Effect Of UV Rays On Pool Chlorine Biology Essay

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Pools have always been a source of leisure for Australians. They provide enjoyment and good times for many households generated in a family or local swimming pool. Despite this, pools have to be maintained by using chemicals or disinfectants, lest unwanted pathogens create health hazards. However, the over use of these chemicals can also result in health hazards such as; serious irritation and breathing problems. A balance or equilibrium, therefore, must be achieved to maintain suitable conditions. This is where chemistry comes into play.

The main type of chemicals used in pools to prevent unwanted pathogens is pool chlorine. Sodium hypochlorite is a commonly used as pool chlorine. It was discovered by Louis Pasteur in the late 19th century that sodium hypochlorite had disinfectant properties. Sodium hypochlorite effectively kills bacteria, viruses and fungi. When pool chlorine is added to water, a reaction takes place where hypochlorus acid is formed.

In the past, most consumer chlorine bleach was sold in a 5.25% solution. Today, some more concentrated solutions are being sold and touted as an improvement over the less concentrated bleaches.

To check for the concentration of the pool chlorine, excess potassium iodide is to be added to it and then it is to be titrated with a solution of sodium thiosulfate. The concentration of the liquid chlorine is then to be calculated using stoichiometry. The net ionic equations of the chemical reactions are:

Source: Chemistry In use Book 2

There are also many factors that affect the effectiveness of pool chlorine such as sun radiation.

Ultra-violet (UV) light degrades hypochlorous acid to hydrochloric acid (HCl).

Ultraviolet (UV) radiation not only destroys light sensitive chemicals such as sodium hypochlorite (NaOCl), it also has the potential to significantly degrade the structural integrity of the storage tank or IBC containing the light sensitive material. 

According to the NSW Ministry of Health (2010) roughly 1/3 of free chlorine exposed to UV lights in a outdoor swimming pool is destroyed every hour. UV light therefore lowers the concentration of the disinfection component of free chlorine.

In this experiment, the concentration of sodium hypochlorite in pool chlorine, left under a UV lamps for different periods of times, was measured.

Aim: To investigate the effects of Ultra Violet (UV) rays on sodium hypochlorite (a bleaching agent used commonly in pools).

2.0 Hypothesis: As pool chlorine is left for longer periods of time under UV rays, the concentration of the pool chlorine will decrease.

This was hypothesised as the theory suggests that UV rays degrade the component in pool chlorine which allows it to disinfect bacteria.

3.0 Equipment/Material


Diluted Sodium hypochlorite

Sodium thiosulfate

Potassium iodide

Starch indicator

Boiling water


2 x 100 mL Beaker

5 x 250 mL Beaker

15 x 100 mL Flask

2 x 1L Volumetric flask

1 x 500 mL Volumetric flask

1 x 100mL Measuring cylinder

2 x 20mL Measuring cylinder

3 x Funnel

5 x 20 mL Pipette

2 x 1mL Pipette

3 x Stirring rod

Senior Balance

Burette and stand

UV Lamp

Permanent marker


4.0 Safety

To prevent any harm to the group during this experiment, safety precautions were taken to ensure the safety of the members. Refer to Appendix A for MSDS (Material Safety Data Sheet).

Lots of glassware was used - Refrain from holding too many pieces of glassware at once. Clean up glass immediately if any is broken.

Handle boiling water with care - Boiling water was used to create a solution of starch. Beaker tongs were used to carry the boiling water.

Potassium iodide is irritating to eyes and skin - Avoid contacting without gloves. Wash with water if contact occurs.

Sodium thiosulfate is irritating to eyes and skin - Avoid contacting without gloves. Wash with water if contact occurs.

Starch gives skin discomfort - Avoid contacting without gloves.

5.0 Procedure

Refer to Appendix 1 for preparation of solutions

Preparation of titration

A burette and stand was filtered with the sodium thiosulfate solution.

The burette was filled up with the sodium thiosulfate solution to around the 0mL mark.

100mL of diluted sodium hypochlorite was poured into 5 individual 250mL beakers.

These beakers were labelled 0min, 15m, 30m, 1hr, 3hrs and 4hrs.

All the beakers, except the 0min beaker, were placed under the UV lamp for the amounts of time labelled on them. At this point the stopwatch was started, once the samples had reach the specified time they were taken out.

For each time sample, 20mL was measured with a pipette and placed into 3 100mL flasks.

10mL of potassium iodide solution and 2mL of starch indicator solution were prepared prior to every titration.


The mark of where the sodium thiosulfate solution in the burette was recorded before the titration occurred.

10mL of potassium iodide solution was added to each time sample of 20mL sodium hypochlorite solution in the 100mL flasks. The new solution was left until it has completed reacted (when the solution turns light yellow).

The solution was then titrated until it turned a very pale yellow.

2mL of starch indicator solution was added to the titrating solution. This should make the solution a dark blue/black colour.

Titration continued at a slower rate until the solution had turned clear.

