Processes of Water Chlorination

Abstract:

The article, “Water Chlorination Principle”, explains the process of water chlorination, a process in which water is treated with chlorine to make it fit for human consumption or safe enough to swim in. The article goes in depth in explaining the processes of water chlorination in terms of what substances are produced when others are added and the properties of these created substances. The main topic of the article surrounds the addition of chlorine to swimming pools, the dissociation it undertakes during reactions and how it is used in water treatment for the disinfection of microorganisms and/or oxidation. It also explains the processes of other substances that are added such as calcium hypochlorite, sodium hypochlorite and ammonia and the effects, in each of the stages, of adding these chemicals to swimming pools.

Chemistry Involved:

When chlorine is added to pure water (H2O), a mixture of hypochlorous (HOCl) and hydrochloric (HCl) acids is formed as shown by the following reaction;

Cl2 + H2O ⇌ HOCl + H+ + Cl- Eq 1

The hypochlorous acid created is used as an oxidising agent/disinfectant to kill any bacteria present in the water. The hypochlorous then breaks down as seen in the following formula.

HOCl ⇌ H+ + OCl- Eq 2

The hypochlorous acid dissociates, as an almost instantaneous reaction, into hydrogen and hypochlorite ions. These hypochlorite ions are what give hypochlorous its name for being an oxidising agent/disinfectant as the hypochlorite ions, due to their low stability, react with many organic and inorganic compounds that may have found their way into the swimming pool.

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There are other ways we can produce hypochlorite ions to treat the water. We can do this by adding calcium hypochlorite or sodium hypochlorite. When added to water, these substances ionize and yield their base element, in this case calcium and sodium respectively, and also produce hypochlorite ions as well as water as shown by the below two formulas.

Ca(OCl)2 + H2O ⇌ Ca++ + 2OCl- + H2O Eq 3

NaOCl + H2O ⇌ Na+ + OCl- + H2O Eq 4

These substances, calcium hypochlorite and sodium hypochlorite, and commonly referred to as chlorine which confuses a lot of people as they assume that the chlorine that they buy from pool shops is Cl2 when in fact it is not. Chlorine (pool chlorine, not Cl2) is the chemical most often used to keep swimming pools and spas free of all micro-organisms, including those that are hazardous to humans. Bromine is another common source of the purification of pools through the process of bromination.

Ammonia (NH3) is added to swimming pools to create chloramines that act as oxidising agents. Ammonia reacts with hypochlorous to form monochloramine (NH2Cl) and water as shown by the following formula.

NH3 + HOCl à NH2Cl + H2O Eq 5

Monochloramine is a highly unstable compound in concentrated form, but because only a dilute solution is used it is stable enough to use in swimming pools and it is often this stability that is the basis of its applications. The monochloramine also reacts with hypochlorous to further breakdown to form dichloramine as shown by the following formula.

NH2Cl + HOCl à NHCl2 + H2O Eq 6

Dichloramine reacts with hypochlorous also break down yet again only to form water and trichloramine which is also referred to as nitrogen trichloride.

NHCl2 + HOCl à NCl3 + H2O Eq 7

Nitrogen trichloride is a common by product when pools are found to contain monochloramine (NH2Cl). In this case monochloramine was also the product of a reaction but we were trying to create this one as seen in Eq 5. The reaction between hydrogen sulphide and chlorine is generally a very rapid one.

H2S + 4Cl2 + 4H2O à H2SO4 + 8HCl Eq 8

Hydrogen sulphide, famously known for its awful odour that resembles the odour of a rotten egg, is commonly found in pools as a result of anaerobic bacteria. The bacteria produce this as a waste product and with the help of chlorine we can break it down to sulphuric acid.

Focus Questions:

• What is Le Chatelier’s principle and how does it incorporate into pool chemistry?

ANSWER: Le Chatelier’s principle states that when a system at equilibrium is disturbed, the equilibrium position will shift in the direction that counteracts the effect of the disturbance. For example, if the pressure acting on the equilibrium system is increased, then the equilibrium position will shift to reduce the pressure. Conditions that can affect the equilibrium are not limited to the pressure acting on the system but can also include; the concentration of the reactant, the temperature acting on the system and the changes of volume of a gaseous equilibrium.

With this knowledge in mind we can assess how this is used in swimming pools. Swimming pools grow algae and contain microorganisms after a period of time. We add chemicals to the pool to clean it and rid it of these microorganisms and algae, this process is chlorination. When we add chlorine to a swimming pool we are increasing the concentration of it as the chlorine that was added a period of time ago has reacted and rid of algae and microorganisms therefore reducing the amount of chlorine left which means that the concentration has been lowered and that is why chlorine needs to be added. The increase in concentration of chlorine will result in the shifting of the chemical equilibrium. The chemical equilibrium of the equation; Cl2 + H2O ⇌ HOCl + H+ + Cl- will shift to the side that decreases the total change in concentration. Using le Chatelier’s principle we can predict that the amount of hypochlorous (HOCl) will increase resulting in a decrease in the total change in chlorine (Cl2). With this happening the system will stay in equilibrium.

• What is chemical equilibrium and how can a swimming pool reach chemical equilibrium?

