What is Free Chlorine, Combined Chlorine, and Total Chlorine?

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Chlorine Quick Answers

  • What is Free Chlorine – Free Chlorine is the amount of chlorine that has not yet combined with chlorinated water to sanitize contaminants.
  • What is Combined Chlorine – Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool.
  • What is Total Chlorine – Total chlorine is the sum of free chlorine and combined chlorine.

When it comes to caring for your swimming pool and keeping your pool water balanced, no chemical is more important than chlorine.

Chlorine is the chemical compound that works to kill contaminants in your pool water. Contaminants include things like bacteria, perspiration, pee, and other microorganisms that could otherwise cause harm. Pool water that is not properly sanitized with chlorine can foster the growth of bacteria that leads to things like ear infections and Legionnaires disease.

There are three types of chlorine: Free Chlorine (FC), Combined Chlorine (CC), and Total Chlorine (TC). In this article we will discuss what each of these types of chlorine are and how you should interpret them when caring for your pool. After reading this article, you may want to check out our articles on How to Test, Raise, and Lower Pool Chlorine and Types of Swimming Pool Chlorine Sanitizer.

What is Free Chlorine?

Free Chlorine is the amount of chlorine that has not yet combined with chlorinated water to sanitize contaminants. In effect, free chlorine is the amount of chlorine that is free to kill harmful microorganisms in your swimming pool’s water. Free chlorine is the most important type of chlorine to monitor because it is responsible for actively sanitizing your pool.

POOL DUDEZ recommends keeping free chlorine levels between 2.0 and 4.0 parts per million (ppm). POOL DUDEZ recommends 2.0 to 3. 0 ppm for a stabilized pool. The easiest way to check your chlorine levels is with test strips. However, ideal free chlorine levels are dependent on cyanuric acid levels. We recommend using the pool calculator to manage your pool’s entire chemistry. When your free chlorine levels get too low, we recommend using calcium hypochlorite (unstablised shock) to raise them back up.

PRO TIP: We recommend testing free chlorine every 2 to 3 days and adding chemicals as you need to keep your free chlorine in recommended ranges. We also recommend adding your chemicals at night. UV rays from sunlight cause chlorine compounds to break down faster. Adding chemicals at night is a great way to save money on chlorine costs.

What is Combined Chlorine?

Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool. Combined chlorine causes the smell many people associate with chlorine pools. If CC is above 0.5, you should shock your pool. Combined chlorine indicates that there is something in the water free chlorine is in the process of breaking down. In an outdoor pool, combined chlorine will normally stay near zero so long as you maintain an appropriate free chlorine level and the pool gets some direct sunlight.

What is Total Chlorine?

Total chlorine is the sum of free chlorine and combined chlorine. Inexpensive chlorine tests, such as the common OTO test, which shows total chlorine as different shades of yellow, measure true chlorine because it is easier to test for than free chlorine and combined chlorine. In normal operation total chlorine can be used as if it was free chlorine because combined chlorine is usually zero. However when you have algae or some other problems, combined chlorine levels can be significant and total chlorine becomes useless.


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How to Test, Raise, and Lower Swimming Pool Chlorine

Chlorine Is The Most Important Chemical When It Comes To Maintaining Sparkling Clean Water That Is Safe To Swim In. Chlorine Sanitizes Your Pool Water By Attacking And Killing Harmful Microorganisms, Called Contaminates, That Could Lead Adverse Health Consequences. Therefore, It Is Critical To Know How To Test The Chlorine In Your Pool Or Hot Tub, As Well As What To Do When Your Chlorine Levels Are Outside Recommended Ranges.


The Short, Short Version

Test strips are the easiest way to test free chlorine but testing in store with an electronic water test is more accurate. Add sanitizer to raise chlorine. Elevated chlorine should drop quickly with use and sunlight.
Chlorine is the most important chemical when it comes to maintaining sparkling clean water that is safe to swim in. Chlorine sanitizes your pool water by attacking and killing harmful microorganisms, called contaminates, that could lead adverse health consequences, including death. For these reasons, it is critical to know how to test the chlorine in your pool or hot tub, as well as what to do when your chlorine levels are outside recommended ranges.

