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One of the most important means of keeping pool water thoroughly disinfected, clean and free of bacteria, algae and other microorganisms is the regular addition of active chlorine.

Unfortunately, the disinfecting effect of chlorine quickly wears off under the influence of harmful UV rays from the sun.

In order not to have to constantly add new chlorine – which would also be associated with some costs – you need a substance that protects the chlorine from UV light.

Cyanuric acid has proven its worth as a chlorine stabilizer for this purpose. It is very durable, so it does not need to be constantly added. This allows costs for the necessary pool chemistry to be noticeably reduced.

On the other hand, cyanuric acid significantly reduces the disinfecting effect of chlorine. So there is a trade-off to be made between the positive and negative effects on chlorine.

A ratio of chlorine to cyanuric acid of 1:14 has proven to be effective. We would like to show you here how to achieve this and what needs to be observed.

What is a chlorine stabilizer?

The main active ingredient of a chlorine stabilizer is cyanuric acid.

Cyanuric acid is an organic acid with the molecular formula C3H3N3O3. In pure form it forms colorless crystals, which enclose water of crystallization in a ratio of 1 : 2.

However, this escapes in dry air. Nowadays, cyanuric acid is obtained on an industrial scale by heating urea up to temperatures between 200 and 300 °C. This process is also used for the production of cyanuric acid.

As a chlorine stabilizer, cyanuric acid is used in liquid or granular form. Cyanuric acid serves to stabilize the disinfecting effect of free chlorine in water, which would otherwise be rapidly degraded under the influence of UV rays from sunlight.

To prevent this, add cyanuric acid to the pool water as a chlorine stabilizer if free chlorine is used for disinfection.

However, many chlorine products already contain cyanuric acid, so there is no need for an additional chlorine stabilizer in the water. These include almost all chlorine tablets (trichloroisocyanurates) and the so-called pool shockers (dichlorosiocyanurates).

A chlorine preduct that already contains cyanuric acid is called stabilized chlorine – in contrast to free chlorine, which does not contain a stabilizer.

Just look on the label of the chlorine product, there must also be the active ingredient: If the name ends with cyanurate, then you do not need an additional chlorine stabilizer in the pool water. This is because these chlorine products already contain cynaric acid as a stabilizer.

However, the disinfecting effect of the stabilized chlorine is also largely inhibited once by the cyanuric acid and can only develop slowly.

In order for free chlorine in the water to retain its disinfecting effect over a longer period of time, the quantity ratio between free chlorine and cyanuric acid must be precisely matched. In addition, the pH value in the water also plays a role.

Those who are not confident in the use of both products and in the dosage of free chlorine and cyanuric acid as chlorine stabilizer should instead use already stabilized products, i.e. chlorine tablets or pool shockers:

With the help of this chlorine stabilizer, the active, free chlorine can be used for longer, the disinfecting effect lasts longer, but is also lower. Overall, however, this chlorine stabilizer reduces chlorine consumption and inhibits chlorine odor.

If the concentration of the chlorine stabilizer in the water is too high, it stabilizes the chlorine and protects it from decomposition by the sun’s UV rays, but the effectiveness of the chlorine is then greatly reduced.

This can go so far that the stabilized chlorine no longer disinfects the water sufficiently and some strains of bacteria survive in the water.

In addition, when the pH is high (when it is above the recommended range for pool water between 7.0 and 7.4), the efficiency of the chlorine also decreases.

If the pool pH is too high, you can lower it in 3 simple steps.

If the chlorine stabilizer, i.e. the cyanuric acid, has been added to the pool water, it “stays there”, i.e. the concentration of the cyanuric acid remains stable over a long period of time.

So, once added to the water with the correct dosage, the cyanuric acid does not need to be re-dosed. However, this can become a problem if you have added too much of the chlorine stabilizer to the water and the concentration of cyanuric acid is too high.

Then there are only 2 ways to lower the concentration again:

  1. One exchanges a part of the pool water for fresh, clean water or
  2. you dilute the concentration of the chlorine stabilizer by increasing the water level in the pool by adding fresh water.

The functioning of cyanuric acid

Free chlorine forms hypochlorous acid (HOCl) in water.

If cyanuric acid is already present in the pool water as a chlorine stabilizer (once in the pool water, the concentration of cyanuric acid hardly decreases, see above), then the cyanuric acid binds to a large extent to the chlorine, which then cannot form active HOCl.

Thus, the disinfecting effect of the chlorine is initially blocked and is only gradually released.

Over time, an equilibrium is established between chlorine bound to cyanuric acid and the active hypochlorous acid. However, the disinfecting effect of the active chlorine is severely limited.

On the other hand, cyanuric acid, as a stabilizer, shields the chlorine from decomposition by the sun’s UV rays. Otherwise, the chlorine would lose its effect in a short time.

With cyanuric acid as a stabilizer, the disinfecting effect of the chlorine lasts three to five times longer than without a stabilizer.

This is also a good thing, because due to the greatly reduced effect of chlorine due to cyanuric acid, it also takes correspondingly longer until the pool water is completely disinfected, bacteria, algae and other microorganisms are killed.

To further limit the growth of algae, you can use algaecide. Algicide pool works differently than many people think and prevents algae from spreading.

How much cyanuric acid should be used?

