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Analysis of Available Chlorine in a Swimming Pool

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Chlorine is the main chemical used in pools to sterilize and destroy harmful organisms like algae and bacteria. It was used as a common disinfectant until the early 2000s when it was industrialized as a pool sterilizer.

The aim of this investigation is to determine the impact UV rays have on hypochlorous acid and hypochlorite ions through numerous titration procedures, to discover the molar concertation of the hypochlorite ion. Essentially the cyanuric acid filling it’s desired role for reducing the dissociation of hypochlorite ion caused by UV rays; hence, increasing the effectiveness of chlorine for longer time frames.

Background Theory

a reaction takes place in order with water in order to form hypochlorous acid (HoCL) and chlorine ions (CL-). This reaction can be written in the following equation:

Cl2 (aq) + 2H2o ↔ HoCl(aq) + Cl-(aq) + H2o (equation 1)

The HoCl then dissociates almost immediately into hypochlorite ions an hydrogen:

2H2o + HoCl(aq) H3o +(aq) + oCl-(aq) (pKa = 7.54) (equation 2)

HoCL and oCL act on micro-organisms by attacking distinctive membranes in the cell walls. However, hypochlorous acid is eight times more effective in destroying organisms as the negative charge on oCl slows down its entry into cell walls.

HoCL and oCL are useless as sterilisers alone in a pool as they are not protected from the effect of sunlight. at a wavelength of between 290-350 nm with the presence of ultraviolet light, both undertake a photochemical process called photolysis.

asxHoClxisxmorexeffective in ridding unwanted organisms, it would be reasonable to decrease the pH so there is more HoCl present to sterilise. (see Figure 1).The free chlorine content is maintained within a range of 1-3 ppm range. When HoCl and oCl react with the ultraviolet light, towards 50% of the free chlorine content is converted to unusable chloride io within 17 minutes (NCBI, 2009). Because of this (Figure 1) degradation of free chlorine content, cyanuric acid (1,3,5 triazine 2,4,6 triol) (C3N3o3H3) is added to reduce photochemical degradation of the available free chlorine ion, and is accomplished by the formation of chloro isocyanurate acid (Chlorinated isocyanurates) that do not absorb UV light.

Because of this is has biological activity. at λ = 290 to 350 nm, according to Doug Le De Matter; about 90% of chlorine consumed by outdoor pools is the lost due to the photolysis


2oCl- + νUV → 2Cl- + o2(g) (equation 3)

2HoCl- + νUV → 2HCl+o2(g) (equation 4)

In order to stabilize chlorine levels, isocyanuric acid is added to the water and reacts with oCL- to give dichloroisocyanuric acid.

Dichloroisocyanuric is not impact but the sunlight and is not broken down, however the equilibrium reaction will shift in order to counteract for losses of oCL-. To achieve optimal photochemical decomposition of available chlorine, a test done by John a.Wojtowicz at Chemcon in 2004 a test was done on samples exposed to sunlight of chlorine demand-free swimming pool water at a PH level of 7, and between a temperature of 29-32°C which contained 2.5ppm av Cl. It was done at 0,25,50 and 100 ppm cyanuric acid, and it showed that cyanuric acid greatly reduced the photochemical decomposition of the available chlorine.

The reaction of hypochlorite ion and the dichloroisocyanuric acid is;

C3o3N3H3 + 2oCl-(aq) 2oH-(aq) + C3o3N3Cl2H (equation 5) as a result of this reaction there is a loss of oCL-. The equilibrium will therefore shift to counteract the losses of oCl- according to Le Chatelier’s Principle. Within the range of 25-50 ppm (the ideal concentration of cyanuric acid in a healthy swimming pool, the equilibrium between the cyanuric acid hypochlorite ion is established. Therefore, both constants K and Q are equal:

according to Le Chatelier’s Principle, if extra cyanuric acid is added to the equilibrium solution the reaction will adjust accordingly by shifting towards the right to compensate for the excess levels of cyanuric acid. Therefore, the concentrations of C3o3N3Cl2H and oH-, will increase, and the concentrations of C3o3N3H3 and oCl- will decrease. as a result of this, it will impact the ability for the pool to disinfect negatively because HoCL provides the majority of the sterilization in a pool. Therefore, an excess of cyanuric acid will negatively impact the sterilizing; therefore, it is important to find an appropriate amount. Because HoCl is more effective in ridding bacteria and organisms, the pH needs to decrease so more HoCL is available to sterilise. However, an extreme low pH cause corrosion and irritation, on the contrary a high pH also has negative effects; hence why a range of between 7.2-7.8 is important (Smith, 2006). as well as this a range between 7.2-7.8 is important because it is the same pH as the human eye and mucous membranes (Pahlen, 2018). This range allows for effective sterilisation without causing any damage.

It is hypothesized that the cyanuric acid and chlorinated water solution which has been exposed. Therefore, it is hypothesized that the pool water sample containing cyanuric acid which has been exposed to the least amount of sunlight will contain the highest amount of hypochlorite moles and the pool water sample without cyanuric acid that has been exposed to the highest amount of sunlight will contain the least amount of hypochlorite moles.

Based on theory, it is predicted that there will be a significant difference in free chlorine concentration between the cyanuric acid trials and the non-cyanuric acid trials for each time interval. These differences are based on the information found that the hypochlorite ion present in non-cyanuric acid solutions degrades at a rate of 35% each hour, which will produce an exponential decay model.

For the cyanuric acid solution trials, the theoretical relationship of concentration being essentially maintained is based on the theory that only 2% of hypochlorite ions will dissociate at each hour when cyanuric acid is added, which will produce a more linear decay model.


It should be noted that all trials were conducted on the same day and during the same time period, but with no control over temperature and the amount of sunlight the solutions were exposed to, there was not complete correlation between each trial.

