Salters Advanced
Chemistry
ES5 The risks and benefits
of using chlorine
Activity sheet
5.1 Which is the most cost-effective brand of
bleach?
Specification reference: Elements from the sea (f)
Aims
In this activity you will work with all the other members of your class to investigate
which is the most cost-effective commercially available brand of bleach.
This activity will enable you to:
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check that you know how to accurately dilute a solution
check that you know how to carry out an ‘iodine’ titration
check that you can calculate the concentration of a solution using data from a
titration.
Introduction
The active ingredient of many brands of bleach is sodium chlorate(I). This may be
mentioned on the label as ‘sodium hypochlorite’. It is made by reacting chlorine with
a cold solution of sodium hydroxide:
Cl2 + 2NaOH → NaOCl + NaCl + H2O
The bleach will therefore contain a mixture including hydroxide ions, chloride ions
and sodium ions.
Household bleach usually contains between 3 and 6% sodium chlorate(I). Sodium
chlorate(I) is a good bleach because it is a powerful oxidising agent. You can make
use of this property to analyse bleach to find how much active ingredient it contains.
In acidic solution, the chlorate(I) ion readily oxidises iodide ions to iodine:
ClO− + 2I− + 2H+ → I2 + Cl− + H2O
You can titrate the iodine produced with sodium thiosulfate solution:
I2 + 2S2O32− → 2I− + S4O62−
You use starch solution as an indicator because it produces a blue–black colour
when iodine is present. The disappearance of this colour tells you that all the iodine
has reacted with the thiosulfate and the end point of the titration has been reached.
You will get the most accurate results if you wait until the iodine has been nearly all
used up and the solution has changed to a straw colour before you add the starch
indicator.
You will be provided with a 0.1 mol dm−3 solution of sodium thiosulfate. Your bleach
solution is probably too concentrated to be used ‘neat’ – it will produce so much
iodine that you would have to use large volumes of the sodium thiosulfate in the
titration. So one of your first steps will be to dilute the bleach before titrating it – try a
ten-fold dilution to start with and modify this if you have to, depending on the volume
of the titre.
© Oxford University Press 2015
www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original
1
Salters Advanced
Chemistry
ES5 The risks and benefits
of using chlorine
Activity sheet
Equipment and materials
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25 cm3 pipette
pipette filler
50 cm3 burette
250 cm3 volumetric flask
25 cm3 measuring cylinder
250 cm3 conical flask
teat pipette
range of different brands of household bleach, with price information
sodium thiosulfate solution, 0.1 mol dm−3
starch solution
sulfuric acid, 1 mol dm−3
potassium iodide solution, 10%
distilled/de-ionised water
Safety
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Eye protection must be worn.
Bleach is an IRRITANT.
Sulfuric acid is an IRRITANT.
What you do
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Dilute the bleach sample using a 25 cm3 pipette and a 250 cm3 volumetric flask.
Pipette 25 cm3 of your diluted bleach into a conical flask.
Using a measuring cylinder, add about 15 cm3 of 10% potassium iodide solution
and about 20 cm3 of 1 mol dm−3 sulfuric acid to the conical flask.
Fill a burette with 0.1 mol dm−3 sodium thiosulfate solution.
Titrate your diluted bleach solution with the sodium thiosulfate solution, adding
1–2 cm3 of starch indicator when the solution just becomes straw coloured. Take
care not to go past the end point otherwise you will have to repeat the titration.
If your titre is not satisfactory you will have to dilute another sample of your
original bleach in a more appropriate way.
If your titre is satisfactory, repeat your titration with further portions of your
diluted bleach until your titres are within 0.1 cm3 of each other. Remember to
add the potassium iodide and sulfuric acid each time you do a titration.
Record your readings in a table like the one below.
© Oxford University Press 2015
www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original
2
ES5 The risks and benefits
of using chlorine
Salters Advanced
Chemistry
Titration
Activity sheet
Trial
First
accurate
Second
accurate
Third
accurate
Final burette reading/cm3
Initial burette reading/cm3
Titre/cm3
Questions
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2
3
4
5
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From the titres that agree to within 0.1 cm3, work out the mean volume of sodium
thiosulfate used in the titration (the mean titre).
Use the mean titre and the concentration of the sodium thiosulfate solution to
calculate the amount (in moles) of sodium thiosulfate which just reacts with the
sample of diluted bleach in the conical flask.
Use the equation for the reaction to work out the number of moles of iodine
which must have reacted with the sodium thiosulfate.
Use your answer to Step 3 and the equation for the reaction to work out the
number of moles of chlorate(I) ions in 25 cm3 of the diluted bleach.
Calculate the number of moles of chlorate(I) ions in 1 cm3 of diluted bleach.
Work out the number of moles of chlorate(I) ions in 1 cm3 of the original,
undiluted bleach.
Find the volume of the bottle of bleach you used and calculate the number of
moles of chlorate(I) in one full bottle of bleach.
Use the price of the bottle of bleach to work out the cost per mole of chlorate(I)
for your brand of bleach.
Your teacher will draw up a table like the one below for the group to write in their
results. Compare the results for your bleach with data about the other brands of
bleach that have been analysed by other students. Arrive at a class decision
about which is the most cost-effective brand of bleach.
Brand of
bleach
Number of moles of
chlorate(I) ion per bottle
© Oxford University Press 2015
Cost of
bottle
Cost per mole of
chlorate(I) ions
www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original
3