Chlorine vs. Shock: What’s the Difference and How Do They Work?

Chlorine is one of those products that comes in so many forms and types that can be confusing for new pool owners to understand. You thought shock was just regular chlorine, right? So did I a few years ago. Chlorine has been the most cost-effective and reliable sanitizer for swimming pools for over 110 years now, and the product itself comes in many shapes and sizes. 

The term “chlorine,” when used to describe the sanitizer that goes into a pool, doesn’t account for the fact that chlorine is a gas in its natural state. What we call shock and chlorine typically contains a certain amount of sodium or calcium hypochlorite, two chlorine carriers, and a boatload of other chemicals that turn it into a granular or liquid form. Or it comes in the form of Trichloro-S-Triazinetrione tablets. We’ll get into these more in-depth soon, but I wanted to address the chlorine name problem first. There’s also a variety of non-chlorine shocks on the market, but we will save those for another time

The goal of this article is to answer some of the most frequently asked questions about chlorine and shock and break down what makes them different, as well as the forms they come in. We’re not going to be talking about saltwater pools, which are just chlorine pools essentially, in this article. We have other educational articles dedicated to saltwater pools that you can read here. Let’s talk about chlorine, shall we?

 

What’s the Difference?

Shock is a granular or liquid form of chlorine sanitizer used to sanitize and, in some cases, oxidize your water. So why’s it called shock? Well, that’s because, as we stated earlier, that shock is not true chlorine. In fact, the amount of active chlorine present in shock can range from 8-75%, depending on if you get liquid or granular shock. These products are typically paired with trichloro-s-triazinetrione chlorine tablets, which can carry up to 90% available chlorine. Tablets dissolve over time and give you a steady stream of chlorine sanitization. When you pour the liquid or granular chlorine into the pool, it shocks all the active chlorine and the chlorine becomes so much stronger and more efficient. That is why we call it shock.

Granular shocks can range from about 68 – 78% calcium hypochlorite. They are highly effective sanitizers that are best used for weekly chlorine needs. They are typically bought in bags and can be deposited directly into the pool. You will occasionally notice a clumping of chlorine on your pool floor with granular shock. It is recommended that granular chlorine be deposited throughout the whole pool or in front of a jet. 

Liquid shock is comprised of around 8 – 12% sodium hypochlorite. The higher the percentage, the stronger the shock. You can usually buy liquid chlorine by the gallon. It is very easy and safe to pour into your pool. Quite convenient, too, since there is no dissolving involved, this allows it to work quicker than granular chlorine. 

How Does Chlorine Work?

Other chlorine products on the market contain higher available chlorine levels that are used on a more daily basis which we refer to simply as chlorine. Chlorine typically comes in a giant bucket or in the form of tablets and sometimes a bucket. Trichloro-s-triazinetrione tablets are what we like here at RPG2. If you put three or four of those tabs into a chlorinator or a floating chlorine feeder, your pool gets constant, steady chlorination. Granular chlorine in a bucket typically involves a weekly schedule to follow that will help your pool get steady chlorination. Chlorine tablets and granular chlorine can carry up to 90% available chlorine, and are stabilized as well. What does stabilized mean? I got you on that one too. 

 

Stabilized vs. Unstabilized Chlorine?

There are two types of chlorine that you are going to buy for a pool: stabilized and unstabilized chlorine. Cyanuric acid (CYA) is a pool stabilizer and acts as protection for chlorine from UV rays allowing chlorine to live longer and work better. Most chlorine tablets you see on the market will be stabilized, as well as stabilized granular chlorine. Stabilized chlorine is ideal for pools that get a lot of sunlight.

Unstabilized chlorine is generally found in the form of shock. Unstabilized chlorine does not contain any cyanuric acid, which in turn causes the chlorine to die quickly. This is why pool tablets give you a steady, slow release of chlorine every day, and shock gives you a quick and heavy chlorine boost. Unstabilized pool shock is great for indoor pools and pools with automatic covers. When using chlorine, regardless of whether it is stabilized or not, it is important to monitor CYA and other chemical levels every few days. CYA levels should hover around 40-50 ppm in a traditionally chlorinated pool. Saltwater chlorine pools require higher CYA levels, but they should never exceed 100 ppm. 

 

How Do Chlorine Tablets Work?

Chlorine tablets are deposited into a floating chlorine feeder or a chlorinator on your equipment pad. We typically recommend 2-3 tablets at a time, but depending on the volume of your pool, you may use more. They work to sanitize your pool for up to a week in most cases. Remember that the chlorine survives longer not just because the tablets are slowly dissolving but also because they are stabilized. Chlorine tablets will also ensure that your chlorine levels are maintained between .5 – 3 ppm, saving you time and money for more chlorine. They also have a very long shelf-life, sometimes up to 5 years, as long as they are kept in a cool, dry environment. 

 

Can They Be Used Together?

I love this question because the answer is a resounding “Of course.” Most pool owners who use chlorine already do this. You have to remember people have been using chlorine in pools for over a century now. We’ve learned some tricks. Water needs daily chlorination, but as tablets dissolve, the chlorine output won’t be as strong as on days 1 and 2. So we use shock to boost that chlorine power and give it the extra push. Combining unstabilized shock and stabilized granular chlorine or tablets will basically guarantee that your pool stays clean and clear so long as your water’s chemistry is balanced.

I cannot stress enough the importance of maintaining proper water chemistry, which is:

Free Chlorine: .5 – 3 ppm

Total Chlorine: .5 – 3 ppm

pH: 7.2 – 7.8

Alkalinity: 80 – 120 ppm 

Calcium hardness: 175 – 350 ppm

Cyanuric acid: 30 – 50 ppm

Free chlorine refers to the amount of active chlorine working in your pool, whereas total chlorine accounts for the free chlorine as well as any chlorine that has died off over time (combined chlorine). Ideally, you want the free and total chlorine to be equal, indicating that there is no combined chlorine in the pool. But if your other chemicals are not in range, the chlorine you’re dumping into your pool is being wasted. Pool chlorine requires balanced water to sanitize efficiently and keep the water clear. What I’m trying to say is this: test your pool chemistry as often as possible!

 

Closing Thoughts

Using chlorine is one of the safest and best things you can do for your pool. It comes in a variety of shapes and sizes and never really fails as long as you take care of your water. Knowing the difference between shock and regular chlorine, as well as stabilized and unstabilized chlorine, will help you better determine which type of chlorine sanitizer will be best for you and your pool. Saltwater is another effective way to chlorinate and sanitize your pool that has its very own benefits. If you’re curious about saltwater pools, check out these articles below.

What Is Pool Shock And How Often Should I Shock My Pool?

Top 5 Chlorine Alternatives For Pools!

Which Chlorine is Best For Your Swimming Pool?

Written By Logan Edgemon