INTRODUCTION
Peroxides occur in a wide variety of situations: organic peroxides, having the general formula ROOR, where R
is an organic group, are often used to start polymerization reactions, such as the ones needed to make
commonly occurring polymers like polystyrene and polythene.
Hydrogen peroxide, HB2BOB2B is readily available in supermarkets and pharmacies since it is a principal
component in bleaches for hair and most recently and controversially it was one of the components used to
make the bombs which exploded in London on 7PthP July 2005.
Peroxides are also thought to contribute to aging of skin and they are also a source of free radicals thought to
be partly responsible for some cancers.
In this experiment you will use a titrimetric method to determine the amount of hydrogen peroxide in a given
solution. Hydrogen peroxide can act as both an oxidizing agent and a reducing agent depending upon the
conditions of the reaction. In this experiment hydrogen peroxide will oxidize the iodide ion, IP-P, obtained from
potassium iodide to iodine, IB2B. The iodine produced reacts with sodium thiosulphate and the amount of
thiosulphate required can be used to calculate the amount of iodine present and hence the amount of
hydrogen peroxide in the solution.
THEORY
Titrimetry, or titrimetric analysis, is an example of a so-called classical method of analysis. Titrimetry is a
convenient method of getting very small amounts of chemicals in to a reaction because the method involves
the use of dilute solutions of reactants.
Hydrogen peroxide reacts with potassium iodide according to the reaction given below:
HB2BOB2B + 2HP+P + 2IP-P = IB2B + 2HB2BO (equation 1)
Equation 1 shows that 1 mole of hydrogen peroxide gives 1 mole of IB2B.
Note: we have omitted the potassium ion, KP+P, from this reaction since it would occur on both sides of the
equation and so it does not take part in the reaction.
The iodine produced reacts with sodium thiosulphate, NaB2BSB2BOB3B as shown in equation 2 below:
IB2B + 2S2O32- 2IP- + S4O6B2- (equation 2)
Equation 2 shows that 1 mole of iodine reacts exactly with 2 moles of thiosulphate.
Combining equations 1 and 2:
1 mole HB2BOB2B gives 1 mole iodine AND 1 mole iodine reacts with 2 moles thiosulphate.
This shows that 1 mole HB2BOB2B is equivalent to 2 moles thiosulphate.
This means that if we know how much (the number of moles) thiosulphate we need to react with the iodine we
can calculate how much hydrogen peroxide was in the solution we started with.
EXPERIMENTAL
In this experiment you will use titrimetric analysis to determine the concentration of hydrogen peroxide in a
solution. The titration is relatively easy to follow because the iodine produced by reaction of hydrogen peroxide
with iodide is iodine, which is purple in colour. Addition of thiosulphate solution gives a colourless solution when
all of the iodine has reacted (the end point). You can make the end point even sharper by adding a few drops
of starch solution when the colour of the solution is pale yellow. The starch changes the colour of the solution
to black but the solution will become colourless at the end point.
The experiment is quite short and so you have plenty of opportunity to repeat the titrations until you get
consistent results.
You have already carried out an acid-base titration and so you should know that “consistent results” means that
the volumes required to completely decolourise the iodine produced should be consistent to ± 0.1 cmP3P.
You are provided with a solution of hydrogen peroxide and a solution of sodium thiosulphate as well as 1.25
mol dm-3 sulphuric acid and solid potassium iodide.
Throughout your experiment, note any colours that are observed in your lab book.
- Note the concentration of the sodium thiosulphate solution supplied.
0.02497 mol dm-3 - Measure out 5 cmP3 of 1.25 mol dm-3 sulphuric acid using a measuring cylinder and place it in a 250
cmP3P conical flask. - Add about 0.5 g of potassium iodide. There is NO NEED TO WEIGH THIS ACCURATELY so just use a top
pan balance. Swirl the flask in order to dissolve the potassium iodide in the sulphuric acid.
0.51g, 0.50g, 0.49g, 0.48 g - When the potassium iodide has all dissolved use pipette and measure out 10.00 cmP3P of the hydrogen
peroxide solution. - Gradually add the hydrogen peroxide solution to the acidified potassium iodide solution with constant
swirling. - Use a cork, rubber bung or parafilm to close the top of the conical flask and allow the mixture to stand for 15
minutes. The solution should become dark orange/brown because of the presence of iodine in solution. - Pour sodium thiosulphate into a burette. Run some of this solution through the burette to remove any
bubbles below the tap. - Record the reading on the burette to the nearest 0.05 cm3 (NOTE: you will have to estimate the second
decimal place). THERE IS NO NEED TO GET THE VOLUME TO EXACTLY 0.00 ON THE BURETTE. - Titrate the iodine solution in the conical flask with the sodium thiosulphate in the burette. You can add the
thiosulphate fairly quickly in the early stages but you will need to add the thiosulphate more slowly as the
colour of the iodine get more and more yellow. - When the iodine solution becomes straw coloured (pale yellow) add a few drops of starch solution. The
iodine solution should now be much darker – almost black. - Continue adding the thiosulphate solution until the iodine solution becomes colourless.
- Record the reading on the burette again to the nearest 0.05 cmP3.P
- The volume of thiosulphate required to react with the iodine solution is the difference between the final
reading and the initial reading. - Repeat steps 2 – 13 until you get THREE consistent results (i.e. where the volume used agrees to ± 0.1
cmP3P.
Group
1 2 3 4
Initial volume (cm3) 1.00 2.00 0.10 0.05
Final volume (cm3) 25.10 25.75 23.75 23.75
volume used (cmP3P) - Calculate the concentration of hydrogen peroxide in the solution provided.
EVALUATION OF RESULTS
Combining equations 1 and 2 to determine the mole ratio of peroxide to thiosulphate:
1 mole HB2BOB2B gives 1 mole iodine AND 1 mole iodine reacts with 2 moles thiosulphate.
This shows that 1 mole (n2) HB2BOB2B is equivalent to 2 (n1) moles thiosulphate (mole ratio).
C = Concentration (mol dm-3), V = Volume (cm3)
Rearrange C1V1/n1= C2V2/n2 to give C2
QUESTIONS - Calculate a value for the concentration of the hydrogen peroxide solution for your best three titrations.
- Calculate the mean, standard deviation and relative standard deviation for your data. (use your best three titrations)
- Comment on your results.
Sample Solution
