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Chemistry
Sensors:
Loggers:
Colorimeter
Any EASYSENSE
Logging time: SnapShot with Asks
for Value function
Teacher’s notes
49 Finding the concentration of chlorine in water
using Beer-Lambert law
Read
Chlorine is added to pool water to prevent the spread of waterborne diseases. The reaction of chlorine
with water produces hypochlorous acid and hydrochloric acid. The hypochlorous acid easily breaks
down to form hypochlorite ions. The chlorine that is present in water in the form of hypochlorous acid
and hypochlorite is known as free chlorine. Free chlorine is a powerful disinfectant.
The problem for pool managers is to get the level of free chlorine correct. If the level is too low then
disease causing organisms (pathogens) can flourish. If the level is too high then chlorine and other
chlorine containing compounds will cause irritation of the eyes. A compromise value of 1.5 to 4.0 mg/L
(1.5 - 4 ppm) free chlorine is normal for public swimming pools.
The colour of a solution depends upon the concentration of the solution. If you prepare a set of
solutions of known concentration and measure the absorbance of each solution you can produce a graph
of absorbance vs. concentration.
The direct relationship between absorbance and concentration is called Beer-Lambert law or more
commonly Beer's law.
In this experiment you will produce a graph of absorbance vs. concentration for chlorine in water.
Chlorine is not very highly coloured in the concentrations used in water, a reagent (DPD) that reacts
with chlorine to produce a pink colour will be used to disclose the chlorine.
Apparatus
1.
An EASYSENSE logger.
2. A Smart Q Colorimeter set to the Absorbance range with the green filter.
3. Cuvettes.
4. Syringe of pipettes for transferring solutions to cuvettes.
5. DPD No 1 tablets (Diethyl-phenylenediamine)
6. Sodium hypochlorite bleach.
7. Calcium hypochlorite (bleaching powder) for standards.
8. Alternative Potassium manganate(VII) solution for standards.
L3 Chemistry
T49 - 1(V2)
Hazard information
Calcium hypochlorite
White powder that decomposes
on contact with water to release
chlorine gas.
Oxidising
Corrosive
Refer to local regulations for calcium hypochlorite. Environmental protection measures vary. Be aware
instructions often refer to industrial / semi industrial quantities.
Set up of software and logger
Use the setup file 49 Chlorine.
If you wish to set the software and logger manually use the information in the tables below,
Recording
method
Functions
Options
SnapShot mode
Pre-log Function, Asks for
Value*
Plot as a point, x-axis as
Reading number, Show Grid
* Click on Tools, select Pre-log Function, General, Ask for Value. Use the details in table below to
complete.
Name
Unit
Decimal places
Y axis Limits
Chlorine conc
mg/L
3
Expand to fit
Notes
Solutions and reference standards
The normal test for chlorine is to use acidified potassium iodide. The chlorine displaces the idodine
and releases the iodide ion, which can be detected. When the concentration of chlorine is low as in
drinking water or pool water the amount of iodide liberated will be so small as to be very difficult to
detect. Diethyl-phenylenediamine (DPD) reacts with free chlorine in solution to produce a pink colour
discernable even when very small quantities of chlorine are present.
If swimming pool water is not available to test, mock pool water can be made using normal household
bleach. 2.5 cm3 of household bleach containing 4% w/v sodium hypochlorite should be diluted to make 1
litre of a stock solution. 10 cm3 of the stock made up to 1 litre with water will make a solution of
approximately 1mg/L of free chlorine, this will be similar to the chlorine levels found in swimming pool
water.
Alternatively 0.01 g of calcium hypochlorite can be dissolved into 1 litre of deionised water to give a
stock solution of 5 mg/L. From this stock dilutions can then be made up to give 1 - 5 mg/L reference
samples.
If you have used the calcium hypochlorite to make up reference solutions you may wish to let the
students add the DPD tablets to make up the standards for the concentration curve. I (one) DPD
tablet should be dissolved in 10 cm3 of each solution to make up a standard.
L3 Chemistry
T49 - 2(V2)
Alternative standards for concentration curve
If you have difficulty getting chlorine free water, reference standards can be made up from dilutions
of potassium manganate (VII) solution. Using manganate (VII) also reduces the requirement for DPD
tablets.
1.
Prepare a stock solution of 891 mg of KMnO4 dissolved in 1 litre of water (the addition of a
small volume of sulphuric acid will help maintain the colour, no more than 25 cm3 of 1 mol dm-3
H2SO4 in the total solution).
2. Dilute 10 cm3 of the stock with water to make up 100 cm3 solution.
3. 1 cm3 of this solution made up to 100 cm3 will give a colour equivalent of 1.00 mg/L chlorine +
DPD, 2 cm3 of this solution made up to 100 cm3 will give a colour equivalent of 2.00 mg/L
chlorine + DPD, etc.
The DPD tablets used in this practical were Lovibond DPD No1 tablets purchased from the Tintometer
Ltd, Taunton, Somerset. A pool agency should have similar tablets available for purchase.
General
Strong chorine solutions will decolourise the DPD tablets. If this is the case the solutions will need to
be diluted. The results will then need to be multiplied back up by the dilution factor to give the true
result.
Tap water can contain quite large amounts of chlorine; the test may become compromised if this
chlorine is not considered. It is worth pointing out that the smell of chlorine in the water does not
make it undrinkable.
If possible, it is best to use a separate cuvette for each of the standards. It might be worth while
making up a set of standards and have the students share them.
L3 Chemistry
T49 - 3(V2)