Mandatory Experiment 9.1

Determination of free chlorine in swimming pool water or bleach using a colorimeter or comparator

(a) Determination of chlorine using a colorimeter

Student Material

Theory

When chlorine compounds are used to sterilise swimming-pool water, the active agent is usually chloric (I) acid, HOCl. It kills micro-organisms by oxidation. Chloric (I) acid and its conjugate base, the chlorate(I) ion, ClO, together make up what is called “free chlorine”. These species react with a solution of iodide ions in the same way as chlorine itself does.

When chlorine reacts with potassium iodide in an acidic solution it liberates iodine:

Cl2 + 2KI  I2 + 2KCl

The intensity of the colour of the iodine solution formed is a measure of the concentration of the oxidising chlorine in water. The concentration of chlorine in a sample of swimming pool water or diluted bleach is obtained by comparing the colour obtained on reaction with potassium iodide solution with those colours obtained by the similar reactions of some standard solutions of chlorine. This comparison can be done very accurately using a colorimeter (Fig. 1).

Fig. 1

Chemicals and Apparatus

2% potassium iodide solution

5% ethanoic acid solution in water

Sample of swimming pool water ordiluted bleach

Milton Sterilising fluid

Deionised water

Colorimeter

440 nm wavelength filter

Cuvettes

50 cm3 volumetric flasks with stoppers

Burettes

Filter funnel

250 cm3 volumetric flask

10 cm3 graduated cylinder

Procedure

NB: Wear your safety glasses.

  1. Add 2.5 cm3 of Milton Sterilising fluid to a 250 cm3 volumetric flask and dilute to the mark with deionised water.
  1. To a series of five 50 cm3 volumetric flasks add 5 cm3 of 5% ethanoic acid solution.
  1. Transfer the diluted Milton solution to a burette. To the first 50 cm3 flask add nothing at this stage; to the second 50 cm3 flask add 1.0 cm3 of this solution. To the third, fourth and fifth flasks, add 2.0 cm3, 4.0 cm3 and 8.0 cm3 of the solution respectively.
  1. Using a burette, transfer 5.0 cm3 of 2% potassium iodide solution to each flask and dilute to the mark with deionised water. Stopper each flask, mix thoroughly, and allow about five minutes for the colour to develop. These solutions are now the working standards. Label the flasks (Fig. 2) as A, B, C, D and E respectively.


Fig. 2

Chemical

/ Flask A / Flask B / Flask C / Flask D / Flask E
5% Ethanoic acid / 5 cm3 / 5 cm3 / 5cm3 / 5cm3 / 5cm3
2% Potassium iodide / 5 cm3 / 5 cm3 / 5 cm3 / 5 cm3 / 5 cm3
Diluted Milton / 0 cm3 / 1 cm3 / 2 cm3 / 4 cm3 / 8 cm3
Concentration of NaOCl in p.p.m. / 0 / 4 / 8 / 16 / 32
Total volume in flask / 50 cm3 / 50 cm3 / 50 cm3 / 50 cm3 / 50 cm3
  1. Switch on the colorimeter and place a 440 nm wavelength filter in the filter slot. Pour each working standard into a cuvette, rinsing each cuvette first with the

solution it is to contain.

  1. Using the operating procedure for the colorimeter, as per the manufacturer’s

instruction book, zero the instrument.

  1. Obtain the absorbance for each standard, starting with the most dilute. Rinse the

sample cells with deionised water after each sample has been used.

  1. Plot a graph of absorbance versus concentration (in terms of chlorine) for

the series of standards.

  1. To a 50 cm3 volumetric flask add 5 cm3 of 5% ethanoic acid solution, and then 5.0 cm3 of 5% potassium iodide solution. Fill the flask up to the mark with the swimming pool water ordiluted bleach. Allow about five minutes for the colour to develop. Label this flask as flask F.
  1. Obtain the absorbance for the solution in flask F.
  1. From the graph, obtain the concentration of NaOCl in the sample.
  1. Multiply by 71 / 74.5 to calculate the concentration of free chlorine in the sample

Table of results

Copy this table into your practical report book

Absorbance of solution in flask A =

Absorbance of solution in flask B =

Absorbance of solution in flask C =

Absorbance of solution in flask D =

Absorbance of solution in flask E =

Absorbance of solution in flask F =

Concentration of NaOCl in the sample =

Concentration of free chlorine in the sample =

Questions relating to the experiment

  1. Why is potassium iodide used in this experiment?
  1. Why is ethanoic acid used in this experiment?
  1. Why is excess potassium iodide used in this experiment?

4. The amount of chlorine in a water sample can also be determined by titrating

the iodine solution formed (on reaction with potassium iodide) with standard

sodium thiosulfate solution. What advantage is there in using a colorimeter for

determining chlorine?

