Free Chlorine

How Important is Free Chlorine in Pool Water?

Maintaining an appropriate FC level is the most important part of keeping your water clear and sanitary. Free chlorine is the primary sanitizer for pool water and one must maintain a proper residual concentration of chlorine in water at all times (it gets used up and must be replenished constantly).

When free chlorine levels drop too low or go to zero, algae and pathogens (bacteria, protozoa and viruses) are able to grow and replicate in your pool water. Pool water can have low FC levels and be hazardous to the health of swimmers even when the water is clear! This is why TFP will always say -

“Clear water is not a sign of clean water!”​

What is Free Chlorine in a Pool?

Free chlorine, as measured by a proper test kits, is the sum of two forms of chlorine in your pool water - active chlorine (the chlorine compounds that directly oxidize bather waste and kill pathogens) and reserve chlorine (the chlorine that is bound to the cyanuric acid stabilizer, or CYA).

What is Active Chlorine in a Pool?

Active chlorine compounds come in two forms - hypochlorous acid (HOCl) and the hypochlorite ion (OCl-). Both forms of active chlorine will oxidize and disinfect but hypochlorous acid is the more potent form of active chlorine while the hypochlorite ion is more susceptible to loss from UV radiation (it absorbs UV and gets turned into the inert form of chlorine, the chloride ion or Cl-).

What is Reserve Chlorine in a Pool?

Reserve chlorine compounds come in many different chemical forms but they can all be classified as chlorinated cyanurates (sometimes denoted as HCy-Cl). Basically, the chlorine atom has attached itself to the cyanuric acid molecule and it is “held” in reserve and protected from UV loss. As the active chlorine compounds get used up by disinfection and oxidation, the chlorinated cyanurates release the chlorine atoms to form more active chlorine.

What is the Differnce Between Active and Reserve Chlorine?

A good analogy for the role of active and reserve chlorine is this - think of the chlorine in your pool as an army. The solders on the front lines fighting the enemy are your active chlorine species. The reserve chlorine species are your soldiers waiting behind the front lines ready to replace an active soldier that dies in combat.

How Does pH and CYA Effect Free Chlorine?

So what determines how much, or the proportion of active chlorine to reserve chlorine in your pool?

Basically, it depends on the pH of your water (more about that later) and the concentration of cyanuric acid (CYA). Typically speaking, most of the free chlorine (more than 90% of it at normal pool pH) is held in reserve by the cyanuric acid and the rest of the free chlorine (hypochlorous acid and hypochlorite ion) break up into proportions dictated by pH (at a pH of 7.5, the amounts of HOCl to OCl- are 1:1 but, in total, only a small fraction of the overall FC).

The most important point to remember though is this - the amount of active chlorine ion your pool water is determined by the concentration of CYA. You need enough CYA in your water to create a proper reserve of chlorine, BUT, too much CYA can reduce the amount of active chlorine in your water to levels too low to fight pathogens and oxidize bather waste. This is called “overstabilization” of pool water and, mistakenly, is referred to as “chlorine lock” by the pool industry (although in TFP we do not use that term “chlorine lock” as it is technically incorrect and completely misunderstands the chemistry of CYA and chlorine).

What is the right Level of Free Chlorine in a Pool?

The level of FC you need to maintain depends on your CYA level and how much you use the pool. See the Chlorine/CYA Chart for guidelines on the appropriate FC level to maintain based on your CYA level.

What Are the Best Ways to Increase Free Chlorine in a Pool?

Recommended ways to raise FC include:

• household bleach (sodium hypochlorite, typically 8.25% in solution)
• liquid chlorine (higher concentrations of sodium hypochlorite)
• salt water chlorine generators (electrochmeical production of chlorine gas from the chloride ion in salt).

Non-Recommended ways to raise FC:

• solid, stabilized chlorine compounds such as calcium hypochlorite (“cal-hypo”)
• lithium hypochlorite
• sodium dichloroisocyanurate (“dichlor”)
• trichloroisocyanuric acid (“trichlor”)

These solid compounds will all add “extra” stuff that you do not want in your water (eg, cal-hypo adds calcium, dichlor and trichlor add excess CYA, etc).