Why do I need to re-Chlorinate so often on a sunny day?

Why do I need to re-Chlorinate so often on a sunny day?

Chlorine is commonly used in pools to kill bacteria and algae; this is because it is a known disinfectant. It works by producing hypochlorite ions, which under the effect of sunlight release oxygen free radicals. The oxygen free radical reacts with all organic matter it encounters, killing the bacteria present in water. Some chlorine molecules that fail to ionize in water also react with organic matter. However, Chlorine does have its disadvantages and one of them is the fact that the hypochlorite ions decompose very quickly under sunlight, on a cloudless sunny day, 90% of the chlorine level in a pool can be destroyed in about two hours. This can mean that pools need to be chlorinated repeatedly to keep the levels at the required level to kill bacteria. In this month’s experiment we are going to investigate the effects of sunlight on Chlorine within water and the way in which thermal blankets and chlorine stabilisers can keep the levels higher over a period of time.

3 x Foam Containers (10L capacity)
1 x Foam Container lid to replicate a thermal blanket
1 x Tub (30L capacity)
1 x EC-C201 - Free Chlorine and Total Chlorine Colorimeter
25 x 94X377001 - Free Chlorine Reagent
1 x 500g Bag of Granulated Chlorine
1 x 2kg Bag of Pool Stabiliser

1) A large tub had water added to it until there was 24L of water available. Granulated Chlorine was then added to the tub and a measurement was under taken using the EC-C201 in the Free Chlorine mode. Measurements were continually performed and Chlorine added until a 1.14ppm of Free Chlorine level was reached (a healthy pool should have a level between 1 and 3 ppm).
2) The water in the large tub was then separated into 3 separate 8L containers. 0.4g of stabiliser was added to one tub as per the manufacturer’s instructions for the 8L volume. The second tub was then covered using a foam lid (to replicate a pool blanket) and the final blanket was left uncovered.
3) The 3 tubs were then taken outside and placed in a sunny area.

Samples outside
Figure 1: All containers outside ready for testing

4) The Free Chlorine levels and UV levels were then measured every hour until a 3 hour period had expired.
Reading of uncovered sample
Figure 2: Reading of uncovered sample after 3 hours

5) All results were recorded and are detailed in the table below.

Results and discussion


Initial Reading

1 hour

2 hours

3 hours
















Table 1: Results of Free Chlorine levels (ppm) and UV levels over time

The results showed that the covered and stabilised chlorine containers remained close to the 1.14ppm level over the course of the measured period, only dropping by 0.10ppm for the covered container and 0.06ppm for the stabilised container. The uncovered container dropped significantly over the course of the 3 hour period. At the end of this period the Chlorine level was down to 0.30ppm which is over a 70% drop from the initial reading. The reason for this is that when UV from the sun hits the Hypochlorite it breaks apart and releases the Chlorine as gas into the atmosphere. 

The reason that this significant drop did not occur with the covered container is that UV cannot enter and as such there is no way for it to break down by way of UV. In saying this the break down that did occur may have been due to the fact the free Chlorine reacted with organic matter and bacteria within the water causing the level to drop. This is also similar to what occurred with the stabilised container, Stabiliser contains Cyanuric acid and when this reacts with Free Chlorine it forms a compound that does not react with UV and as such maintains chlorination levels in outdoor pools that have a high degree of contact with sunlight.

This experiment shows that in order to keep your Chlorine levels higher over a period of time you are best to either cover your pool to stop or reduce contact with UV or add stabiliser to your pool to keep levels higher over a period of time.

If you would like to discuss this experiment further please contact one of our friendly Scientists via email or phone on 1300 737 871. By Daniel Craig McGaffin
By Daniel craig McGaffin

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