Teacher Material

  • If a sample of swimming-pool water is to be tested, the operation must be carried out within a short period of time of taking the sample from the pool. The level of free chlorine drops rapidly as it reacts with organic matter in the sample, so any delay would result in an inaccurate reading.
  • Colorimeters can be analogue (as indicated in Fig. 1) or digital (as in Fig. 3).

Fig. 3

  • Most of the time for this experiment is taken up with making up the working standards. Only one colorimeter is needed for a class, as colorimeter readings can be taken very quickly.
  • If the experiment is being repeated, the cuvettes should be refilled from the flasks, rather than reusing the same samples.
  • The cuvettes need to be rinsed out with deionised water immediately after use, as otherwise they will become coloured by the iodine.
  • The concentration of free chlorine (in the form of OCl-, and expressed as Cl2) is found by multiplying by 71 / 74.5, as this is the ratio of the molar mass of Cl2 to that of NaOCl.
  • Milton Sterilising Fluid is used because it contains a 2% sodium hypochlorite solution. If left unopened after purchasing until the day of the experiment, it should be very satisfactory. The diluted solution has a sodium hypochlorite concentration of approximately 200 mg per litre.
  • In general, sodium hypochlorite solutions have a concentration that is only approximately known. Their shelf life is limited. To find the precise concentration of a diluted hypochlorite solution, proceed as follows: To 25 cm3 of the diluted sodium hypochlorite solution in a 100 cm3 volumetric flask, add 5 cm3 of 2% potassium iodide solution and 5 cm3 of 5% ethanoic acid solution, and dilute to the mark with deionised water. An iodine solution is formed. Titrate 25 cm3 samples of this solution with a standard (0.0025 M) sodium thiosulfate solution, adding starch indicator when the solution in the conical flask goes pale yellow. The diluted hypochlorite solution has a concentration of chlorine c = 4 x the concentration of iodine in the solution used in the titration. The working standards B, C, D and E have iodine concentrations of 0.01c, 0.02c, 0.04c and 0.08c respectively; the concentration of A is 0.

Extension Work

  • If swimming pool water has been used in this experiment, a diluted bleach sample could also be used.
  • If a comparatoris available, the samples could also be analysed by this method, and the two sets of results compared.

Preparation of reagents

Ethanoic acid solution(5%): Measure out 50 cm3 of concentrated ethanoic acid using a graduated cylinder. Transfer with washings to a 1 litre volumetric flask, and dilute to the mark with deionised water. Stopper the flask, and invert the flask a number of times to ensure thorough mixing.

Potassium iodide solution(2%):

Using an electronic balance, measure out 20 g of potassium iodide. Add the solid to about 500 cm3 of deionised water in a beaker. Transfer with washings to a 1 litre volumetric flask, and dilute to the mark with deionised water. Stopper the flask, and invert the flask a number of times to ensure thorough mixing.

Diluted bleach solution: A suitable bleach solution could be made by diluting household bleach by a factor of between 10,000 and 100,000. It would be advisable to test the suitability of this solution before the commencement of class.

Sodium thiosulfate solution (0.0025 M): Dissolve 0.62 g of Na2S2O3.5H2O in deionised water and make up to 1 litre of solution.

Quantities per working group

40 cm3 2% potassium iodide solution

40 cm3 5% ethanoic acid solution

20 cm3 diluted sodium hypochlorite solution

40 cm3 sample of swimming pool water or diluted bleach

Safety considerations
  • Safety glasses must be worn.
  • The use of gloves is recommended.

Chemical hazard notes

Dilute ethanoic acid is slightly corrosive.

Concentrated ethanoic acid is corrosive to skin and tissue. The vapour is very irritating to lungs.

Sodium hypochlorite G.P.R. solution is corrosive and causes skin burns.

i Domestic bleach is an irritant.

Disposal of wastes

Add 1 M sodium thiosulfate solution to the waste. When colourless, dilute with water and flush to foul water drain.

Specimen results and calculations

Absorbance of solution in flask A = 0.00

Absorbance of solution in flask B = 0.06

Absorbance of solution in flask C = 0.12

Absorbance of solution in flask D = 0.24

Absorbance of solution in flask E = 0.43

Absorbance of solution in flask F = 0.18

Concentration of NaOCl in the sample = 12.6 p.p.m.

Concentration of free chlorine in the sample = 12.6 X 51.5 / 74.5 p.p.m.

= 8.71 p.p.m.

Suggested answers to student questions

  1. Why is potassium iodide used in this experiment?

Potassium iodide is readily oxidised to iodine by chlorine. Chlorine solutions do not themselves have sufficient colour to be analysed using a colorimeter, whereas the iodine solutions formed do.

  1. Why is ethanoic acid used in this experiment?

Acidic conditions are necessary to ensure complete reaction of chlorine with potassium iodide.

  1. Why is excess potassium iodide used in this experiment?

To ensure that all of the chlorine reacts, and that all of the iodine formed dissolves.

