Chlorine in swimming pools

A simple redox 'titration'

Introduction

Chlorine is an oxidizing agent and is added to swimming pool water in order to disinfect it[1]. The problem is that there needs to be enough chlorine present to disinfect the water but not too much to cause problems to users. This means that the water needs regular testing. This can be achieved easily using a standard redox titration. Excess potassium iodide is added to a sample of the water. The free chlorine in the pool water oxidizes iodide ions to iodine and the amount of iodine formed can be determined by titration with a standard solution of thiosulfate ions. This could be achieved using a burette and a pipette using the normal titration procedure in a school laboratory. However most people owning or using a swimming pool will not have access to sophisticated equipment. This simple experiment still uses the principle of titration but volumes are obtained by counting drops so that the only equipment necessary is a measuring cylinder and a dropping pipette. The procedure could easily be carried out by the side of the pool and saves buying expensive commercially available kits.

Teachers notes

This practical is suitable for both SL and HL students. SL students may need more guidance to obtain the correct redox equations so these should probably be given although according to sub-topic 9.1 they are supposed to be able to handle them. HL students should be able to arrive at the correct equations by constructing and using the relevant half-equations. Essentially the calculation is an exercise in stoichiometry. It is also a good practical example of the use of ppm which occurs on the syllabus in Topic 1.3 Reacting masses and volumes. The values obtained usually do fall in the region of 1 - 3 ppm but may be more if the chlorine has just been added or may be less (particularly for outdoor pools) due to the chlorine escaping into the atmosphere. You might like to get your students to research chloramines on the Internet. The rather unpleasant swimming pool smell of 'chlorine' is not actually due to chlorine but to chloramines which build up when the added chlorine reacts with perspiration, oil and urine. Stress the importance of taking a shower before swimming as this helps to prevent chloramine formation.

Standard Level Higher Level Student worksheet

CHLORINE CONTENT OF SWIMMING POOL WATER

Chlorine is used in swimming pools to stop the development of bacteria which would otherwise make the water both unhealthy and unsafe to swim in. The level must be carefully controlled. It must be high enough to inhibit bacterial growth but low enough to stop skin and eye irritation. The normal safe level is around 1 - 3 ppm (parts per million). The following method for determining the chlorine content is very simple but gives surprisingly good results. Although you will be carrying out the procedure in the laboratory and understanding the chemistry involved it could easily be done by the side of any swimming pool by a non-specialist chemist.

ENVIRONMENTAL CARE:

This method uses very small quantities of relatively harmless substances and the waste can be disposed of down the sink.

SAFETY:

As the method has been designed for use by non-chemists there are no special hazards to be aware of. Chlorine is also added to drinking water in small quantities to make it safe to drink. You might like to think of the disadvantages as well as the advantages of adding chlorine to drinking water and suggest why ozone is now often used instead of chlorine by the water authorities.

PROCEDURE:

Obtain fresh samples of the water from your local swimming pool (you might also like to determine the chlorine content of tap water). Measure 300 cm3 of the water sample into a beaker, add 1 gram of solid potassium iodide and stir with a glass rod until the solid has dissolved. A brown colour of free iodine shows that chlorine is present in the sample. Add three or four drops of starch solution which acts as an indicator for the titration by producing a blue/black colour in the presence of iodine.

Using a dropping pipette, add dropwise sodium thiosulfate, Na2S2O3(aq), solution with a concentration of 0.025 mol dm-3 making sure you count the drops. The colour of the solution should turn from black to green to blue and finally to colourless. Stop when the blue colour just disappears and record the number of drops added.

Now by an appropriate method determine the volume of each drop.

EQUATIONS AND CALCULATION.

An excess of potassium iodide is added so that all the available chlorine in the water is converted to iodine according to the redox equation:

2KI(aq) + Cl2(aq) → 2KCl(aq) + I2(aq)

( or ionically 2I−(aq) + Cl2(aq) → 2Cl−(aq) + I2(aq) )

The iodine produced reacts with the sodium thiosulfate by another redox reaction:

I2(aq) + 2Na2S2O3(aq) → 2NaI(aq) + Na2S4O6(aq)

(or I2(aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)

The amount of chlorine originally present can be determined from the amount of sodium thiosulfate used. Although the result could be given as a molarity it is best to convert this into grams of chlorine per dm3 of solution. If expressed as milligrams per dm3 then this is the same as the value in parts per million.

Is your swimming pool safe to swim in?

This worksheet can also be downloaded from:

  Chlorine in pool water

Footnotes

  1. ^ In fact, disinfecting water by using chlorine or ozone is mentioned under International-Mindedness in sub-topic 9.1.

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