ANSWER: Chemical equilibrium is the ability of chemical reactions to resist change, despite any changes that may be imposed on them. Changes that may be imposed upon can include; the pressure acting on the equilibrium system, the temperature of the equilibrium system and even the concentration of the substances. If any of these conditions are changed the equilibrium will move in the opposite way to counteract what was changed. For example, if the total pressure acting on the reaction system is increased, then the equilibrium moves in the direction that will reduce the pressure to counteract it.

Chemical equilibrium can be reached, in swimming pools, by chlorination. Chlorination is the process of adding chlorine to water to purify it so that it is suitable for human consumption or in this case, suitable for swimming in. Shock chlorination is another method that can be used to reduce the bacterial and algal residue in the water. Shock chlorination is performed by mixing large quantities of sodium hypochlorite into the water. Any water that has undergone shock chlorination should not be swum in or drunk until the sodium hypochlorite count in the water reduces to at or below 3ppm. It is common for calcium hypochlorite to be used for disinfection of swimming pools as well. However, it is very important that the sodium hypochlorite and calcium hypochlorite do not get mixed together as there is a risk of an explosion occurring that could result in serious injury.

Chemical equilibrium is reached in swimming pools once all the different chemical equilibria have been established to the right state. Once this happens they tend to remain that way for a while before needing to be balanced out again. The chemical equilibriums in pools that need to be balanced are shown in the reaction equations above (Eq 1-8). Each of these reactions must be at equilibrium which means that the reaction has occurred. Some of them react in the opposite way (backwards) to form an equilibrium. This is enabled by the fact that they are reversible reactions. After this equilibrium has been found the reaction is stable and will stay that way for a period of time until chlorination needs to be repeated to restore the chemical equilibrium.

• What is the difference between the three chloramines (monochloramine, dichloramine and trichloramine more commonly known as nitrogen trichloride) and what are they used for?

ANSWER: The main differences between the three chloramines are that they are all used for different things and that the further the chloramine has been dissociated the more dangerous it becomes. For example, monochloramine is added to pools as a disinfecting agent while dichloramine is a by product of the monochloramine reacting with hypochlorous. Nitrogen trichloride is formed when monochloramine reacts with hypochlorous.

Monochloramine, in dilute solution, is quite stable and it’s because of this feature that it is the reason for its applications. Monochloramine is primarily used as a disinfecting agent in pools as a secondary chemical only to chlorine.

Dichloramine is commonly found in pools when they have been disinfected by monochloramine as dichloramine is a by-product of the reaction between monochloramine and hypochlorous. Nitrogen trichloride is the same, in being that it is created as a by-product of the reaction between dichloramine and hypochlorous.

Nitrogen trichloride is very dangerous as it has the same effects as that of tear gas, although it has not been used for that purpose. Nitrogen trichloride is very unstable and it is very sensitive to light, heat and organic compounds. Nitrogen trichloride can be found in swimming pools in small amounts but can gradually generate to large amounts if the pool is not maintained properly and not carefully looked after. Nitrogen trichloride forms when chlorine reacts with most types of ammonia. Ammonia is commonly found in urea which is found in urine, therefore meaning that if people urinate in the pool, nitrogen trichloride will be present. Nitrogen trichloride can also form when monochloramine is found to be present in the water. This is common as monochloramine is used as a disinfectant in swimming pools. Even if only small traces are present in pools, nitrogen trichloride can be irritating to mucous membranes.

• What are the positive and negative effects of human exposure to pool chlorine (not Cl2, but NaOCl and Ca(OCl)2)?

ANSWER: The concentration of pool chlorine that is found in swimming pools is generally not strong enough to be harmful to humans. However, if there is an excess amount present in the water it will burn the body tissues, which causes damage to air tracts, the stomach, the intestines and the eyes. Eyes will get inflamed if exposed to high levels of sodium hypochlorite but symptoms will disappear after a short period. Chloramines will be formed if there are traces of ureum (a mixture of urine and sweat) present within the pool. Chloramines irritate mucous membranes and cause the pool to smell of the so called ‘chlorine smell’. These problems in swimming pools are prevented by water purification known and ventilation of indoor swimming pools.

Not all effects of chlorine are negative. Chlorine has natural antibacterial properties to kill algae’s and microorganisms in pools. As a result of this feature, a person entering the water with any external wounds will find that the wound will heal quicker if it has been exposed to chlorine. This is because the chlorine penetrates the wound killing any bacteria present. As the bacteria dies, the infection is negated and the wound gradually heals. The same feature of chlorine is also useful to facial sores such as acne. The chlorine has a drying effect and this combined with the antibacterial effect is great. The chlorine penetrates deep into the pores and removes any grease and bacteria. The chlorine then dries the pore and it heals. However, excessive exposure to chlorine can make the skin become too dry. The body then overcompensates and produces more oils reversing what was done and making it worse. It is best to only use pools moderately and it is also good to apply a moisturiser after swimming as this will prevent the skin from drying out too much.

Humans with external wounds that are fresh or open should not enter the water as the wound is more likely to become further infected from any bacteria present in the pool. It is only after open wounds have formed a scab that can protect it from bacteria that it is safe to use the healing effects of chlorinated water. Chlorinated water should NEVER be used to treat internal wounds such as those inside the eyes and mouth as the soft tissues lining the inside of the mouth and eyes are soft tissues capable of absorbing very harmful bacteria. In addition to this the chlorine can produce irritation to these areas in the form of a burning sensation.