  1. The amount of chlorine in a water sample can also be determined by titrating the iodine solution formed (on reaction with potassium iodide) with standard sodium thiosulfate solution. What advantage is there in using a colorimeter for determining chlorine?

The colorimeter reading can be taken very quickly. Once the calibration curve is available, the concentration of chlorine can then be very quickly found. The colorimetric method would be particularly useful where a number of different water samples have to be analysed. The alternative would be to carry out a number of titrations for each sample, which would be very time-consuming.

(b) Determination of chlorine using a comparator

Student Material

Theory

When chlorine compounds are used to sterilise swimming-pool water, the active agent is usually chloric (I) acid, HOCl. It kills micro-organisms by oxidation. Chloric (I) acid and its conjugate base, the chlorate(I) ion, ClO, together make up what is called “free chlorine”.

Free chlorine oxidises DPD No.1 tablets (N,N-diethyl-p-phenylenediamine in the form of its sulfate) to a soluble red product. The intensity of the red colour is a measure of the concentration of the oxidising chlorine in water.

Chemicals and Apparatus

DPD No.1 tablets n

Sample of swimming-pool water or diluted bleach solution

Comparator

Procedure

NB: Wear your safety glasses.

  1. Rinse the compartments of the comparator with portions of the water sample under test and discard the rinsings.
  1. Fill each compartment to their graduation marks with fresh portions of the sample.
  1. Add a DPD No.1 tablet to each appropriate compartment, and crush each tablet with a stirring rod.
  1. Fit the lid and shake until the tablets have dissolved completely.
  1. Compare the colour produced in the sample with the pre-calibrated standards, using daylight rather than artificial light as an aid.
  1. Choose the best colour match, and read the free chlorine concentration of the selected standard in p.p.m. (mg l-1).

Questions relating to the experiment

  1. Why should the comparator be rinsed with the water sample being tested?
  1. Why should the compartments be filled exactly to the graduation marks?
  1. What precaution should be taken with the stirring rod before the crushing operation?
  1. Why should the tablets be completely dissolved before comparison with the standards?

Teacher Material

  • The procedure may vary depending on the particular type of comparator.
  • If a sample of swimming-pool water is to be tested, the operation must be carried out within a short period of time of taking the sample from the pool. The level of free chlorine drops rapidly as it reacts with organic matter in the sample, so any delay would result in an inaccurate reading.
  • A suitable bleach solution could be made by diluting Parazone by a factor of between 10,000 and 100,000. It would be advisable to test the suitability of this solution before the commencement of class.
  • Alternatively, Milton sterilising fluid (2.5 cm3 diluted to 250 cm3 with deionised water) may be further diluted as in the colorimeter experiment. Agreement between results obtained by the colorimeter and comparator methods is better at lower concentrations, as the following data indicate:

Chemical

/ Flask A / Flask B / Flask C / Flask D / Flask E
Diluted Milton / 0.1 cm3 / 0.2 cm3 / 0.25 cm3 / 0.5 cm3 / 2 cm3
Concentration of NaOCl in p.p.m. / 0.4 / 0.8 / 1 / 2 / 8
Total volume in flask / 50 cm3 / 50 cm3 / 50 cm3 / 50 cm3 / 50 cm3
Concentration of free chlorine in p.p.m. (comparator method) / 0.2 / 0.4 / 0.6 / 2.0 / 8.0
Concentration of free chlorine in p.p.m. (colorimeter method) / 0.28 / 0.55 / 0.69 / 1.38 / 5.5

(Note that the free chlorine concentration values in the last row of the table above are obtained by multiplying the corresponding NaOCl concentration values by 71 / 74.5.)

  • The DPD No.1 tablet should not be handled as grease from the skin interferes with its performance.

Quantities per working group

  • About 120 cm3 of water sample per working group would be sufficient for rinsing and testing.
  • Two DPD No.1 tablets would probably be sufficient, but this could vary depending on the type of comparator.
Safety considerations
  • Safety glasses must be worn.

Chemical hazard notes

n The DPD No.1 tablet is made from a compound that is harmful if inhaled in powder form, though such a situation seems unlikely in this experiment.

Disposal of wastes
Flush to foul water drain with excess water.

Suggested answers to student questions

  1. Why should the comparator be rinsed with the water sample being tested?

To prevent contamination or dilution.

  1. Why should the compartments be filled exactly to the graduation marks?

Because the standards are designed for comparison with a solution resulting from

the reaction of a tablet with an exact volume of the water sample.

  1. What precaution should be taken with the stirring rod before the crushing operation?

The rod for each compartment should be clean and dry.

  1. Why should the tablets be completely dissolved before comparison with the standards?
Because the standards are designed for comparison with the resulting solution

when a specified amount of DPD is oxidised by the free chlorine.

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