“Free Chlorine” vs “Total Chlorine” in Pools

April 28, 2020

What is free chlorine vs total chlorine?

When it comes to chlorine levels in pools, you may be confused by the different types of chlorine present.

Here at Fibre Tech, our customers in Florida and around the country are sometimes unsure of which type of chlorine to test for in their pool. Read below if you want to learn more or just drop us a line if you want to discuss your pool today.

Swimming pool water generally contains three types of chlorine commonly known as Free Chlorine, Combined Chlorine, and Total Chlorine.

To understand the difference between the three types of chlorine, consider this simple formula:

FC + CC = TC

Understanding the formula

Free Chlorine is the type that we commonly test for to determine the proper chlorine levels in pool water. Free Chlorine is also the chlorine that is still available to sanitize your water. Combined Chlorine is the chlorine that has already been “used up” sanitizing your water. And Total Chlorine is the sum of the two.

Some test kits display only free and total chlorine. To find your combined chlorine, just use the formula. 

Think of it this way: when a chlorine compound is added to swimming pool or spa water, it reacts with water to form the compounds known as hypochlorous acid and hypochlorite ion. Together, these compounds are known as “free available chlorine” or “free chlorine.”

The primary reason for adding chlorine to swimming pool water is to disinfect or kill possibly harmful microorganisms. But once the Free Chlorine has joined with ammonia and nitrogen compounds to form Combined Chlorine, its ability to disinfect is hindered.

It takes 25 parts of Combined Chlorine to do the work of one part of Free Chlorine. If the Total Chlorine in your pool is higher than the Free Chlorine reading, then the difference between the two represents the level of Combined Chlorine in the water. If the readings are the same, then no Combined Chlorine is present. The Total Chlorine level cannot be less than the Free Chlorine level.

Once you know how much Combined Chlorine is in the water, you add about 10 times that amount of Free Chlorine to get rid of it. This overdosing – or super-chlorination – is also referred to as “breakpoint chlorination.”

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