Record the finishing titre mark on the burette.

These steps were repeated 3 times for each time sample (total of 18 titrations).

Figure 1: Diagram of titration in progress.

6.0 Results

Table 1: Table showing the amount of sodium thiosulfate solution added to sodium hypochlorite and potassium iodide solution at different times left under the UV ray.

Refer to Appendix 2 for initial volume and final volume calculations and averages.

Time (mins)

Volume (mL)

Average of 3 titrations













Table 2: Table showing the concentration of pool chlorine

Refer to appendix 3 for converting of average volume of titration to concentration.

Amount of time left under UV lamp (mins)

Concentration of OCl (M)













Figure 1: Graph showing the concentrations of pool chlorine left under a UV lamp for different periods of time.

7.0 Discussion

An experiment design was made to test and compare the effects of UV rays on pool environments, particularly on the impacts of chlorine concentration. In doing this, a total of 18 pool chlorine samples were put under a UV lamp for different periods of time. The concentrations of the chlorine when left untouched by UV rays were stable at around 0.03M. By exposing the pool chlorine samples to ultraviolet lights for given periods of time, the concentration of the chlorine decreased. The graph shows that as the sodium hypochlorite is more exposed to UV rays, the lower concentration of the pool chlorine was. A line of best fit was drawn to display the trend in the data. It showed that there is a linear relationship between the concentration and time left under a UV lamp. The data collected agrees with the theory that UV rays disrupt the structural integrity of the pool chlorine. The photodecomposition of chlorine is apparent as the initial levels of hypochlorite ion decreased by a significant amount when exposed to sunlight. The photochemical reaction that represents the situation of the experiment is:

The main errors of the experiment were the inaccuracies of most aspects dealt with when implementing the experiment.

One anomaly occurred when one sample of chlorine was left under the UV lamp for 60minutes. There is a major drop in concentration from 30min. This suggests that there was experimental error during the investigation.

8.0 Conclusion

The results collected from this experiment suggest that the more pool chlorine is subject to UV lights, the lower the concentration of the chlorine will be. It has partially supported the hypothesis


Deb Smith, D. R. (2006). Chemistry in use - Book 2. Sydney: Queensland Chemistry syllabus.

Date retrieved: 05/09/12

Fletcher, D. J. (N/D). (The Sodium Hypochlorite Story). Retrieved September 5, 2012, from south shore gunite pools: http://www.southshoregunitepools.com/resources/htms/naocl.htm

Gina A. Ishida, B. ÷. (N/D). IMPACT OF CHLORINE AND MONOCHLORAMINE ON ULTRAVIOLET. University of North Carolina, Chapel Hill, NC: N/A.

Date retrieved: 05/09/12

N/A. (N/D, N/D N/D). Chlorination of pool water. Retrieved September 6, 2012, from pested: http://www.pested.msu.edu/resources/bulletins/pdf/2621/e2621chap7.pdf

N/A. (1968). Stabiliser (Cyanurate) Use in Outdoor Swimming Pools. Retrieved September 5, 2012, from NSW Government Health: http://www.health.nsw.gov.au/utilities/copyright.asp


Appendix A

Going to add MSDS later…

Appendix 1

Diluting sodium hypochlorite

100mL of sodium hypochlorite was measured and put into a 1000mL volumetric flask.

The flask was filled with distilled water up to the 1L mark

The flask was inverted several times

Sodium thiosulfate solution

8.82g of sodium thiosulfate was placed into a 1L volumetric flash

The flask was filled with distilled water up to the 1L mark

The flask was inverted several times

Potassium iodide solution

5g of potassium iodide was weighed and placed into a 250mL beaker

Distilled water was poured until it reached 105g on the scale

The solution was thoroughly mixed

Starch solution

1g of starch indicator was weighed and placed into a 250mL beaker

100mL of water was boiled and placed into the 250mL beaker

The solution was thoroughly mixed and was left to cool down

All solutions were labelled with the name of the solution, the concentration, a group member's name and the teacher's name.

Appendix 2

Initial volume and final volume of sodium thiosulfate left under a UV lamp at different periods of time.

0 min - 23.35-0.04 15 min - 45.63-23.29 30min - 22.34-0.07

= 23.31mL =22.34mL =22.27mL

46.76-23.31 23.04-0.14 44.78-22.32

= 23.45mL =22.90mL =22.46mL

23.54-0.13 46.66-23.04 23.01-0.21

=23.29mL =23.62mL =22.80mL

Avg = = =

=23.35mL =22.95mL =22.51mL

60 min - 41.22-23.01 3 hour - 15.62-1.23 4hours - 13.23-0.12

=18.21mL =14.39mL =13.11mL

18.62-0.33 30.59-15.62 26.35-13.23

=18.29mL =14.97mL =13.12mL

37.26-18.72 49.32-30.59 38.21-26.35

=18.54mL =18.73mL =11.86mL

Avg = =

=18.35mL =16.03mL =12.70mL