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 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 (electrochemical 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).

How Can Free Chlorine be Lowered in Pool Water?

In outdoor pools free chlorine will naturally decline 2-4 ppm a day.

There are chlorine neutralizers that most likely contain sodium thiosulfate.

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 Difference 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).

How Can Algae Grow in Water With Chlorine?

Algae replication rates in warm water with insufficient sanitizer can be high enough such that there is a colony doubling every 2 to 4 hours (depends on species, light requirements, etc). So in a 24 hour period with light available for more than 12 hours per day (summer time), algae grows very fast. The kill rate of any sanitizer is concentration dependent.

There's not a lot of hard data on the CT kill rate for algae with chlorine, at least no easily available data, but once your active chlorine levels (that would be the hypochlorous acid part of the FC) fall below 100 part per billion (0.1ppm), the kill rates are well below the colony doubling times. So basically, algae can freely grow and thrive when there is (insufficient) sanitizer around.

Despite what you might think, your water is not one, homogenous mixture. While your water sample might say there's 0.5ppm or 1ppm FC, there's a very good chance that water right at the surface of the pool has no chlorine in it whatsoever (UV light destroys chlorine quickly) and the water nearest the walls probably has much lower FC levels.

Does Chlorine Consumption Change with Water Temperature?

Warmer water losing more FC due to sunlight than colder water is not true.

Chlorine loss from the UV in sunlight does not depend on temperature because it only depends on the number of photons per area entering into the pool and on the concentration of chlorine. The photons of light are traveling much faster (at the speed of light) than the molecules containing chlorine so the temperature which relates to the speed of those chlorine molecules is irrelevant. From the point of view of the photons, the molecules of chlorine are essentially standing still so the cross-section of those molecules which is the area with which the photon has a quantum probability of reacting with the molecule is independent of the temperature and only related to the concentration of such molecules in the water.

Chlorine consumption that is related to temperature is for chemical reactions with chlorine such as oxidizing pool covers, bather waste, pollen, leaves, algae, etc. And yes, algae grows faster in warmer water but such consumption won't matter if there is sufficient FC/CYA since algae will get killed before it can reproduce so the rate will be based solely on the rate of blown-in algae spores which is usually not measurable (pollen, on the other hand, can be voluminous as can pods and other material dropped from trees).

A pool with a solar cover that is opaque to UV would lower the loss of chlorine from sunlight that is not temperature dependent but would increase the loss of chlorine from oxidizing the cover which is temperature dependent.

There can be a temperature dependence on the subsequent chemical reactions that occur after the photochemical reaction occurs. But for the breakdown of chlorine by UV, it's pretty much all over with such breakdown since the probability of having the OH• and Cl• reform compared to other reactions that lead forming chloride and oxygen gas is low and not temperature dependent. At most, the intermediate concentrations of some intermediate species such as hydrogen peroxide may be higher at lower temperatures, but by that time it's too late and the chlorine is already on it's way to becoming chloride.

There can be a small dependence on temperature in terms of the rate of relaxation from excited states so there is a small but negligible temperature dependence on having the HO-Cl vibrational state be excited and not break apart as often because of low temperature, but it takes a very low temperature before that effect would be seen in practice.

Can FC Targets be Lowered for Pools Open in Cold Weather?

Though the growth rate of algae slows down as water gets colder, the reaction of chlorine killing algae also slows down. So lowering the FC level too much could be risky.

The main advantage of the cooler water, especially if the sun isn't on the pool due to clouds or a cover, is the lower daily FC usage. I'd keep the FC at whatever level you normally need to do to prevent algae.

Though it's possible that at some cold water temperature the algae growth plummets, why take the chance that it doesn't?