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Free Vs. Combined Vs. Total Chlorine: What’s The Difference?

  • Posted on
  • By Atlas Scientific
Free Vs. Combined Vs. Total Chlorine: What’s The Difference?

Chlorine is a chemical element that belongs to the family of halogens. It is an extremely effective disinfectant as it can kill disease-causing bacteria and viruses, along with microorganisms that live in water, such as algae. Water chlorination has become common practice to prevent the spread of waterborne diseases, and chlorine is routinely added to the public water supplies, water reservoirs, pools, and hot tubs in order to keep the water clean. Whether you are curious about how drinking water is disinfected, or how to keep your pool water clean and see-through this summer, a good starting point would be understanding the notions of free, total, and combined chlorine, and being able to differentiate between them. 

What is free chlorine? 

You have probably heard the words “free chlorine” mentioned before. It is simply a term used to describe the amount of chlorine, that is available to disinfect the water from any potentially dangerous or unpleasant contaminants. Once free chlorine is added to the water, it will form hypochlorite ions and hypochlorous acid, which are the active disinfecting compounds. These compounds rid the water of bacteria, viruses, and microorganisms by damaging their cell membranes and essentially, disturbing the processes necessary for their replication and survival. As you can imagine, free chlorine is very important to monitor, as not having enough will allow bacteria and other contaminants to grow, and having too much can cause skin irritation and itchiness. You can measure the amount of free chlorine in your pool or other forms of water reservoir indirectly by using an ORP sensor. The value that you want your free chlorine levels to be at is around 2-4 parts per million, or ppm for short. 

What is combined chlorine?

Combined chlorine refers to the chlorine that has been already been “spent” on sanitizing the water. It is an intermediate product that is formed when the pool is being disinfected. It is called combined chlorine as it is essentially free chlorine that has bound itself to a contaminant or organic material, such as oil from the skin, sweat, or urine. You will also find combined chlorine being sometimes called chloramine. When measuring chlorine levels in your pool, the amount of combined chlorine should be below the threshold value of 0.5 ppm. If you have higher amounts of combined chlorine in your pool, you might want to postpone your swimming plans, as a pool with higher than normal levels of combined chlorine will have an unpleasant odor, and will cause eye redness and skin irritation. 

So, a perfectly clean and fully disinfected pool should have no combined chlorine present. The presence of combined chlorine indicates that the free chlorine is actively neutralizing the bacteria and contaminants in the water. The key to keeping the combined chlorine levels at zero in your pool is to closely monitor the amount of free chlorine, and adjusting the value when necessary. You can remove the excess of combined chlorine from your pool via water filtration, chemical balancing, evaporation and/or UV light

Free vs. Combined vs. Total Chlorine: What’s the Difference?

What is total chlorine? 

Total chlorine is simply a sum of the free and combined chlorine. It is very easy and inexpensive to test the level of total chlorine, using an OTO solution, which turns yellow when added to chlorinated water. The darker the color, the more chlorine is present. If your pool is clean, the combined chlorine value can be assumed to be 0, so then the total chlorine amount should be made up of just free chlorine. However, there is no sure way of knowing if your pool is completely clean, without monitoring free and combined chlorine levels, as contaminants, such as bacteria and other microorganisms are not visible to the human eye. A presence of algae or a characteristic lingering chlorine smell is both indicators of the excess of combined chlorine.

How to test for free, combined, and total chlorine levels? 

So now that you are equipped with knowledge on how chlorine works to disinfect your pool, and what type of chlorine you want around, it is good to know how to actually test for the different chlorine levels. First things first, in order to make sure that your pool is clean, disinfected, and safe, it is essential to routinely monitor and maintain the chlorine levels. You can test your pool’s chlorine levels by using test strips, test kits, or an electronic chlorine tester. Test strips are the easiest and most intuitive to use. They are also cheaper than test kits or an electronic chlorine sensor. They often come in a small, cylindrical container and are made from plastic with a small chemical pad attached at the bottom. This chemical pad will react with the water in your pool and will change color, depending on the amount of chlorine present. You might need to keep the test strip in the water for a certain period of time and allow it to rest after the water exposure as well in order to obtain the most accurate results. 

If you are up for a little chemistry experiment, you could opt for a chlorine test kit. They are a little more complicated to use and are also more expensive than test strips, but they are more accurate, as they use a larger sample of water, therefore better representing the actual chlorine levels in your pool. Test kits often include a testing tube for the collection of the water sample, a saturation index tool, and reagents, which will be added to the water sample to measure the chlorine levels. Once a sample of water is taken, a few drops of a reagent are added to the water sample, which will result in a color change. Just like with the test strips, by comparing and matching the color that your water sample has obtained with the color panel provided, you will be able to deduce the amount of chlorine present. 

