Chemistry of Swimming Pools

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Swimming pools are a brilliant way of cooling off during a hot summer. Of course, this isn’t a particularly original idea, and hundreds of people might use a particular pool every day. Chemistry is on hand to help prevent us from swimming in water that harbours potential water-borne infections. It can also help out with the cardinal sin of pool-peeing, though not without consequence. It does this, as you likely already know, through the chlorination of pool water – although it’s less simple than you might think!

Firstly, how is water chlorinated? You might expect that it’s accomplished using chlorine, but it’s actually become quite rare to chlorinate pools using chlorine itself. This is because of the toxic nature of chlorine gas, which makes it tricky to store, and potentially hazardous to health if an accident were to occur. Instead, other chemicals which can also accomplish water chlorination are used instead.

Chief amongst these are the hypochlorites, and some of the most commonly used are sodium hypochlorite and calcium hypochlorite. These compounds have an advantage over chlorine, in that they are solids at room temperature, and can be dissolved in water, making them much easier to store and use. Sodium hypochlorite is a compound you’ve probably come across in your household, too, as it’s a component of chlorine based bleaches. It’s sold in solutions of slightly higher concentration to chlorinate swimming pools, and can also be obtained in tablet form. Of course, once it’s in the water, it’s at a much lower concentration than that found in bleach, so the fact that it’s found also there isn’t a concern.

Both chlorine and hypochlorite salts react with water to produce a different compound called hypochlorous acid. This is a relatively weak acid, but also a strong oxidising agent, and actually largely responsible for the bactericidal effects of water chlorination. Exactly how it helps kill bacteria has been debated, with it thought to effect a number of factors at the bacteria cell membrane including suppressing the metabolic function of the cell, preventing its DNA replication, and stopping proteins in the cells from being able to group together.

Hypochlorous acid partially dissociates (splits up) in water, forming the hypochlorite ion. This is an oxidant around 60 times weaker than hypochlorous acid, so it isn’t as good at helping remove bacteria from water. Luckily, the dissociation of hypochlorous acid is reversible, and we can tweak it in our favour by monitoring the acidity of the pool. Whilst we want to keep it at a pH that’s still pleasant for us to swim in, by keeping it between 7.2–7.8, we favour most of the hypochlorous acid staying put, rather than breaking up to form the hypochlorite ion. The combined concentrations of the two are often referred to as ‘free available chlorine’ (FAC).

Another issue, particularly with outdoor pools, is that of UV photolysis. This is the break up of chemical compounds in the presence of UV light. As we well know, the sun gives off UV light, which is what we try to protect ourselves from using sunscreen. This UV light can also cause the break-up of hypochlorite ions (and, to a lesser extent, hypochlorous acid). This causes 90% of the FAC loss from outdoor pools, and means that outdoor pools require more frequent chlorination. Other chemicals can also be added to the pool water to help prevent this, which is something we’ll discuss shortly.

A common side effect of a day of swimming in a chlorinated pool is stinging eyes. Often, this is blamed on the levels of chlorine in the pool being ‘too high’; in fact, the opposite is true, as we’ll discover. Firstly, it’s not actually the hypochlorous acid or hypochlorite ions that cause these sore eyes. In fact, it’s the result of compounds produced by the reaction of these with chemical compounds contained in your sweat – or in your urine.

Both you sweat and urine contain ammonia or ammonia-derived compounds. Urea is a compound we associate with urine, but it’s actually also found at very low levels in sweat too. Uric acid is another compound that’s found in both. When these compounds react with the hypochlorous acid in chlorinated water, a range of compounds are produced, including some known as chloramines.

Chloramines are the compounds responsible for the eye irritation that any frequent swimmers reading this have no doubt experienced at one point or another. They’re also responsible for the smell we associate with swimming pools. Though we often refer to this as the smell of the chlorine, it’s actually these byproducts that produce it, so if a pool smells strongly of ‘chlorine’, it indicates a higher level of these compounds in the water – which is obviously not a good thing!

The percentage of people willing to confess to peeing in the pool is actually higher than you might expect: a whopping 19% of Americans asked in a 2012 survey admitted they have, at some point, relieved themselves in swimming pool water. In light of this, the fact that 79% of those asked in the same survey suspected that other people urinate in the water. This isn’t great news, because urine increases the amount of trichloroamine present in the pool water. Trichloroamine has been accused of causing respiratory symptoms in frequent swimmers and pool workers, with debate on whether it might be responsible for inducing asthma in some.

Another chemical produced as a consequence of pee in the pool is cyanogen chloride, a chemical which can also have some pretty unpleasant effects – although, at the concentrations produced in swimming water, it’s been questioned as to whether any ill effects would be seen. Of course, there’s a simple way to help prevent production of these compounds, and that is, obviously, not peeing in the pool. If you’re a dedicated pool pee-er, you may want to reconsider your position! Michael Phelps, take note.

The chlorine contained in these byproducts of chlorination is referred to as ‘combined chlorine’ (CC). The total amount of chlorine in the pool is the sum of the free available chlorine (FAC) and combined chlorine, and it’s recommended that the FAC level should remain between 1-4 parts per million. An Olympic swimming pool contains 2,500,000 litres of water, so this is actually an incredibly small amount.

Other compounds can be added too, as well as those added for the purpose of chlorination. Once such compound is calcium chloride. This is added to prevent the slightly soluble calcium sulfate, a component of the grouting between the tiles in swimming pools, from slowly dissolving away. It prevents this via something known as the common ion effect. Essentially, the calcium ions present in calcium chloride raise the concentration of calcium ions in the water, preventing much of the calcium sulfate from dissolving.

Another compound that can be added is isocyanuric acid. This compound is a herbicide, and so levels must be kept below 200 parts per million; it’s usually present at a much lower level. The reason that it’s added is that it can help stabilise chlorine levels, particularly in outdoor pools where they’re depleted by the action of UV light. Isocyanuric acid reacts with hypochlorite ions, producing dichloro(iso)cyanuric acid. However, this is a another reversible reaction, and as hypochlorite ions are depleted by UV photolysis, the breakdown of this compound back to isocyanuric acid and hypochlorite ions is promoted. It therefore acts as something of a chlorine ‘reservoir’, replenishing the lost hypochlorite ions.

It’s probably become clear through the course of this article that there’s a lot of chemistry behind keeping a swimming pool clean. The chemical landscape in a swimming pool is a constantly changing one, and careful management of it is required to maintain a safe, clean pool.

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