Part of my job is to help people with water care questions and water quality struggles, which means I regularly talk to people who have difficulty understanding what free, combined and total chlorine are, and how to remember what each one is.
Sometimes it’s customers but far too often, it’s people in the industry, including everyone from new employees to seasoned pros. I recently had a spa retailer with 25 years of experience reach out for help, saying that he could not maintain a chlorine reading in his spa, which didn’t make sense to him since he could smell the chlorine.
So let’s explore the differences and relationships between the types of chlorine for those who aren’t sure, and for those who are, I’ll share how I teach others to remember each designation.
Free chlorine
Chlorine that hasn’t done any work yet, meaning it hasn’t broken down or sanitized anything, is known as free chlorine. It is the chlorine you add to the water or that is produced by a saltwater chlorine generator and is often referred to as FCl (where the “F” stands for “free” and “Cl” is the symbol for chlorine) and is sometimes known as “free available chlorine,” or FAC.
We use a phrase to help people remember what free chlorine is: Free available chlorine is FREE and AVAILABLE to do its job. It’s just hanging out in the water waiting for something to come along that it can deal with, including inorganic contaminants such as iron, manganese and copper and organic contaminants including bacteria, dead skin, human waste, leaves, twigs and insects.
Combined chlorine
When contaminants are in the water, they combine with the free chlorine, which goes to work killing and/or breaking the contaminant down.
Once the free chlorine has done its work, it is no longer free chlorine — it is converted to combined chlorine, or chloramines. To remember what combined chlorine is, think that once free chlorine COMBINES with a contaminant, it converts to COMBINED chlorine.
Combined chlorine doesn’t do anything but sit around and give off that chlorine smell people complain of, and it can cause itchy skin, burning eyes and decreased sanitizer efficacy. When you walk into a public pool and smell the chlorine or feel your eyes burn and think, “Wow, they have a lot of chlorine in the water,” the opposite is true. They have a lot of used-up (combined) chlorine in the water doing nothing. Do not confuse chlorine odors with an excess of chlorine. A properly maintained spa will not have any combined chlorine and regardless of how high the free chlorine reading may be, won’t smell at all like chlorine.
Total chlorine
Total chlorine is the sum of combined chlorine and free chlorine. We teach people to remember this as TOTAL chlorine being the TOTAL of combined and free chlorine.
Customers, and a few in the industry, get confused and believe total chlorine is an important reading — and it can be — but on its own, it doesn’t tell you anything. You need to subtract the free chlorine reading from it to determine the combined chlorine level. To demonstrate, here are a few sample readings and calculations, all showing total chlorine as 10 ppm:
| Total chlorine | 10 ppm |
| Minus free chlorine | – 0 ppm |
| Equals combined total chlorine | 10 ppm |
| Total chlorine | 10 ppm |
| Minus free chlorine | – 10 ppm |
| Equals combined total chlorine | 0 ppm |
| Total chlorine | 10 ppm |
| Minus free chlorine | – 6 ppm |
| Equals combined total chlorine | 4 ppm |
Removing combined chlorine
Using chlorine will create combined chlorine, which can be harmful.
To remove combined chlorine, you need to add an oxidizing agent to the water in a large enough dose to break down the combined chlorine. Oxidizing agents include chlorine or nonchlorine shock.
Ideally, total chlorine should equal free chlorine, which means you have no combined chlorine. Our industry has varying recommendations as to how much combined chlorine should be present to necessitate shocking, with ranges from 0.2 to 0.75 ppm.
Many test strips don’t allow a customer to calculate combined chlorine, in which case recommending a weekly shock, and/or shocking if your client can detect chlorine odors, is a good practice.