Chlorine Types: When Shocking Your Pool

May 26, 2020

Did you know there are several different types of chlorine in your pool? Free Chlorine, Combined Chlorine, and Total Chlorine can all be found in a typical swimming pool. Although these terms may be new to you, they are worth keeping in mind.

If you want to properly monitor and shock your pool, you’ll need a solid understanding of how these types of chlorine work. Fortunately, it’s easy to understand.

Remembering the Terminology 

The three types of chlorine work together in the following formula:


In other words, Free Chlorine + Combined Chlorine = Total Chlorine.

How to Shock Your Pool

The formula above is important for shocking your pool, since Free Chlorine disinfects and kills microorganisms and bacteria.

As Free Chlorine is added to the water, the reaction reduces the amount of chlorine while cleaning the pool. The chlorine that remains once the reaction is complete is called Combined Chlorine. Although it’s no longer “cleaning” the pool in the form of residual chlorine, it still kills microorganisms in the water.

Shocking your pool, which is also called super-chlorination, is overdosing the pool with Free Chlorine to kill unwanted microorganisms. Determining the amount of Combined Chlorine at work will help you determine how much Free Chlorine you need to shock the pool.

If the amount of Total Chlorine in the pool is greater than the amount of Free Chlorine, the difference is the amount of Combined Chlorine. To get rid of the Combined Chlorine, super-chlorinate the pool by adding Free Chlorine — ten times the amount of Combined Chlorine present.

This is commonly called “break-point chlorination,” and it will clear your pool of all contaminants.

To figure out the Free Chlorine level of your pool, use a FAS-DPD test kit to determine the exact amount. Then add enough chlorine to bring the Free Chlorine count to break-point chlorination.

You may need to repeat these steps until the Combined Chlorine level of your pool drops below 0.5 ppm, or if after leaving the shock overnight, the Free Chlorine test shows your pool has 1.0 ppm of Free Chlorine.

You will also want to give it a thorough look-over to make sure the water is clear and ensure there’s no visible algae buildup.

When to Shock Your Pool

There are a wide range of reasons to shock your pool. But when it comes to frequency, it’s recommended you shock once a week during peak swimming season. Although this is a general rule, you may want to shock more frequently depending on how often your pool is used and by how many people.

Across the board, you should shock your pool if:

  • You see any algae growth
  • A test measures your Free Chlorine level at zero, or
  • You note a Combined Chlorine level above 0.5 ppm.

If you’re looking for other ways to keep your pool in tip-top shape, contact Fibre Tech. Our pool preservation process will ensure your pool is well-maintained at a cost you can afford. Give our professionals a call today!

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