Using test strips and test kits is a fast, affordable, and easy way to test for chlorine levels in your pool. However, if a really precise measurement is required, you might want to opt for an electronic chlorine sensor. The only downside is that electronic chlorine testers are substantially more expensive than chlorine test strips or test kits. 

If you already have an ORP sensor, you can use it to indirectly measure chlorine levels as well. An ORP sensor will measure the electron activity in the water. An oxidative water environment with higher ORP readings indicates a clean, sanitized pool. Chlorine, as well as oxygen, adds to the higher oxidative reduction potential level due to the breakdown of unwanted impurities and pollutants in the water. Hence, a high ORP value is associated with a higher level of water disinfection, and consequently, a necessary amount of chlorine present for this disinfection to occur. For the pool water, the ORP value has to be between 700 mV and 750 mV for sufficient sanitation. It is important to remember that ORP is not a direct measurement of water chlorination, but rather an indication of water sanitation level, which is interlinked with the chlorination levels. 

Free vs. Combined vs. Total Chlorine: What’s the Difference?

How to increase the chlorine amount?

If the free chlorine level in your pool is lower than 2.0 ppm, it is strongly recommended to raise the chlorine levels in order to avoid the occurrence and transmission of water-borne diseases and infections. There are several chemicals you can use, which include chlorine gas, chlorine tablets, liquid bleach (sodium hypochlorite), cal-hypo (calcium hypochlorite), dichlor (dichlorine) and trichlor (trichloroisocyanuric acid). 

How to decrease the chlorine amount? 

It is best to keep your pool’s chlorine levels above 2 ppm and below 4 ppm. If you find the chlorine level to be below this threshold, the water will become a good environment for bacteria, viruses to grow and for contaminants to accumulate. On the other hand, if the chlorine level of your pool is too high, then swimming in it will cause eye redness and skin irritation, and itchiness. In order to lower the water chlorine levels, you can either try and just wait it out or replace the existing water with new water. Giving your pool some time to get back to normal chlorine levels is a foolproof method to get rid of extra chlorine. The extra free chlorine will simply react with the contaminants that are being produced continuously, and will eventually subside to normal levels. This could work really well especially on a sunny day, as the sun’s UV rays speed up the free chlorine dissolution. If you would like to get rid of the extra chlorine faster, you can always just replace some of the pool’s water with new, unchlorinated water. The new water should have no chlorine in it but will have some contaminants, which will make the existing free chlorine get consumed even faster. 

Summary 

There are 3 types of chlorine: free, combined, and total. Free chlorine refers to the amount of chlorine that is readily available to disinfect any bacteria or other contaminants, as soon as they appear in the water. The optimal levels of free chlorine are between 2 and 4 ppm. The amount of free chlorine should be monitored closely, which can be done by using a test strip, test kit, an electronic chlorine tester, or indirectly by using an ORP sensor. 

Combined chlorine is a term used to describe the free chlorine that has bound itself to a contaminant or organic material, such as skin oil, sweat, or urine. While it is acceptable to have combined chlorine levels below 0.5 ppm, a clean and disinfected pool will have no combined chlorine present. Having too much combined chlorine can result in a strong chlorine odor, eye redness, and skin irritation. Total chlorine is the sum of free and combined chlorine. It is easy to monitor the total chlorine levels, but this measurement is only useful if there is no combined chlorine present. 

You can measure chlorine levels, using a test strip or a test kit, where the strip or a water sample will change color depending on the amount of chlorine you have in your pool. The most accurate way of measuring water chlorination is done by using an electronic chlorine sensor. However, if you have an ORP sensor, you can also use that to indirectly measure the chlorine levels and estimate the water sanitation levels. If the chlorine level in your pool is below 2 ppm, it is highly recommended to add more free chlorine in order to avoid contaminants and water-borne diseases and infections. You can increase the chlorine levels by adding a chlorine-containing chemical, such a liquid bleach, cal-hypo, dichlor, trichlor or, simply chlorine gas or tablets. If the chlorine levels in your pool are above the threshold of 4 ppm, it is necessary to reduce the amount of free chlorine in order to avoid eye and skin irritation. You can either wait it out by letting the extra free chlorine react with the new contaminants, or purge some of your pool’s water and replace it with fresh non-chlorinated water. 

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