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What Does Chlorine Do In a Pool? (Simply Explained)

Ever wondered why pools need chlorine, what chlorine does in a pool, or how chlorine actually works in the water?

This article will take you through exactly why swimming pools need a sanitizer like chlorine, and how it works on a chemical level to keep your pool water clean and clear.

Why Do Pools Have Chlorine?

Most pools use chlorine because it’s a cheap and effective sanitizing agent.

Aside from its low cost, tablet, granular or liquid chlorine is also relatively safe to handle in the amounts needed for pool sanitation.

While alternative pool sanitizers like bromine and biguanide are sometimes used in place of chlorine, they aren’t nearly as widespread due to being both considerably more expensive and less effective.

What Does Chlorine Do In a Pool?

Chlorine’s primary role in a pool is to disinfect the water by attacking organic and inorganic compounds it comes into contact with.

Without chlorine, your swimming pool would be a breeding ground for microscopic contaminants such as bacteria, viruses, parasites, and algae – making the water extremely hazardous to humans.

Chlorine is also an oxidizer, which means it helps to remove the waste left behind after neutralizing contaminants in the pool. It does this by breaking them apart and forcing them out of the water in the form of a gas.

It also raises or lowers the pH level depending on the type of chlorine you’re using, but we’ll talk more about that later.

How Does Chlorine Work in a Pool? (Chemistry Explained)

If the above explanation wasn’t quite enough, we’re going to break down the basic chemistry of how chlorine works in a swimming pool.

Here’s exactly what happens when adding chlorine to your pool:

Phase 1: Transformation

When you add chlorine to your pool, whether from a salt generator or straight chlorine, it immediately transforms into free chlorine.

Free chlorine is made up of two compounds:

  1. Hypochlorous acid (strong disinfectant)
  2. Hypochlorite ion (weak disinfectant)

Both of these compounds will sanitize your water, but hypochlorous acid is both significantly faster and around 80 times more effective at disinfecting compared to hypochlorite ion.

The amount of hypochlorous acid (strong disinfectant) you get during this transformation depends on the pH level of your pool, with lower pH water yielding more.

Put simply, the lower your pH level, the higher your chlorine’s effectiveness:

  • A pH level of 7 results in around 75% hypochlorous acid
  • A pH level of 7.5 results in around 50% hypochlorous acid
  • A pH level of 8 results in around 25% hypochlorous acid

Fortunately, a balanced pH level of between 7.2 to 7.8 nets you more than enough hypochlorous acid to get the job done while still maintaining the rest of your pool chemistry.

Phase 2: Search and Destroy

Once your chlorine has been converted into free chlorine, it will begin to seek out all the nasty stuff in your pool water.

This includes things like algae, insects, animal waste, and dirt. It also looks for organic material brought in by swimmers such as sweat, saliva, body oils, hair, urine, and even fecal matter.

While you might think of chlorine as a reckless killer, the sanitation process is actually a little more calculated than it seems.

Free chlorine (or, more specifically, hypochlorous acid and hypochlorite ion) inactivates germs and other contaminants by penetrating the cell wall and disrupting it from the inside.

What happens next depends on the type of contaminant in question.

If the compound contains ammonia or nitrogen (mostly waste brought in by swimmers), the chlorine will combine with it to create combined chlorine, also sometimes referred to as “chloramine” or “chloramines”.

Most other compounds in the water will oxidize and burn off, leaving behind very little (if any) combined chlorine in the process. This includes metals such as copper and iron, as well algae and other plant-based matter.

Speaking of oxidation, that brings us to the next and final phase…

Phase 3: The Breakpoint

At this stage, free chlorine is being used up in fighting contaminants in your water, and much of it is being converted into combined chlorine.

Now, combined chlorine (chloramine) is pretty useless as a swimming pool disinfectant, and having too much of it will eventually lead to other issues with the quality of your water.

Namely, a buildup will:

  • Create a pungent chlorine smell
  • Irritate the eyes and skin of swimmers
  • Lead to rubber degradation (rubber o-rings, seals, etc)

To get rid of combined chlorine in your water, you need to force it to oxidize.

Direct sunlight will do some of this for you, but since chlorine is both a sanitizer and oxidizer, this final stage involves adding a large dose of chlorine to your pool in order to force the combined chlorine to break apart.

This is called shocking your pool, and it involves raising the free chlorine level to 10x the amount of combined chlorine to reach something known as “breakpoint chlorination” – or the point at which combined chlorine oxidizes.

This effectively resets your pool water, allowing for a more stable free chlorine level… at least until the combined chlorine builds up again.

What Does Chlorine Do To Your pH Level?

It depends on the type of chlorine you use because different types of chlorine have different pH levels.

For example, if you use trichlor chlorine tablets, their low pH level of around 3 will slowly reduce the pH of your pool water as they dissolve.

If you use granular chlorine, however, this could be in either trichlor (same as the tablets), dichlor, or calcium hypochlorite.

Dichlor has a pH of around 6.5, which is close enough to neutral that it’s going to have minimal impact on your pools pH level. Calcium hypochlorite has a much higher pH level of around 12, so you can expect a noticeable boost in your pool’s pH after using it.

Finally, liquid chlorine or bleach, which is sodium hypochlorite, has an even higher pH level of around 13. Unlike other types of chlorine, though, liquid chlorine only has a short-term impact on your pH.

Chlorine Kills the Bad Stuff

Now you know what chlorine does in a pool.

If it wasn’t for chlorine or similar pool sanitizers, we certainly wouldn’t be able to maintain them the way we do. It plays such a vital role in pool chemistry that, without it, most swimming pools would resemble murky, infested swamps.

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