Chlorine is the primary chemical agent for maintaining pool water health and clarity. When added correctly, it rapidly neutralizes harmful waterborne pathogens, such as bacteria, viruses, and algae. Continuous chlorination prevents the growth of these microorganisms, ensuring a safe swimming environment. The chemical process chlorine undergoes requires careful monitoring to sustain its effectiveness.
Essential Roles: Sanitizer and Oxidizer
Chlorine performs two distinct functions in pool maintenance: sanitization and oxidation. As a sanitizer, its primary job is to deactivate and kill living contaminants and pathogens, which is crucial for public health.
Oxidation involves breaking down non-living organic materials like sweat, oils, cosmetics, and urine. These contaminants consume chlorine, reducing the amount available for sanitization. When chlorine reacts with these wastes, it forms chloramines, which cause the strong odor often mistakenly associated with high chlorine levels. Oxidation destroys these compounds, ensuring chlorine remains available for sanitization.
The Chemistry of Cleaning: Free Available Chlorine
The effectiveness of chlorine depends on a chemical equilibrium that occurs when it is introduced into water. The total measured amount of active chlorine is known as Free Available Chlorine (FAC), which consists of two simultaneous forms: hypochlorous acid (\(\text{HOCl}\)) and hypochlorite ion (\(\text{OCl}^-\)).
Hypochlorous acid (\(\text{HOCl}\)) is the fast-killing, highly potent form of chlorine, substantially more effective at destroying microorganisms than the slower-acting hypochlorite ion (\(\text{OCl}^-\)).
The relative concentration of these two forms is heavily influenced by the water’s pH level. A lower pH favors the formation of the highly active \(\text{HOCl}\), increasing the overall killing speed of the FAC. Conversely, a higher pH shifts the balance toward the less effective \(\text{OCl}^-\). For example, if the pH rises from 7.5 to 8.0, the percentage of strong \(\text{HOCl}\) drops significantly from 50% to around 20 to 25%.
Types of Chlorine Used in Pools
Pool owners choose between two categories of chlorine products: unstabilized and stabilized.
Unstabilized chlorine, such as liquid sodium hypochlorite and granular calcium hypochlorite (Cal-Hypo), lacks protective additives. These products are fast-acting for immediate sanitation but are vulnerable to sunlight, as UV rays can break down the free chlorine quickly.
Stabilized chlorine products, like trichloroisocyanuric acid (trichlor) tablets or dichloroisocyanuric acid (dichlor) granules, incorporate cyanuric acid (CYA). CYA acts as a “sunscreen,” shielding chlorine from UV degradation and allowing it to remain in the water longer, making stabilized products preferred for outdoor maintenance.
Although CYA extends chlorine life, its continuous buildup can diminish sanitizing power. Pool owners must monitor CYA levels, as high accumulation necessitates partially draining the pool for dilution. Unstabilized chlorine is preferred for indoor pools, where UV degradation is not a concern, or when a quick spike in chlorine levels is needed without adding CYA.
Safe Handling and Storage
Chlorine products require strict adherence to safety protocols during handling and storage. Never mix different types of chlorine products, nor combine chlorine with acid, as this can trigger a dangerous chemical reaction and release highly toxic chlorine gas. Always add the chemical to water when dissolving or mixing, rather than adding water to the chemical, to prevent a violent reaction.
All chlorine products must be stored in their original, labeled containers in a cool, dry, and well-ventilated area, away from other pool chemicals, especially acids. When handling concentrated forms of chlorine, wearing personal protective equipment, such as safety goggles and chemical-resistant gloves, is mandatory to protect the skin and eyes.