydrogen peroxide using permanganate (MnO4-) as
the strong oxidant in the form of KMnO4. Since MnO4 - is intensely violet in color, a selfindicating titration is possible.
Several points require attention when using MnO4- as the titrant. Under certain
conditions, the MnO4 - can oxidize chloride to chlorine, requires special measure.
Permanganate solutions, especially dilute ones, are not very stable for storage. In addition,
the reduction of MnO4 - may proceed to different oxidation states of Mn, depending on the
solution condition.
In strongly acidic medium, MnO4 - undergoes a five electron reduction to colorless
manganous, Mn(II), ion:
MnO4- + 8 H+ + 5 e- → Mn2+ + 4 H2O
PART I. Preparation of permanganate solutions
Potassium permanganate is not pure enough to be used as a primary standard. In
addition, the oxidizing power of MnO4- is sufficiently great to oxidize water as follows:
4 MnO4 - + 2 H2O → 4 MnO2 (s) + 4 OH- + 3 O2 (g)
An aqueous permanganate solution is therefore intrinsically unstable. Fortunately,
this reaction is extremely slow and a reasonable stability is obtained if catalytic influences are
excluded. The reaction is catalyzed by light, Mn(II) ion, and by any Mn(IV) oxide present.
In practice, therefore, a solution of approximate strength required is prepared with special
precaution and then standardized.
A. Preparation of 0.02 M potassium permanganate
1. Weigh 1.6 g of potassium permanganate to the nearest hundredth of a gram and dissolve
in 500 mL of deionized water in a 1L beaker.
2. Heat the solution to boiling, keep hot for about 30 minutes over low heat and then allow
it to cool. Heating of the solution hastens the oxidation of any organic matter present and
also aids in the coagulation of the Mn(IV) oxides.
3. Filter through a sintered glass or porcelain filter of the fine porosity under suction. Care
should be exercised to perform all operation with clean glassware and with precautions that
no additional organic matter such as particles from rubber stopper or tubing is introduced.
Under no circumstances, of course, use filter paper for filtration.
4. Transfer the filtrate to a clean, glass-stopper bottle and store in the dark.
PART II. Standardization of permanganate solution against sodium oxalate
Anhydrous sodium oxalate, Na2C2O4, is available in high purity and is stable in air.
For the standardization of permanganate solution, an accurately weighed amount of this salt
is dissolved in sulfuric acid and titrated with the permanganate solution. In the acidic
medium the oxidation is best described in terms oxalic acid:
H2C2O4 → 2 H+ + 2 CO2 + 2 eThe overall reaction between permanganate ion and oxalic acid may be expressed as
2 MnO4- + 5 H2C2O4 + 16 H+ → 2 Mn2+ + 10 CO2 + 8 H2O
When permanganate is first added to a solution of oxalic acid, the solution immediately
becomes pink and remains so for some time, indicating the slowness of the reaction. As the
titration is continued, the decolorization becomes rapid, because the Mn2+ formed by the
reaction catalyzes the reduction of permanganate ions. In the vicinity of the end point the
decolorization is almost instantaneous.
A. Preparation of 0.05 M sodium oxalate
1. Weigh 1.7 g of sodium oxalate to the nearest 0.1 mg.
2. Transfer the salt quantitatively to a 250 mL volumetric flask, add about 200 mL of DW,
swirl until complete dissolution, and dilute to mark and mix well.
3. Calculate the molarity of the solution.
PART III. Standardization of permanganate according to McBride
1. Transfer 25.00 mL of sodium oxalate standard solution into an Erlenmeyer flask, and add
20 mL of 1:10 sulfuric acid and then 75 mL of water.
2. Heat the solution slowly on a hot plate, with stirring, to about 80 °C (use a thermometer
to determine temperature, but not as stirring rod!)
3. Remove the beaker from the hot plate and begin the titration with the 0.02 M potassium
permanganate. Add the initial portions of titrant slowly, allowing complete decolorization to
occur before the next portion is added. Note the temperature from time to time; if it falls
below 70 °C, reheat the solution. Titrate until a faint pink is attained that persists for at least
20 seconds.
4. Establish a blank by bringing 120 mL of water and 20 mL of 1:10 sulfuric acid to the
same persistent pink by the addition of the titrant solution.
5. Take the blank into account and calculate the molarity of the permanganate solution
6. Repeat the titration with at least two additional aliquots of the oxalate standard solution.
7. Calculate the average molarity and standard deviation (< 0.5% rsd) of at least three
results.
PART IV. Determination of hydrogen peroxide
Hydrogen peroxide reacts with permanganate ion in acidic solution according to
5 H2O2 + 2 MnO4- + 6 H+ → 2 Mn2+ + 5 O2 + 8 H2O
A permanganate titrant serves well for the determination of hydrogen peroxide even
in dilute solutions. As in the titration of oxalic acid, the decolorization of the initial portion
of titrant added may be somewhat slow. Since hydrogen peroxide decomposes at an
elevated temperature the titration should be performed at room temperature.
1. Receive your sample (5 mL) in a well-cleaned 50 mL beaker. Keep a watch glass over the
top of the beaker to prevent contamination. Because of the low stability of hydrogen
peroxide sample, the titration must be performed during the laboratory period in which the
sample is received; preferably all arrangements for the titration should be completed before
the sample is issued.
2. Pipet a 1.00 mL of aliquot of the sample solution into an Erlenmeyer flask, dilute with
100 mL of water, and add 20 mL of 1:3 sulfuric acid.
3. With gentle swirling of solution, titrate with permanganate solution until a faint pink is
attained that lasts for at least 30 seconds.
4. Establish a blank by titrating 100 mL of water and 20 mL of 1:3 sulfuric acid to the same
pink color.
5. Take the blank into account and express the titration result as grams of hydrogen
peroxide present in 100 mL of the sample.
6. Repeat the titration at least twice and calculate the average and percent relative standard
deviation of your result.
QUESTIONS
1. To what oxidation state is permanganate reduced in acidic medium? In alkaline medium?
2. Elaborate on the necessity of using blank in a permanganate titration.
3. Write the equation for the decomposition of hydrogen peroxide and the equations for the
two half-reactions involved.
4. The concentration of a hydrogen peroxide solution is sometimes expressed as "volume
percent," which here means the number of 100 mL volumes of oxygen gas (under STP – if
you do not remember what STP stands for LOOK IT UP!) liberated from 100 mL of the
given solution upon complete decomposition of hydrogen peroxide into water and oxygen.
What form would the calculation take if the result of titration in the present experiment were
to be expressed in this way?