Swimming Pool Chemistry


Most people still assume that swimming pools are just holes in the ground, filled with water. In reality, there are actually many chemical processes, which occurs in a swimming pool, to keep the water clean and safe. Pathogens such as adenoviruses, Hepatitis A viruses, Shigella and Escherichia coli 0157, which can cause fever, diarrhoea and vomiting, must be eradicated. [3] There are many other chemicals involved in balancing the water as well. [4]


Chlorination is the practice of adding chlorine to pool water, to destroy disease-producing microorganisms and other contaminants. [2,4] All forms of chlorine ionise in water to form hypochlorous acid (HOCL). [1,2,4] The HOCL formed, retains the oxidising property of chlorine, and can kill microorganisms by passing through their membranes and destroying their organelles. [1,4] However, HOCL further dissociates into hydrogen and hypochlorite ions, where the degree of dissociation depends on pH and temperature. [1,4] This makes pH control important, as hypochlorite ions are not as effective as HOCL in killing bacteria because of their negative charge. [1,4] At pH levels below six, HOCL dissociates poorly. [1,2,4] Then, between a pH of 6.0 and 8.5, there is a very sharp increase in the dissociation of HOCL to form hypochlorite ions. [1,2,4] However, acidic water is corrosive. [1,2,4] Hence, a pH level of 7.5 is perfect, having 50% of dissolved chlorine as HOCL. [4]


Chlorine compounds, in a solid or liquid state, are added to the pool manually or by erosion feeders. [2,4] Elemental chlorine, a poisonous gas, requires special equipments though. [2,4] It is cheap but extremely acidic, requiring a lot of base to counteract it. [1,4] Using an electrolysis cell, HOCL can be generated continuously if salt is added. [1]


Dissolved chlorine exists as “free available” chlorine if it has not bonded with another compound. [2,4] When it has combined with nitrogen compounds, creating chloramines, it loses its ability as a sanitizer. [1,2,4] This compound, which causes eye burn and strong chlorine smell in pools, is eliminated by superchlorination. [2,4] These compounds are now called combined chlorine. [4] Superchlorination is a process to oxidize and remove all ammonia compounds in a pool by raising the chlorine level. [2,4] A threshold called breakpoint chlorination is reached when chlorine levels are ten times the level of chloramines. [4] All dissolved chlorine exists as free available chlorine at this point because the additional chlorine oxidises chloramines to nitrogen gas and nitrate ions. [1,4]


HOCL dissipates under sunlight. [1] Cyanuric acid, used as a stabilizer, reacts with chlorine to produce dichloro(iso)cyanuric acid, which does not absorb the sun’s ultraviolet light. [1,4] When chlorine is used up in a pool, hypochlorite ions are released from the dichloro(iso)cyanuric acid to replace it, maintaining a constant amount of chlorine in the pool. [1]


Water balance is the adjustment of the pH and Total Alkalinity of pool water. [4] The pH of pool water is a measurement of how acidic or basic the water is. [2,4] The pH is adjusted, using acids and bases, to reach an ideal pH range of 7.2-7.8. [2,4] This is because acidic water causes corrosion and eye irritation where else basic water causes the formation of scale. [1,2,4] Alkalinity level is the ability of the water to resist changes in pH and is known as the buffering capacity of the water. [1,2] Alkalinity is raised by bases. [4]



Chlorinated pools do have drawbacks. They release trihalomethanes and nitrogen trichloride, resulting in accumulated concentrations around the pool. [3,5,6] Swimmers who inhale these toxins have increased risks of developing lung diseases, such as asthma. [5,6] Nitrogen trichloride triggers three proteins that destroy the cellular barrier protecting the lungs, unleashing an asthma attack. [6] Low pH levels in pool water can also cause dental enamel erosion. [5]



There are other sanitizers available other than chlorine. [4] Ozone water treatment produces ozone molecules, which are very powerful oxidisers, by passing air over an ultraviolet bulb or creating a small lightning in an air-filled chamber. [4 Catalytic sanitation uses a non-electric ionizer, which doubles as an ozonator. [4] This self-eroding metallic cartridge releases silver cations and produces ozone molecules too. [4] Enzymes can also be used to break down organic matter in pools. [4] Ultraviolet light was once a certified disinfection process for pools but