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New pool owners are first confronted with a whole barrage of technical terms. It quickly becomes confusing. Chlorine is at the top of the list of pool care chemicals. And already things get tricky, because the very first purchase gives you a choice between two different types of chlorine: stabilized chlorine and non-stabilized chlorine.

OK. Only two different types still sounds quite good.

A little later, however, the terms free chlorine, combined chlorine, total chlorine and active chlorine come up. So what’s the deal with all this different chlorine. What is the difference between the various types of chlorine? When is which chlorine needed and how high should the values be?

An overview: free chlorine, combined chlorine, total chlorine, stabilized and non-stabilized chlorine.

Before we go into more detail about each type of chlorine, it is important to understand the terms. Pool maintenance uses all of these terms, so it’s important to keep the types apart.

  • Free Chlorine: Free chlorine means the amount of HOCl acid in the water. This substance acts as a disinfectant to keep the pool free of foreign matter.
  • Combined Chlorine: Combined chlorine occurs when free chlorine binds to bacteria or other foreign matter, thereby “consuming” it.
  • Total Chlorine: Total chlorine (also called total chlorine) is the sum of free and combined chlorine.
  • Stabilized Chlorine: Stabilized chlorine means chlorine to which cyanuric acid has been added. This chemical protects the chlorine from decay due to UV radiation from the sun and ensures that the chlorine stays in the water longer.
  • Non-stabilized chlorine: Non-stabilized chlorine does not contain cyanuric acid and is broken down by UV radiation. The non-stabilized chlorine does not stay in the pool water for long.

Now that we’ve learned the basics, it’s time to talk about how to use each type of chlorine.

Chapter 1: Stabilized and non-stabilized chlorine.

We learned that the addition of cyanuric acid makes the difference between stabilized and non-stabilized chlorine. Now we will go into the different uses of these two chemicals.

Non-stabilized or unstable chlorine sounds like a dangerous chemical. In fact, it just means that the chlorine does not contain cyanuric acid. Non-stabilized chlorine is broken down by the sun’s UV rays and evaporates.

Essentially, there are three different uses of non-stabilized chlorine:

  • After a heavy use of the pool (e.g., pool party, heavy rainfall, water replacement), you should shock chlorinate. Non-stabilized chlorine is ideal because it does not stay in the pool water for long.
  • If your pool is regularly exposed to a high load and you use chlorine accordingly often, but do not want the chlorine level to continue to rise.
  • Non-stabilized chlorine is used in indoor pools because they are not exposed to UV radiation.

The bottom line is that non-stabilized chlorine is used whenever you do not want the chlorine to remain in the pool for long periods of time.

Note: Treatment with non-stabilized chlorine is always done at night because sunlight burns the chlorine too quickly. With non-stabilized chlorine is already after 17 minutes is only half of the free chlorine in the water.

After explaining non-stabilized chlorine, it is not difficult to conclude stabilized chlorine. Essentially it is exactly the same, only now cyanuric acid has been added.

The UV radiation from the sun would dissolve the chlorine until there was nothing left. A constant level of chlorine in the pool, is not achievable this way.

According to one study, it would take up to eight times the amount of non-stabilized chlorine to achieve the same effect as with stabilized chlorine.

So what are the reasons for using stabilized chlorine:

  • Stabilized chlorine and the cyanuric acid it contains reduce the need for chlorine because it is no longer broken down by the sun’s rays.
  • Using stabilized chlorine saves you time in readjusting chlorine levels and costs in purchasing, because you need less chlorine overall.
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You must be careful when handling cyanuric acid and chlorine products that contain cyanuric acid, as the chemical inhibits the disinfecting effect of chlorine and also can no longer be broken down.

Regularly test the water with special pool test strips for the ratio of free chlorine to cyanuric acid. The ideal ratio is 14 to 1, so for every 3 to 4 ppm of free chlorine, there is about 50 ppm of cyanuric acid.

You should not go higher than 50 ppm because this does not further increase protection from UV radiation, but can lead to unsightly effects such as cloudy pool water, bacterial proliferation and algae growth.

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Note: Dichlor or trichlor based chlorine products already contain a small amount of cyanuric acid. The stabilized chlorine lasts longer in the water. For precise control of the concentration of cyanuric acid, these small amounts are not suitable. For this purpose, you should buy cyanuric acid in its pure state.

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Chapter 2: Free chlorine, combined chlorine and total chlorine

Chlorine content is one of the most important measurements of a pool. The relationship of free chlorine, of combined chlorine and the total chlorine (also called total chlorine) is quickly explained.

