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Differences between Total and Free Chlorine in Swimming Pools

Chlorine is an important substance for keeping the water clean, but you must make sure that the water has a proper chlorine level. New pool owners often feel overwhelmed with all the chlorine-related terms.

What is free chlorine? How about total chlorine? Are they different from combine chlorine? How can they help me understand the suitable chlorine level in my pool?

Before diving into the terms, you must understand the basics of chlorine and how it works. Chlorine (Cl) is the basic chemical element in products used to sanitize a swimming pool.

It works by destroying the lipid cells of organisms, removing bacteria, germs, and algae from the pool water. In short, this chemical component prevents your pool water from becoming green, disgusting, and dirty.

Sodium hypochlorite (NaOCl) is one of the most common chlorine compounds for cleaning a swimming pool. You can find it in liquid chlorine products, and it has similar components to regular bleach. There are also granular chlorine products, which use calcium to bind the chemical components.

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How much chlorine should you use to maintain swimming pool water?

The standard amount is half-cup (0.1 liters) or two cups (around 0.5 liters) per 50,000 gallons of water. However, they are not strict measurements. You can use a higher or lower amount of chlorine to treat a swimming pool.

The amount of pool using can affect the amount of chlorine you must use. More usage means an extra amount of chlorine to treat bacteria and germs.

Rainfall and the size of the pool can also affect the use of chlorine. This doesn’t mean that adding extra chlorine is the best action. You must test the water regularly to keep the chlorine level in a balanced proportion.

Free, Combine, and Total Chlorine

If you decide to have a swimming pool, make sure to understand the important chlorine terms, especially for testing and applying. Free, combine, and total chlorine are the first three common terms to understand the state of chlorine in pool water. 

Free Chlorine

Free chlorine refers to the “new” chlorine product, which is not yet reacted with bacteria, germs, algae, and other contaminants in the water. The chemical components are still “free” from unwanted chemical components in the pool. Free chlorine is also known as free available chlorine (FAC).

When a chlorine product is added to water, it will react with the water components to form hypochlorite ion and hypochlorous acid. They are considered as free chlorine until they react with ammonia, nitrogen, nitrate, and organic compounds in the water.

Combine Chlorine

Combine chlorine refers to the chlorine components that have reacted with water contaminants. Combine chlorine components have done their job reacting to ammonia, nitrate, and organic contaminants in the water. It is responsible for the typical chlorine odor and effects such as red eyes and skin irritation.

When combine chlorine is formed, you need to add extra free chlorine to finish the job. This is because the cleaning process has “used up” the free chemical components in chlorine, creating new components called chloramines with the contaminants. They can no longer do the water sanitizing job. 

Total Chlorine

Total chlorine refers to the amount of combine and free chlorine left in the water. When the combine chlorine part is higher than the free one, the water will become odorous and irritating for the eyes or skin. You will also notice algae growth, murkiness, and other signs of unhygienic pool water.

Testing the amount of total chlorine is important to keep the balance of chlorine in the water. The level of combined chlorine must always be lower than the free one.

Ideally, combine chlorine must be less than 0.2 parts per million (ppm), while the free one is between 2 and 4 ppm (but not below 0.1).

How to Test the Chlorine Level in Pool Water

Testing Chlorine level (illustration)

Swimming pool owners must test for chlorine two or three times per week. If the pool is uncovered or often used, daily testing is a must.

There are several types of chlorine testing kits available in the market. You should use several testing kits to get whole information about the chlorine level in your pool.

Here are several recommended chlorine test kits you can easily buy.

Strip Test Kit

The strip test kit consists of several plastic strips with chemical pads. They change colors based on the type and amount of chlorine detected. The kit has color indicators for easier reading. They can test the amount of free, combine, and total chlorine in the water. 

Titration Test Kit

The titration test kit uses reagent, a liquid chemical substance that causes reactions when dropped into a specific compound. The kit comes with color indicator charts that show you the condition of the water chlorine. The kit can read the amount of total and combine chlorine. 

Automatic Chlorine Tester Kit

Automatic chlorine tester kit is the most expensive in the market and often claimed to be the most accurate. However, since the claim is hard to prove, you should use the other test kits for a more accurate reading.

This kit combines an electric strip tester and an automatic chlorine release function. If the reading for free chlorine is too low, the kit immediately releases the proper amount of chlorine.

What should you when all the kits show a high amount of combined chlorine in the water? It is time to conduct “shock treatment”.

What is Pool Shock Treatment?

Pool shock treatment refers to a drastic step to reduce the amount of combined chlorine (chloramines) from the pool.

You do this by adding more chlorine into the pool water. This way, the amount of free chlorine will be bigger than the contaminated one, removing remnants of bacteria, germs, algae, ammonia, and nitrogen from your pool.

There are several recommended times to conduct shock treatment. They are:

After the Rain

Heavy rain can bring unwanted contaminants into your pool, especially if it is uncovered. The water brings airborne particles into the pool, such as dust, pollen, spores, and elements of air pollution. Running water from the surrounding grounds also brings dirt, bacteria, and germs.

Algae Bloom Season

When standing waters (including outdoor pools) start to show the algae bloom, make sure to shock the pool. Green algae are the easiest to remove compared to the yellow or red ones.

Winter Start-up and Close-down

A winterized pool needs to be shocked when the cover is closed and opened. The close-down process requires a shock treatment for a sanitizing process.

The start-up process needs it to oxidize the particles, remove possible contaminants, and make the water clear again.

Heavy Pool Use

There are times when a pool is used more intensely than before, such as during a pool party.

The water is likely to get contaminated by more bacteria, germs, and contaminants than usual (not to mention things like feces traces, urine, and vomit). Shock treatment is important to return the water condition to normal.

Hot Weather

Hot weather and intense sun exposure can reduce the level of chlorine in the pool. If your pool is uncovered, and the weather is exceptionally hot, make sure to shock the water with more chlorine.

Normally, a pool shocking treatment is done every three to four weeks. You must consider things like the weather, frequency of use, pool covering (if any), and the condition of pool elements such as a filter.

You don’t need to wait until the water becomes murky or dirty. If a shock treatment is necessary, do it as soon as possible.

Chlorine-based Products for Shock Treatment

There are several products available in the pool maintenance market for a shock treatment. They are:

Calcium Hypochlorite

Calcium hypochlorite is chlorine powder, which combines chlorine with calcium to bind its molecule. Also known as the bleaching powder, calcium hypochlorite has 12 pH level and is available in 65 and 73 percent of strength. It can add the calcium present in the water.

Sodium Dichlor

Sodium dichlor consists of chlorine and cyanuric acid. This shock treatment product comes in grains, considered as the most expensive. However, the acid can protect the water chlorine level from the sun effect. It has a neutral pH level.

Potassium Monopersulfate

Potassium monopersulfate is a chlorine-free treatment product. It adds oxygen to the water, with no other residues. 

Calcium hypochlorite is the most common shock treatment products. However, if your pool has the characteristics of hard water, choose between sodium dichlor and potassium monopersulfate.

Conclusion

So what is a free chlorine? What about the total and combine chlorine?

Everyone who wants to have a swimming pool must understand these terms to treat the water. Choosing the right time to apply chlorine will keep your pool water clean, fresh, and free of contaminants.

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