Bleaching powder is a mixture of basic hypochlorite and a basic
chloride.
On standing, particularly in sunlight, the solution evolves oxygen;
because of this bleach solutions are normally supplied in opaque
plastic containers.
The use of chlorine in the water supply is an example of destroying
micro-organisms, such as substances are called disinfectants. A
Scottish doctor, Joseph Lister (1827-1912) was the first to use a
disinfectant to kill bacteria during a surgical operation. Surgery in
those days was extremely dangerous, because even if the patient
survived the shock of the operation, there was great likelihood of
death soon after, due to the wounds becoming septic. Lister
believed that sepsis was due to bacteria in the air getting into the
wound and causing the tissues to decay just as broth had done in
Pasture’s flask. In 1865 he put his theory to the test by carrying out
his operation under a fine spray of carbolic, a very strong
disinfectant. The result showed that he was right, the wounds did not
become septic, but the carbolic was so strong that it destroyed the
tissues and healing was slow. After further experiments using milder
solutions it became accepted practice to use what we then called
antiseptics in surgery.
After researching into the background of bleach and considering
various experiments to carry out I came up with the idea of testing
different bleaches, I would carry out experiments which include
finding the available chlorine content in each bleach, and I would
devise and experiment to find the effectiveness of each bleach on
bacteria. I found four different brands of bleach with which to carry
out my experiments, they are Parazone, Tesco, Somerfield and a
bleach I intend to make myself as part of my investigation.
Bleaching powder liberates chlorine when reacting with diluted acid,
this chlorine is available for bleaching and is known as ‘available’
chlorine. Bleaching powder may deteriate because it attacted by
carbon dioxide of the air, a hypochlorine tends to decompose on
standing. Both of these changes release the available chlorine
content and in an old sample of bleaching powder may almost be
worthless for bleaching.
The following experiments estimate available chlorine content in
each of the four bleaches I have chosen to investigate, but I firstly
have to make my own bleach.
Making Bleach
I know that I can make sodium hypochlorite by bubbling
chlorine gas through sodium hydroxide.
2NaOH + Cl2 2NaOCl + H2
there are many way in which chlorine gas can be produced but for
my experiment I will attempt to oxidise hydrochloric acid. For this I
will need oxidising agent of those available I will use potassium
permanganate.
apparatus
* 250cm3 Conical flask
* Thistle funnel
* 3 Boiling tube
* Delivery system (4 bungs with glass tubes)
* Bunsen burner
* Bench mat
* Sodium hydroxide
* Potassium permanganate
* Conc. Hydrochloric acid
* safety goggles
Method
The apparatus was set up as shown above ensuring all connections
were secure preventing any Cl2 gas from leaking into the room. This
was set up in the fume cupboard so the gas escaping the
experiment would be extracted from the room.
The boiling tubes were each filled approximately half full with the
required liquid.
In the conical flack I put 20ml of water mixed with 10g potassium
permanganate and made sure that the delivery tube from the thistle
funnel goes below the water level to stop the chlorine gas travelling
back up the tube. Next I added 50ml of conc. HCl however the
reaction did not start immediately so I used a Bunsen burner to
gentle heat the mixture when ever needed to speed up the reaction.
In the conical flask the potassium permanganate is reacting with the
hydrochloric acid in this reaction:
2KMnO4 + 16HCl 2KCl + 2MnCl2 + 5Cl2
this gas given off then travels through the H2O and into the sodium
hydroxide where it forms bleach the second test tube with NaOH is
to neutralise any excess Chlorine gas before being released out of
the system.
I stopped the reaction when I noticed the first test tube
containing the sodium hydroxide had turned a green shade. I then
tested the bleach with red litmus paper by dipping it in the solution.
The paper was bleached by the solution proving that I had made a
bleach.
PLANNING OF FINDING AVAILABLE CHLORINE.
I used two experiments to find the amount of available chlorine in the
four bleaches I have used in my investigation.
Experiment 1.
apparatus
3 cm3 bleach
5 cm3 hydrogen peroxide (20 vol)
100 cm3 glass syringe
50 cm3 conical flask
50 cm3 burette
Method
Pipette 3 cm3 of bleach into a conical flask then place 5 cm3 of 20
volume hydrogen peroxide into a burette attached to the conical
flask, then drop by drop add the hydrogen peroxide on to the bleach.
The hydrogen peroxide readily decomposes to give off oxygen, the
equation of the process is shown below;
NaOCl + H2O2 NaCl + H2O + O2
The oxygen evolved by the reaction between the bleach and the
hydrogen peroxide is measured by the syringe . This in turn
measures the amount of available chlorine in the bleach because the
amount of oxygen evolved is equivalent to the amount of chlorine.
This is demonstrated in the equations above and below.
NaOCl + HCl NaCl + H2O2 + Cl2
The apparatus should be set up as shown below;
PLANNING OF FINDING AVAILABLE CHLORINE
Experiment 2
apparatus
5cm3 bleach
25-50cm3 potassium iodide 0.5mol
25cm3dilute sulphuric acid
1cm3 starch
standard solution of sodium thiosulphate 0.5mol
100cm3 burette
250cm3 conical flask
white tile
Method
The titration measures the amount of chlorine, because the number
of moles of iodine is equivalent to the number of moles of chlorine.
Shown in the equation below;
2OCl2 + 2I- 2Cl- + I2
Also the number of moles of iodine are equivalent to that of sodium
thiosulphate, shown in the equation below;
I2 + 2S2O3 2- S4O6 2- + 2I-
Add the bleach and the potassium iodide to the conical flask, the
chlorine is displaced. Then add the dilute sulphuric acid to the
solution. The equation of the solution is shown below;
2OCl- + I2 + 4H+ H2O + 2Cl- + I2
This solution is black in colour, starch is then added to increase the
black colour of the solution.
Then drop by drop add the standard solution of sodium thiosulphate
into the solution until the colour changes from black to a colourless
solution.
Therefore the number of moles of the tiosulphate I measure from
the buretteis also the number of moles is also the amount of
available chlorine. This method has to be repeated several times so
that the measurement will be accurate.
CONCLUSION FROM EXPERIMENTS 1 + 2
If I compare the results of the two experiments to find the
percentage of available chlorine you will see the difference between
them.
BLEACH EXPERIMENT 1
EXPERIMENT 2
Laboratory 2.32 % 3.98 %
Tesco 3.14 %
3.86 %
Somerfield 6.43 % 7.24 %
Parazone 1.06 % 1.88 %
Even though the results are very different I cannot actually see why
that should not be similar as they are both textbook methods, but I
believe the second experiment to be more reliable. The results from
experiment two are the results I will be using when comparing them
with the results of the bacterial experiment.
I think the first table of results is more likely to be inaccurate because
unlike the second experiment, you cannot tell when the reaction has
finished. You know when the reaction has taken place in the second
experiment because the solution has changed colour. If I had let the
reaction go on for a longer time there could have been a higher
amount of oxygen given off, but I am not sure how long this reaction
takes to work and can only assume it happens within the first few
moments. There could also have been a leak in the experiment but
because it is a gas I would not be able to see it, where as if I had
dropped some of the sodium thiosulphate I would have known. Also
there was some gas escaping out of the burette when I tried to drop
somehydrogen peroxide into the bleach, there was also a build up of
pressure because the glass syringe kept sticking and the gas would
bubble up through the hydrogen peroxide. After two trial attempts I
discovered this problem, so I placed a rubber bung at the top of the
burette but the empty burette would have been filled with oxygen as
well. On the next page is a bar chart displaying the two different
results.