  • Free chlorine: The free chlorine is the portion of the substance that is truly active. The disinfectant keeps your pool clean. The concentration of the chemical should be between 1 and 4 ppm.
  • Combined chlorine / chloramines: The combined chlorine is spent chlorine. It has formed compounds with foreign bodies and has only a greatly reduced disinfection effect. The proportion of combined chlorine should be less than 0.2 ppm. Combined chlorine is the cause of the typical “swimming pool smell”, reddening of the eyes and irritation of the skin and respiratory tract.
  • Total Chlorine (Total Chlorine): Total chlorine is the sum of free and combined chlorine.

The formula is: free chlorine + combined chlorine = total chlorine.

In pool maintenance, the proportion of free chlorine and that of combined chlorine in the water is important. The free chlorine serves as a disinfectant, while the combined chlorine is a product of compounds with foreign matter.

The combined chlorine or chloramines are also responsible for the typical “swimming pool smell“, which is often confused with chlorine odor. In public pools, the odor is often unavoidable because large amounts of contaminants (dirt, cosmetic products, bodily fluids, etc.) enter the pool daily. Your private pool, on the other hand, does not have to have an odor and still contains chlorine, which acts as a disinfectant.

The solution is shock chlorination with free chlorine (“shocking“). When the pool water reaches a concentration of free chlorine ten times higher than the combined chlorine, the free chlorine destroys the molecular structure of the combined chlorine and the odor disappears.

The problems such as the unpleasant smell as well as the irritation for eyes, skin and respiratory tract disappear. For this reason, shock chlorination of the pool is an important part of pool maintenance and should be performed approximately weekly.

After heavy pool use (such as a pool party), a water exchange, or a lot of rainwater in the pool, you should also perform unscheduled pool shocking.

Shocking the pool with free chlorine (the associated pool chemicals are also called pool shockers) must be done after sunset, because – as we have learned – non-stabilized chlorine is used.

The one-time allowance of chlorine is very high with a shock and you don’t want the pool shocker to mess up your pool chemistry.

To shock chlorinate your pool, you need to determine the proper amount of pool shocker required. In your calculation, you need to consider the concentration of combined chlorine, the amount of free chlorine already present, and the pool volume.

Note: With a test kit you can only determine the values of total chlorine (TC) and free chlorine (FC). You have to determine the percentage of bound chlorine by using the formula (TC – FC = CC).

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Chapter 3: Active effective chlorine, pH value and cyanuric acid

Active effective chlorine means the amount of free chlorine. The term often comes up in context when describing the effect of pH and cyanuric acid on free chlorine.

When you add chlorine to water, the increase in the concentration of free chlorine depends largely on the pH. The pH determines the balance between HOCl, the free chlorine with disinfecting effect, and OCl, which has only little disinfecting effect.

Some examples of the concentration of free chlorine as a function of pH:

  • pH value: 6.5: HOCl 85%, OCl 15%.
  • pH-value: 7.0: HOCl 74%, OCl 26%
  • pH value: 7.5: HOCl 50%, OCl 50%
  • pH value: 8.0: HOCl <20%, OCl >80%

A pool with the pH in the range of 7.0 to 7.4 prevents the growth of algae, bacteria, but also increases the effectiveness of the added chlorine.

It is an equilibrium of HOCl and OCl that shifts depending on the pH. If you lower the pool pH, the concentration of free chlorine also increases.

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Cyanuric acid serves as a stabilizer. When combined with cyanuric acid, it is called stabilized chlorine. The stabilized chlorine remains in the water significantly longer. In outdoor pools, the UV radiation from the sun’s rays breaks down the chlorine and evaporates it with the pool water.

The disinfection effect of the stabilized chlorine is inhibited. For this reason, stabilized chlorine is assigned to combined chlorine. The ideal ratio of cyanuric acid and free chlorine is 14 to 1. 50 ppm of cyanuric acid are thus added to a concentration of 3 to 4 ppm of free chlorine.

More than 50 ppm of cyanuric acid can cause pool water turbidity and promote bacterial and algae growth. Due to health risks for children, the WHO advises against concentrations above 100 ppm anyway.

When chlorine is added, the effects of pH and cyanuric acid overlap. If the concentration of cyanuric acid is 50 ppm and the pH is 7.0, adding 1 ppm of chlorine increases the amount of free chlorine by only 0.22 ppm (1 ppm x 35% x 74%). You can use online calculators to calculate these numbers.

Note: If you are using a salt water electrolysis system, the addition of cyanuric acid is not necessary because your system will always replenish chloride ions.

That was a lot of information in a short time. However, every pool owner should be able to raise the chlorine level as well as lower it. Now have an initial overview of the different types of chlorine. In the beginning, pool maintenance seems like a mountain that gets steeper and steeper, but once you have made it to the crest, everything runs by itself.

Larry has been a true water rat since childhood. Pure pleasure turned into a passion. That's why he is the first point of contact for friends and acquaintances when it comes to pool-related problems. He is an integral part of the PoolHandbook editorial team.