How to Test Pool Chlorine with Test Strips

There are two ways to test your pool chlorine: test strips and test kits. Test strips are a bit easier and more convenient to use than test kits. Test strips tend to be the method of choice for non-professional, pool owners.
Test strips often come in small, cylindrical containers that feature color scales on the outside of the container that correspond to each of the chemicals tested on the strip. To conduct a test, simply take one of the strips from the container and dip it in your pool water. The container should include instructions on how long to hold the strip under water, as well as how long to let the strip rest before comparing the test colors to the color scale.
To test chlorine, match the appropriate test on the strip with the chlorine color scale on the container. The color will likely not be an exact match with any of the benchmark colors on the testing container. Use your best judgement to estimate the tested ppm based on the color of your test strip compared to that of the lighter and darker colors that most closely match yours.

How to Test Pool Chlorine with Test Kits

Test kits are a bit more cumbersome to use than test strips. They are also more expensive than test strips, on average. However, they are also considered more accurate than test strips because they use a larger sample of water. Therefore, test kits tend to be pool professionals’ preferred method of choice for testing pool chlorine.
Test kits often come in a small box that contains a guide to pool chemistry, a testing tube to take the water sample, a saturation index tool, and reagents to add to the water to measure a chemical, such as free chlorine.
As with test strips, follow the test kit’s instructions on how to properly take the water sample and add the reagent to test your chlorine level. It is important to take exactly the recommended amount of water. Too little water could result in a reading that overestimates the amount of chlorine. Too much water could result in a reading that underestimates the amount of chlorine.
After waiting the appropriate amount of time for the reagent to activate in the water sample, compare the color of the water to that of the color scale provided by the test kit. Like with test strips, it is unlikely your color will match the benchmark colors of the test kit exactly. Use your best judgement to determine the exact amount of chlorine in the water.

How to Raise Pool Chlorine

The Association of Pool and Spa Professionals recommends keeping free chlorine between 2.0 and 4.0 ppm. The Center for Disease Control recommends free chlorine be kept above 1.0 ppm for pools and 3.0 ppm for hot tubs. If you find your free chlorine to be below these recommended ranges, it is important to add the appropriate chemicals to your pool to increase free chlorine back to the desired level.
There are several chemicals that may be used to raise your free chlorine. These include chlorine gas, chlorine tablets, , calcium hypochlorite (commonly referred to as cal-hypo), trichloroisocyanuric acid (commonly referred to as trichlor), dichlorine (commonly referred to as dichlor).
To determine how much sanitizer to add to raise your free chlorine to the desired levels, we recommend using the Pool Calcualtor’s free chlorine test. More information about how to use the test can be found in the video below. When your free chlorine levels get too low, we recommend using calcium hypochlorite to raise them back up.

How to Lower Pool Chlorine

It is much better to have too much free chlorine than not enough free chlorine. Water with not enough free chlorine may allow harmful bacteria to grow that could lead to life-threatening illness, including E Coli. By contrast, too much free chlorine may cause irritation to the eyes and skin. In the worst scenarios, too much free chlorine may result in a rash that lasts several hours to a day or two. Do not be alarmed if your test results indicate your water contains too much free chlorine.
Nonetheless, it is no fun to swim in irritating conditions. If you would like to lower the amount of free chlorine in your pool, there are two easy ways to do so. The first is to wait it out. Free chlorine will interact with the contaminants in your pool and convert into combined chlorine before deconstructing altogether. New contaminants, such as bacteria from the natural environment, are constantly being introduced into the water. Moreover, UV rays from sunlight expedite the dissolution of free chlorine. Depending on how much free chlorine is in your pool, free chlorine may be down to a suitable level within a few hours.
If you would like to expedite the removal of free chlorine from your pool or hot tub water, the best way is to replace some of your pool’s existing water with new water. The new water should have no free chlorine, but also have contaminants for the remaining chlorine to attack. In effect, replacing some of the water packs a one-two punch by reducing the amount of chlorine and making the remaining chlorine work even harder.

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