What Chemicals Do: Chlorine

One of the largest areas of expertise required of an aquatics professional is pool chemistry knowledge. But what do you do if you don’t have your AFO, or CPO, and can’t readily get one? At least at this moment. That doesn’t mean the information inside them isn’t important, or that you shouldn’t have access to at least some of it!

Over the coming weeks we will be releasing a series of posts covering the essentials of not only pool chemistry, but of a number of important topics for you as an aquatics professional, and for your staff as well.

Let’s start our training with the most basics of pool chemistry!

Pool Chemical Basics - Chlorine


When we talk about pool chemicals the first, and most important ones are the ones everyone thinks of, or should think of; chlorine and pH.

More than any other chemical, this is the one that is absolutely essential to running your facility effectively. While others are, of course, important, this is the one that everything else builds around.

So let’s get into the pool chemical most people are familiar with...


What is chlorine?

Chlorine, aside from being the smell people most often associate with pool environments, is the most essential singular chemical in keeping our pools safe for people to use.

Most chlorine today is added in one of two forms:

  • Liquid; Sodium Hypochlorite

  • Dry; Calcium Hypochlorite


Among these two choices, dry chlorine in the form of calcium hypochlorite, is the most commonly used. Why, you might ask? There are a couple of reasons behind that, but essentially it comes down to liquid chlorine being essentially bleach. Although, industrial sodium hypochlorite is more potent than the kind you would buy for cleaning which has about 5.25%, versus around the 10% concentration used for pool treatment.

Whereas most dry chlorines have around 70% available chlorine. That’s 7x as much potency! That right there is enough reason. The space you save by having to have up to 7x less on hand makes it a fairly easy decision.

But beyond that, liquid chlorine doesn’t keep well. And can lose up to a third of its potency in as little as a month. Dry chlorine however is capable of maintaining its potency near 100% for a year.

So it’s clear to see that dry chlorine is the best choice for potency, space and longevity.

What does it do in our pools?

Whichever type of chlorine you use though, they do the same jobs. And those two jobs are:

  • Disinfecting

  • Oxidizing


Disinfecting in your pool using chlorine serves the same function as it does in any setting. Chlorine kills any bacteria it comes into contact with, sterilizing the pool.

That’s why it’s important to keep a chlorine residual in your pool at all times, so that way any bacteria introduced to the pool will be immediately killed before it can have any ill-effects on your patrons.

Disinfecting is only one half of the most important chemical used in your pool.


Oxidizing is the second half of chlorine. Once the chlorine has disinfected the water, by killing any bacteria in it, the second half of its job begins; oxidation. The easiest way to think of oxidation is it’s the process of taking the disinfected bacteria, and disintegrating it, or burning it off.

In order to keep a pool clean it’s important that this second half of chlorine’s job is able to be performed. But how do you do that?

Have you ever had the experience of going to a pool, maybe at a hotel, and as soon as you enter the pool area your eyes start to burn? Most people will say it is because there is too much chlorine in the pool. However, the opposite is true! That stinky pool smell is actually a byproduct of there not being enough chlorine in the pool to properly disinfect and oxidize the compounds that have been disinfected.


When taking a chemical reading, whether using a titration kit, or a photometer, keeping a chlorine reading of between 1 and 4 ppm (parts of chlorine for every million parts of water) will keep your pool properly disinfected, and will aid in oxidizing the neutralized organic materials. But there are three types of chlorine readings that are commonly done. The free available chlorine test (FAC), the total chlorine test (TC) and the combined chlorine reading (CC). They each are different numbers, and will mean different things about your pool.

FAC Free available chlorine is essentially a measurement of how much chlorine is floating around your pool waiting to disinfect something. This should be kept in a range of 1-4ppm.
TC Total chlorine is a reading that will be the same or higher than your FAC reading. This measures all the chlorine in the pool, whether it is free chlorine that can disinfect, or chlorine that is already combined with organic matter and cannot disinfect anymore.
CC Combined chlorine is a measurement of how much of the total chlorine in the pool is combined and unable to disinfect. This number will always be the difference (if any) between the free chlorine, and the total chlorine.

There is a lot more at work here than what we’ve laid out, but we’re just covering the basics.

Further information about eye burn can be found here.

How dangerous is it?


Chlorine is a very hazardous chemical, and should be handled with care. Proper use of personal protective equipment is essential in handling any of the dangerous chemicals associated with a pool environment.

For chlorine specifically, it is important to utilize a respirator, either a half mask or full mask. If only a half mask, when handling chlorine it will be important to also put on eye protection.

Beyond eye protection, and a respirator, if handling chlorine protective rubber gloves are advisable. Chlorine, especially when dry will not burn your skin, or anything so dramatic. But it leaves a lasting smell. Remember how bleach is just chlorine, well accordingly, chlorine, whether the wet stuff, or the dry stuff when it’s dissolved/dissolving will bleach out clothing.

Chlorine in daily pool operations

During the normal day-to-day operations of your facility, it is important to keep your free-available chlorine in proper range (1-4ppm) at all times to ensure your pool is clean and safe for patrons.

It’s a good idea, if not legally required, to test your primary chemicals (chlorine and pH) multiple times per day, usually 3 times. When taking your chlorine and other chemical reading, be sure to keep detailed and accurate logs. Not only is it the law if you run a commercial swimming pool, but it’s part of keeping your pool and your patrons healthy. Read our post on 5 Mehods for Keeping Pool Logs. If you want to track your pool logs digitally, be sure to check out DigiQuatics

For further information about chlorine, check here.

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Have any experience or tips about dealing with chlorine? Have any questions? Let us know in the comments!


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