التجارب المعملية لمعالجة وتبييض وتطهير مياه حمامات السباحة

SWIMMING POOL WATER TREATMENT / BLEACHING

• Oxidation and Reduction…. Swimming Pool Water Treatment

• Oxidation numbers…. Bleaches

• Volumetric analysis…. Redox titrations

-Experiment

.
% (w/v) of hypochlorite in bleach

• Water analysis…. Colorimetry

-Experiment

…. Colorimetric experiment to estimate free chlorine in swimming pool water or bleach.

Bathers bring dirt and impurities to pool water, together with microorganisms, which can be harmful.

Swimmers hope for water that is clear, warm, and free from dangerous microorganisms and from algae.

Pool systems are designed to circulate the water through a heating system and treatment plant

Water in a swimming pool is circulated and constantly reused. One regime is to keep the water at 29 –30oC with about 1/8 of the pool water being replaced each week with fresh water.

The water treatment includes:

• Filtration

• Destruction of microorganisms

• Prevention of corrosion

The water in swimming pools is kept sterile by the addition of oxidizing agents, chlorine or chlorine compounds, which kill microorganisms by oxidation.

The active agent is usually chloric(1) acid (HOCl). It may be formed in two ways

1. Direct chlorination of the water:

Cl2(aq) +H2O(l) HOCl(aq) + Cl−(aq) + H+(aq)
0 +1 -1

Note that when the Cl2 reacts with the water it is both oxidized and reduced

2. The addition of sodium chlorate(I) [sodium hypochlorite]:

NaOCl(s) + H2O(l) Na+(aq) + OH−(aq) + HOCl(aq)

Nowadays chlorine is not used, mainly on grounds of safety.

Pools are sterilized with chlorine compounds, which produce chloric(I) acid when they dissolve in water.

These compounds act in essentially the same way as chlorine.

Sodium chlorate(I) is one such compound.

Chloric(I) acid and pH

Chloric(I) acid is a weak acid. It ionizes to form chlorate(I) ions, which have very little power to kill germs.

HOCl + H2O H3O+ + ClO−

Chloric(I) acid is a more powerful oxidizing agent than chlorate(I) ion.

The acid kills bacteria by oxidising the cell chemicals and the oxidising agent is reduced to Cl−.

Because the reaction of chloric(I) acid with water is reversible and the formation of ClO − ions is favoured at higher pH, pool managers have to check the pH of the pool water carefully.

They are advised to keep the pH in the range 7.2 to 7.8 to maintain a high proportion of un-ionised chloric(I) acid.

While low pH would favour the undissociated HOCl this would damage the fabric of the plant.

This pH range is a compromise. While being comfortable to bathers, it limits corrosion of plant and equipment and keeps the un-ionised chloric(I) acid at an acceptable level.

Water Analysis

Certain pool test kits contain two types of tablets.

Those used to test for pH are based on phenol red indicator which changes colour over a narrow pH range (6.8 – 8.4) from red to yellow.

The tablet contains a measured amount of indicator, which the tester adds to 10cm3 of pool water.

The tester compares the shade of yellow or red with a colour chart and reads the pH
.
FREE CHLORINE

The chlorine in chloric(I) acid and chlorate(I) ions together makes up what is called ‘free chlorine’

A pool manager has to make sure that more than half of the total chlorine in the pool is in the form of free chlorine.

If this level is too low bacteria can survive. If too high, compounds of ‘combined chlorine’ such as the volatile nitrogen trichloride are formed.

The recommended level of free chlorine is in the range 2 to 4 ppm.

TESTING FOR FREE CHLORINE using the second tablet type called DPD tablets and a colour chart.

The main ingredient in this tablet used by the tester is DPD, ( N,N-diethyl-p-phenylenediamine in the form of its sulphate).

Chloric(I) acid and the chlorate(I) ion oxidize DPD sulphate to a soluble red product.

The test must be done within a few minutes of the reagents being mixed to make sure it
measures only the free chlorine.

The deepness of the red colour is a measure of the concentration of the oxidizing chlorine in the water.

Experiment 9.1, the colorimetric experiment to estimate free chlorine in swimming pool water or bleach can be carried out using a colorimeter or a comparator.

Additionally, data logging equipment may be used in conjunction with this experiment.

BLEACHING

Bleaching is a redox reaction.

Bleaching agents are chemical substances that remove unwanted colours by either oxidation or reduction.

Colour in organic materials is often due to alternating double covalent bonds.

Oxidising bleaches break up these double bonds.

Most domestic bleaches contain sodium chlorate(I), NaOCl.

This is an example of an oxidizing bleach. When it bleaches by oxidation it is reduced to chloride
NaOCl → NaCl
+1 −1

Sodium hypochlorite has marked germicidal properties.

In aqueous solution, sodium hypochlorite oxidises essential enzymes and proteins present in bacteria and renders them harmless.

Chlorine is used for commercial bleaching of dyed cotton or linen but it is not suitable for use with wool or silk, which contain proteins, the >NH groups of which would react with the chlorine.

Reducing bleaches convert the double bonds to single bonds.
Reducing bleaches include sulfur dioxide used in solution where it forms SO32− and sodium sulfite, Na2SO3.

Because of environmental considerations the use of chlorine-containing bleaching agents has been restricted in many countries over recent years.

the syllabus deals with redox titrations.

Associated
The NaOCl in the bleach liberates iodine from acidified KI solution.

Therefore sodium chlorate(I) can be determined by titration with sodium thiosulphate solution.

Looking at the chemistry involved in this titration, and assigning oxidation numbers:

oxidised

NaOCl + 2I− + 2H+ → I2 + NaCl + H2O :
+1 −1 0 −1

reduced
Here the Cl (+1) in NaOCl is reduced to Cl− (−1) in NaCl
and the I− (−1) is oxidized to I2 (0)

This iodine can then be titrated against a standard solution of sodium thiosulphate

general.doctor

bahaa badr

water treatment analysis.

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