You want your swimming pool or hot tub water to stay crystal clear and clean. And chlorine is the essential additive used by most pool and tub owners to keep it that way. Yet chlorine can be tricky to work with, so understanding the differences between free chlorine vs. total chlorine, for example, is crucial to keep your water clean and you and your family safe.
Chlorine in pool and spa water takes care of a host of problems. It kills pathogens such as algae and mold spores, as well as some parasites and bacteria. It can also take care of swimmers' byproducts such as blood, urine, and sweat, plus oxidizing and neutralizing soaps, oils, and deodorants. There are three types of chlorine worth understanding:
- Free chlorine: Effective but unstable type of sanitizing chlorine
- Combined chlorine: Less effective but a more stable type of chlorine
- Total chlorine: The amount of free and combined chlorine in the water
Here's more information on each category of chlorine.
What Is Free Chlorine?
Free chlorine is the amount of beneficial chlorine present in chlorinated water that is available—or free, that is—to combine with that chlorinated water. Once the free chlorine does combine, it will begin to kill and neutralize the water's contaminants. Free chlorine is also known as chlorine residual, free chlorine residual, and residual chlorine.
What Is Combined Chlorine?
Combined chlorine is the amount of less-than-desirable but more stable type of chlorine, also called chloramines. This type of chlorine can also bind to and work on the water's contaminants. The presence of some combined chlorine in your pool or spa water can be a good sign: It means that contaminants are being broken down.
Excessive amounts of combined chlorine can work against you. If the amount of combined chlorine reaches 0.4 ppm (parts per million) or above, the level is too high to adequately eliminate bacteria and often results in that characteristic chlorine smell. If this happens, you will need to shock the water, a chemical process that adjusts chlorine levels to function again. Shock raises free chlorine levels and breaks apart the ineffective combined chlorine.
What Is Total Chlorine?
Total chlorine is the sum of the addition of free chlorine and combined chlorine. Use the formula F + C = T to help you with basic free chlorine vs. total chlorine concepts.
If you have a total chlorine reading, but you are barely registering a free chlorine level, it means the combined chlorine level is too high in the water and can't be properly sanitized.
How Chlorine Helps Your Pool Water
If you have ever seen a dark green pond or lake, you have seen the natural work of algae. Countless billions of algae, both the green, brown, or red microalgae or the blooms of blue-green algae called cyanobacteria, grow rapidly, just like the living organisms they are.
Chlorine is the additive to water that kills algae and other harmful organisms. Municipal water is treated with chlorine but not at the level required for pool water or with the same regularity.
How Free, Combined, and Total Chlorine Work
When you test your pool or spa water, it's more important for the free chlorine level to be higher than the combined chlorine level in order to sanitize the water. Think of free chlorine as the arsenal you need in order to keep the contaminants at bay. This arsenal needs to always be well-stocked and must exceed the force of the contaminants that it is combatting.
You can look at the interplay of free, combined, and total chlorine in a number of ways. For one, if the total chlorine level and free chlorine level are equal (T - F = 0), no combined chlorine is present in the water. This means that none of the combined chlorine has been used, which means the water should be clean.
Another way to figure out the combined chlorine level is to subtract the total chlorine level from the free chlorine level, provided the total chlorine is higher than the free chlorine. Apply the formula F - T = C.
Acceptable Chlorine Levels For Safety
Test kits usually test several factors within the same kit: pH, chlorine, acid demand, and total alkalinity. More inclusive six-in-one kits test chlorine, bromine, pH, total alkalinity, total hardness, and cyanuric acid.
Test at least two to three times a week for chlorine levels (as well as for pH levels). After dipping the combined vials at least 18 inches below the surface, drop the indicator solution (or tablets) into the chlorine level section of the vials.
Most testing kits color-code the results, as well as show the reading. For most pools, the free chlorine level should remain between 1 to 3 ppm. Free chlorine levels lower than 1 ppm place swimmers at a high risk of contamination; never let the free chlorine dip below this level. High free chlorine levels of 3 to 9 ppm can still be safe to swim in and continue to kill contaminants, but the chlorine will be an eye and skin irritant.
Water Treatment and Testing. Centers for Disease Control and Prevention.
Swimming Pool Service Technician's Apprentice Study Guide. Los Angeles County Department of Public Health.
Swimming-related Illnesses. Centers for Disease Control and Prevention.