Manufacturers and distributors of the chlorine stabilizer recommend a concentration of cyanuric acid in the water between 10 and 100 ppm, i.e. 10 to 100 mg/liter. The WHO specifies 100 ppm as the maximum limit.

However, at this high concentration, children can experience health problems if they swallow water while splashing around and swimming in the pool.

In addition, the efficiency of the free chlorine at 100mg cyanuric acid/liter is only 29% according to the table. Instead, 50 mg/liter has proven to be effective, in which case the efficiency is still around 35%.

At a concentration of 50 mg cyanuric acid liter, the chlorine content should be 3 to 5 mg chlorine/liter, so the ratio of free chlorine to cyanuric acid should be 1: 14.

This achieves a good compromise between the protection of the cyanuric acid from the decomposition of the free chlorine by the harmful UV rays and the reduced disinfection effect of the chlorine under the influence of the cyanuric acid.

In any case, the achieved concentrations of free chlorine, cyanuric acid and pH in the water should be measured and corrected if necessary.

There are several measuring methods available for this purpose(Testing pool water: The 3 best methods in comparison).

This professional chlorine quick test can be used to determine the free chlorine and bromine content in ppm, the total alkalinity and the pH value of the pool water.

The color scales of these water parameters are printed on the storage can. The package contains 60 test strips.

This small, mains-independent photometer enables precise determination of the following parameters: pH, free and combined chlorine, alkalinity and cyanuric acid.

The measured values can be read directly on the display.

Instructions: Lower cyanuric acid

If the disinfecting effect of the free chlorine is too low to kill certain bacteria, parasites and pathogens, or if you have accidentally added too much chlorine stabilizer to the water, then the concentration of cyanuric acid in the water must be lowered.

This is because the cyanuric acid content will not decrease on its own. After all, it should retain its effect as a stabilizer over a longer period of time! Unfortunately, there are no pool chemicals that could bind or inactivate the cyanuric acid.

Then there are only 2 ways to lower the concentration of cyanuric acid in the pool water:

  • Either replace some of the pool water with fresh, clean water.
  • Or you can dilute the concentration of the chlorine stabilizer by increasing the water level in the pool while adding fresh water.

Chlorine stabilizer: When and how often to use?

In contrast to many other pool chemicals, which are consumed in the pool water over time and have to be repeatedly checked and re-dosed, the concentration of the chlorine stabilizer, i.e. cyanuric acid, remains stable over a long period of time.

It does not decompose on its own, nor is it attacked, inactivated or destroyed by other chemicals in the pool water.

It is usually enough to renew the chlorine stabilizer once a year. The best time is in the spring before the resumption of the bathing season.

Unless, during a long hot summer period, the pool has lost a lot of water due to high evaporation. This increases the concentration of cyanuric acid in the water (the cyanuric acid does not evaporate from the water!).

Then the level of cyanuric acid must be measured and the water evaporation must be compensated by adding fresh water until the concentration of cyanuric acid is reduced to the normal value of 50 mg /l again.

If the concentration of cyanuric acid cannot be reduced in this way or even continues to rise: Then there is nothing else to do, the pool water must be completely replaced. Then, in the future, you only use a chlorine product that already contains cyanuric acid as a stabilizer.

For example, chlorine tablets with the active ingredient trichloroisocyanurate or pool shockers with dichloroisocyanurate.

Add cyanuric acid properly to the pool water

The chlorine stabilizer with the cyanuric acid must never be simply dumped into the pool. This is because it dissolves only slowly in water. It would then first sink to the ground, where it would attack the soil material, bleaching and making it brittle.

Likewise, do not slowly dose the chlorine stabilizer into the water through the pool filter. This is because its acid content can damage parts of the filter system.

Instead, slowly dissolve the chorus stabilizer in a tight-fitting, 20-liter plastic bucket filled three-quarters full with warm water.

How much chlorine stabilizer to dissolve in this plastic bucket, of course, depends on the volume of water in the pool. As a rule of thumb, to gradually increase the concentration of cyanuric acid in the pool water by 10 mg/liter at a time, you need 0.47 kg of cyanuric acid per 10,000 liters of pool water.

In any case, you should also read the instructions for use on the packaging of the chlorine stabilizer or follow the manufacturer’s online information on the Internet.

It can take a long time, sometimes even hours, for the chlorine stabilizer to completely dissolve: only then can you dump the solution into the pool water without causing any damage.

To ensure that the chlorine stabilizer is evenly distributed in the pool water, run the pump for a few hours. And that is before resuming swimming in the pool.


Since free chlorine rapidly decomposes under the influence of the sun’s UV light and loses its effectiveness, it must be protected with cyanuric acid as a chlorine stabilizer. However, cyanuric acid reduces the disinfecting effect of chlorine.

To achieve a workable compromise between the two processes, a balanced concentration ratio between chlorine and chlorine stabilizer must be found.

A ratio of 1: 14 with 3 ppm chlorine to 50 ppm cyanuric acid has proven effective. Cyanuric acid is a long-lived substance and does not disappear by itself in water.

Too high concentrations of cyanuric acid can only be corrected with some effort. Those who are still unsure about pool chemistry should instead use chlorine tablets or pool shockers for disinfection, which already contain a chlorine stabilizer.

The editorship of Wellenliebe.de consists of real water sports fans. From kayakers, sailors and divers to stand up paddlers, (almost) all water sports are represented. Our content has been read millions of times and has been picked up in many newspapers.

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