Pool chlorine tablets are a concentrated, solid form of chemical sanitizer designed to maintain water hygiene over an extended period. These compressed compounds dissolve slowly, providing a steady release of disinfectant to combat bacteria, algae, and other microorganisms in the water. Understanding the chemical components of these tablets is important for pool owners, as their composition dictates effectiveness, water chemistry adjustments, and safe handling procedures.
The Primary Active Ingredients
The sanitizing action of most chlorine tablets comes from one of two main stabilized compounds: Trichloroisocyanuric Acid (Trichlor) or Dichloroisocyanuric Acid (Dichlor). Trichlor is the most widely utilized form in slow-dissolving tablets, offering a high concentration of available chlorine, typically around 90%. Dichlor is more often found in granular applications and has a slightly lower available chlorine content, approximately 56% to 62%.
Both Trichlor and Dichlor are organic compounds that release the sanitizing agent when they dissolve in water through hydrolysis. This reaction liberates hypochlorous acid (HOCl), which is the active form of chlorine that oxidizes and destroys pathogens. In the case of Trichlor, the three chlorine atoms attached to the cyanuric acid ring are slowly replaced by hydrogen atoms from the water, allowing for the sustained release of HOCl.
This sustained-release mechanism makes the tablets practical for continuous pool maintenance, unlike the rapid, short-term impact of liquid chlorine products. The high concentration of available chlorine means a small tablet can provide significant disinfection capability, ensuring the water remains protected between manual additions.
Why Cyanuric Acid is Included
The structure of both Trichlor and Dichlor is based on Cyanuric Acid (CYA), making them “stabilized” chlorine sources. CYA is not the primary sanitizer but serves a protective function once dissolved in the pool water. Its purpose is to shield the active hypochlorous acid from the rapid degradation caused by the sun’s ultraviolet (UV) radiation.
Without this stabilizer, the sun can break down unprotected chlorine molecules in a matter of hours, making it nearly impossible to maintain a consistent sanitizer level in an outdoor pool. Cyanuric acid forms a weak, temporary bond with the hypochlorous acid, effectively acting as a chemical sunscreen. This protective action significantly extends the lifespan of the disinfectant.
In a standard Trichlor tablet, Cyanuric Acid makes up a substantial portion of the tablet’s mass, often around 50 to 52% by weight. As the tablet dissolves, both the active chlorine and the stabilizer are released simultaneously into the water.
Effects on Water Balance
The chemical makeup of chlorine tablets, particularly Trichlor, has predictable consequences for pool water chemistry, requiring regular monitoring and adjustment. Trichlor is highly acidic, possessing a low pH level, often around 2.9 to 3.0. As the tablets dissolve, they continuously introduce this strong acidity into the pool, which drives down the water’s overall pH and total alkalinity levels.
A steadily decreasing pH can lead to a corrosive environment that may damage pool surfaces, such as plaster and grout, and accelerate the deterioration of metal equipment like heaters and pump seals. Low pH water can also cause discomfort for swimmers, leading to eye and skin irritation. Pool owners must periodically add alkaline chemicals, such as soda ash or sodium bicarbonate, to counteract the acidic effect and maintain the water within the recommended pH range of 7.4 to 7.6.
A second major consequence is the continuous accumulation of Cyanuric Acid. Since CYA is not consumed during sanitization, every time a Trichlor tablet dissolves, the CYA level in the pool increases. If the concentration of CYA exceeds 80 to 100 parts per million, it can over-stabilize the chlorine. This is sometimes referred to as “chlorine lock,” where the bond between the chlorine and the CYA prevents the hypochlorous acid from effectively sanitizing the water. The only way to reduce excessive CYA levels is to partially drain the pool and replace the stabilized water with fresh water.
Safe Storage and Handling Practices
Given the concentrated nature of the chemicals, proper storage and handling of chlorine tablets are necessary to prevent dangerous chemical reactions and accidental exposure. Tablets should always be stored in a cool, dry, and well-ventilated area, away from direct sunlight, heat sources, and moisture. Allowing water to enter the storage container can initiate a rapid, uncontrolled chemical reaction that produces toxic chlorine gas and heat, potentially leading to a fire or explosion.
Chlorine tablets must never be stored near or mixed with any other pool chemicals, especially non-chlorine sanitizers, muriatic acid, or organic materials. The accidental mixing of these concentrated compounds can create highly reactive gases or lead to violent reactions. When handling the tablets, personnel should wear protective gear, including rubber gloves and eye protection, to shield the skin and eyes from chemical burns and dust irritation.
Tablets should only be added to a dedicated floating dispenser, an automatic chlorinator, or a skimmer basket designed for this purpose. Direct inhalation of the strong fumes should be avoided.