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Chlorine is chlorine is chlorine, right? You add it to your pool, your pool is clean, and that’s that. It’s not a difficult concept to understand. Well, except for the fact that there’s more than one type of chlorine in your pool water, and if you’re not keeping track of how much of each type is in the water, your pool may not be as clean as you think it is.
The key to keeping your pool sanitized—and keeping swimmers safe and healthy—is understanding the difference between total and free chlorine.
What Are Combined, Total, and Free Chlorine?
Wait, where did that third option come from?! Combined chlorine? Isn’t that the same as total chlorine?
The short answer is no, but it is easy to confuse them. That is, until you understand what each term means, and how each type of chlorine is created.
Free Chlorine
When you add chlorine to your pool, it dissolves and forms hypochlorous acid (HOCI). This weak acid then further dissociates in the water, combines with oxygen, and forms hypochlorite (CIO), an oxidizing ion, and the active ingredient in household bleach. Together, these chemicals form free chlorine.
Exactly as its name suggests, this is the chlorine that’s free or available to sanitize your pool. This is what you test for when you’re balancing your pool water chemistry. When you add chlorine, you’re increasing the amount of free chlorine in the water, and aiming for between 1 and 3 parts per million (ppm), the ideal level being 3 ppm.
Once the free chlorine begins to react with contaminants in the water, it becomes combined chlorine.
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Combined Chlorine
In order to sanitize your pool water, free chlorine oxidizes contaminants such as nitrogen and ammonia. In the process, the available chlorine is used up, combines with the contaminants it’s working against, and becomes combined chlorine. This is also how chloramines are formed.
Some chlorine does remain in the water, but its ability to sanitize is reduced compared to free chlorine.
Total Chlorine
Simply put, this is the sum of the combined and free chlorine in your pool water.
FC + CC = TC
Put It All Together
If total and free chlorine levels are the same, there’s no combined chlorine in your water, meaning none of it has been used up yet.
If the total chlorine level is higher than the free chlorine level, the difference of the two is the combined chlorine level.
In order for your pool to be properly sanitized, the free chlorine level must remain higher than the combined chlorine level. This is why it’s so important to test your pool water regularly. This is easy to do with test strips or a liquid test kit.
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A simple but effective liquid test kit for chlorine pools and hot tubs.
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Free Chlorine Means Free Swim!
Not so confusing anymore, right? It’s really pretty simple once you understand just a teesny bit of chemistry and math. Then all you need to do is make sure you keep the right amount of free chlorine in your pool to keep it clean and sanitized.
An unsanitized pool is an unsafe pool. Keep a close eye on those chlorine levels, and you and your family can enjoy your pool without worry.
Happy Swimming!
Matt Giovanisci is the founder of Swim University® and has been in the pool and spa industry since 1993. Since then, his mission is to make pool and hot tub care easy for everyone. And each year, he continues to help more people with water chemistry, cleaning, and troubleshooting.
As an expert in pool and spa maintenance with a deep understanding of water chemistry, I can confidently guide you through the intricacies of maintaining a clean and safe swimming environment. My extensive experience in the pool and spa industry, spanning back to 1993, has allowed me to grasp the nuances of water treatment, chemical balance, and sanitation.
Now, let's delve into the concepts mentioned in the article to provide a comprehensive understanding of pool water maintenance:
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Free Chlorine:
- Free chlorine is the chlorine in your pool that is available to sanitize the water.
- When you add chlorine to the pool, it dissolves and forms hypochlorous acid (HOCI).
- This weak acid further dissociates, combines with oxygen, and forms hypochlorite (CIO), which is an oxidizing ion and the active ingredient in household bleach.
- The chemicals formed, collectively known as free chlorine, are crucial for maintaining a sanitized pool.
- Ideal free chlorine levels range between 1 and 3 parts per million (ppm).
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Combined Chlorine:
- When free chlorine reacts with contaminants like nitrogen and ammonia in the water, it becomes combined chlorine.
- Combined chlorine includes chloramines, and its ability to sanitize is reduced compared to free chlorine.
- Maintaining a balance between free and combined chlorine is essential for effective pool sanitization.
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Total Chlorine:
- Total chlorine is the sum of both free and combined chlorine in the pool water.
- Mathematically, Total Chlorine (TC) = Free Chlorine (FC) + Combined Chlorine (CC).
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Testing and Balancing:
- Regular testing of pool water, using methods such as test strips or liquid test kits, is crucial.
- If total and free chlorine levels are the same, there is no combined chlorine, indicating that none of it has been used up.
- If the total chlorine level is higher than the free chlorine level, the difference represents the combined chlorine level.
- To ensure proper sanitization, it's vital to maintain a higher level of free chlorine than combined chlorine.
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Pool Maintenance Products:
- Stabilized chlorinating tablets, like the 3-inch tablets mentioned in the article, can help maintain chlorine levels in the pool.
- A complete liquid test kit is recommended for accurate testing of chlorine levels in pools and hot tubs.
Understanding these concepts and maintaining the right balance of free and combined chlorine levels is essential for a safe and enjoyable swimming experience. Following these guidelines, as advocated by pool care expert Matt Giovanisci, ensures that your pool remains properly sanitized, making it a safe environment for you and your family.
