The active agent that works to sanitize pool water is known as Free Chlorine (FC). This chemical form is the primary defense against bacteria, viruses, and algae that can make water unsafe for swimming. When the FC level drops too low, the water loses its ability to cleanse itself, leading to cloudy water and the potential for pathogen growth. Restoring this level is the immediate goal for maintaining a healthy and clear swimming environment.
Understanding the Difference Between Free and Combined Chlorine
Chlorine in pool water exists in three measurable forms: Free Chlorine (FC), Combined Chlorine (CC), and Total Chlorine (TC). Free Chlorine is the active sanitizer, composed of hypochlorous acid and hypochlorite ions, which are available to destroy contaminants. This is the portion of chlorine that actively purifies the water.
When Free Chlorine successfully reacts with organic materials like sweat, oils, or urine, it becomes “used up” and forms Combined Chlorine, also known as chloramines. Chloramines are significantly less effective at sanitizing and are responsible for the strong, unpleasant “chlorine smell” often associated with pools, along with causing eye and skin irritation. Total Chlorine is simply the sum of both the active Free Chlorine and the inactive Combined Chlorine (TC = FC + CC). If your test kit shows a Total Chlorine level much higher than your Free Chlorine level, the difference indicates a buildup of ineffective chloramines.
The Immediate Fix: Calculating and Applying a Chlorine Shock
The most effective way to rapidly raise Free Chlorine is through superchlorination, commonly called shocking the pool. This involves adding a high dose of unstabilized chlorine to raise the FC level high enough to destroy all chloramines and other contaminants, a point known as breakpoint chlorination. A general rule for shocking is to add enough chlorine to reach a level ten times the current Combined Chlorine reading.
To determine the correct amount of product, you must know your pool’s volume in gallons and the concentration of the shocking agent being used. You must use a pool calculator or consult a dosing chart specific to your product and pool volume to ensure accuracy. The chemical product must be added slowly, usually in the evening, and the pool pump should run for at least eight hours afterward to circulate the shock.
Liquid sodium hypochlorite, often sold as pool chlorine, is a fast-acting, unstabilized option that does not add Cyanuric Acid. Calcium hypochlorite (Cal-Hypo) is another popular choice, typically available in granular form with a higher chlorine concentration.
Maintaining Stability: pH and Cyanuric Acid Management
The effectiveness of Free Chlorine is strongly controlled by the water’s pH level. Chlorine works best in its most active form, hypochlorous acid, which is dominant when the pH is lower. As the pH rises above the recommended range of 7.2 to 7.8, the chlorine converts into the less potent hypochlorite ion, drastically reducing its sanitizing power.
Cyanuric Acid (CYA), often called a stabilizer, acts as a sunscreen for chlorine in outdoor pools, protecting it from being rapidly broken down by the sun’s ultraviolet rays. Without CYA, an outdoor pool can lose half its chlorine in minutes, making it impossible to maintain a residual. The optimal range for CYA is between 30 and 50 ppm, which provides stabilization without severely inhibiting chlorine activity.
However, high levels of CYA can form strong, temporary bonds with the Free Chlorine, essentially “locking up” the sanitizer and slowing its ability to kill contaminants. If CYA levels climb much higher than 50 ppm, the chlorine becomes sluggish, and the time required to kill bacteria lengthens. Since CYA does not evaporate and is added by stabilized chlorine products, the only way to lower excessive levels is through a partial drain and refill of the pool water.
Essential Safety Practices for Handling Pool Chemicals
Safe handling of concentrated pool chemicals is necessary to prevent dangerous reactions and injury. Never mix different types of chemicals, particularly different forms of chlorine or chlorine with acid, as this can generate highly toxic gases like chlorine gas. Even common pool chemicals are inherently incompatible and must be stored and handled separately.
When dissolving or diluting granular chemicals, always add the chemical to a bucket of water, never the other way around, to prevent a violent splash-back reaction. Personal protective equipment, including gloves and safety goggles, should be worn to avoid skin or eye contact. All applications should take place in a well-ventilated area.
The pool should remain unoccupied until the Free Chlorine level has returned to the safe swimming range of 1 to 4 ppm. Accurate testing, preferably with reagent-based test kits, is essential for ensuring precise adjustments of FC, CC, and TC.