First, using a pipette, 40 ml of 12.5% Sodium Hypochlorite was dispensed into 960 mL of deionized water in a 1L volumetric flask, until water level was at calibration line. Secondly, 100ml of solution was added into 5 beakers, then immediately covered with a watch glass. The solutions, apart from the control (0 minutes) were then placed in direct sunlight for 30, 50, 120 and 180minutes. a burette which contained standard sodium thiosulfate solution was measured for the initial volume of liquid, then mixed with 5 ml of 1.0M potassium iodide solution and 5 ml of 1.0M sulphuric acid until a dark brown colour formed, as iodine is formed. The mixtures were then titrated with the sodium thiosulfate solution, until the yellow brown colour had become pale. Finally, 1-2 ml of starch indicator was added until the solution turned blue. The titration was continued until the blue colour disappears, and the final volume was recorded in the burette and this procedure was repeated until concordant result was achieved.


Trends and Patterns

Graph 1 displays numerous relationships between the time exposed to UV rays and the concentration of Hypochlorite ions. Firstly, the graph depicts an exponential decay model for the non-cyanuric solution as the concentrations decrease increasingly for each time period. Secondly, the linear relationship occurs for the cyanuric acid sample as the concentration and by approximately the same amount for each time period. The cyanuric and non-cyanuric acid solutions, relationships, are different to each other which ultimately displays the effect cyanuric has as a stabiliser for the available chlorine

Cyanuric acid Solution

It is evident that Cyanuric acid in the solution steady concentrations of hypochlorite ions which stayed essentially constant over the 180 minutes. Hence, in graph displayed above, the cyanuric acid solution data are nearly fully linear between the hypochlorite ions and the duration the solutions were exposed to UV light. a big anomaly is evident with cyanuric acid as at 30 and 60 minutes the concentration of oCl- is greater than to begin with; a difference of 0.000500 and 0.000480, this impossibility is due to human error. Because the difference is extremely minimal the effect is having on the overall results is minimal and the result can still be used. The remainder of the results however comply with 1% decrease principle theory. These results effectively show the effect of cyanuric acid on the photodissociation of hypochlorite ions.

Non-Cyanuric acid Solution

The non-cyanuric results, showed a decrease in the content of hypochlorite content as a direct result of increase UV exposure. as hypothesised in the research journal, the moles per litre of non-cyanuric solution trials being substantially smaller than the mole per litre of cyanuric acid solution, remained constant throughout the entire procedure. The non-cyanuric acid solution displayed a decrease in hypochlorite content as a direct correlation to more UV exposure, as predicted.

Suggested Improvements

The most difficult variable to control was the strength of the UV rays which causes photolysis upon the hypochlorite ions. This could be controlled by controlling the UV rays, using a UV lamp.

However, it should be noted that both solutions were exposed to that same inconsistent strength of UV rays and therefore there was no difference in the amount of UV rays each solution was exposed to. Furthermore, it is important to realise that this experiment more closely follows a chemical system of an actual outdoor swimming pool, in regards to the amounts of UV radiation it is exposed to and therefore is useful for real life situations. In addition, a test on the level of pH in the two solutions should have been conducted prior to the experiment to ensure that the pH range was between 7.2 and 7.8. To better eliminate and limit the effect of errors and potential risks in data, the number of titrations could be increased in order to get more concordant results, as well as multiple more time periods. additionally, validating the time period of exposure to UV light (which would be easier to control if a UV light was used) and establishing the pH level being used prior to experimentation would ensure more exact figures as well as the reduction of expected error.

Finally, there are also several factors that must be considered when applying these findings to society. This investigation was completed on one day with a unique amount of UV rays effecting the concentration of hypochlorite ions and the cyanuric acid solution was a saturated solution. Therefore, in order to obtain more accurate results, experiment across a number of days with varying UV intensity and weather conditions which way alter the data on one particular day. a saturated solution of cyanuric acid is not realistic l for practical applications as then the entire swimming pool would have to be fully saturated cyanuric acid solution; which is a human safety hazard because of contact with eyes and cause serious skin irritation. To further the accuracy of this experiment, in order to accurately replicate realistic pool conditions, urea could be added in the future. *Tests have been conducted on the effect of cyanuric acid at different temperatures in pools. The results show that the loss at 13-15.5 degrees C were 1.9% per day but rose to 12.55% per day at 26.6-29.40C This means that the chlorine loss increases by a factor of 2 for every 100F(5.60C) rise in temperature (Wojtowicz 2004). Urea, Co(NH2)2, can be introduced into pools via urine and sweat. Urea does not appear to affect sterilisation but should be destroyed in pools as it is a nutrient for bacteria and algae. oxidation of urea by free chlorine is a slow process that gives rise to transient ammonia chloramines (e.g. di- and trichloramine) (De Laat et al (2011). Urine contains approximately 9.3g/L of urea. Investigations in outdoor pools showed that with a urine input of in very large pools the loss of chlorine increased from 14 to 26 % per day (Wojtowicz 2004).


This investigation’s results showed that ultraviolet light has a significant effect on the free chlorine concentrations of hypochlorite ions. This effect being that as time increases, and therefore amount of time exposed to UV rays increases, the concentration of hypochlorite ions decreases. In addition, the use of cyanuric acid results in this decrease being dramatically reduced, therefore allowing the hypochlorite ions to fulfil their purpose of destroying bacteria and algae for longer time periods. However, the exact amounts of cyanuric acid that should be applied could not be established, only the knowledge that it reduces hypochlorite ion dissociation. Swimming pools are one of the most popular additions to the public, to schools and to homes and an understanding of the effect of UV rays on unprotected free chlorine is essential for ensuring safe practises are applied.

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