Pool Chlorine Vs Shock: What’s The Difference?


by Rick Patterson

Learning about all the disinfecting chemicals that go into pool water, and the balancing of them, can be confusing.

When it comes time to understand the difference between pool chlorine vs shock, they might seem like the same thing.

But are they?

Yes, and no. We lay it all out for you in this article.

What Is Pool Chlorine?

Pool chlorine is a sanitizer and oxidizer that keeps your pool water free from germs, bacteria, viruses, and organic pollutants.

Breaking it down, chlorine releases two chemicals into the water: hypochlorous acid (HOCl) and hypochlorite ion (OCl-). They attack the enzymes and structures inside the cells of pollutants, destroying them.

The result is water that’s clean and safe to swim in.

When talking about chlorine and how it relates to swimming pools, there’s three terms that are used to describe the different states.

Free available chlorine (FAC), often shortened to “free chlorine” or “available chlorine”. It’s what most people think of when chlorine is mentioned, because it’s chlorine in its purest state; unused and ready to sanitize your pool water.

Combined chlorine (CC), often referred to as chloramine, is chlorine that has been used up in your pool after attacking (and combing with) pollutants, making it ineffective at keeping your pool water sanitized.

Total chlorine (TC). It probably doesn’t take a genius to figure out this is the sum of the pool water’s free available chlorine and combined chlorine levels. This is useful to know when testing pool water, as many test kits only measure total and free chlorine levels.

Are There Different Types of Chlorine?

Chlorine comes in 2 categories: unstabilized and stabilized.

Unstabilized chlorine is chlorine in its purest state, whereas stabilized chlorine contains cyanuric acid (CYA). CYA, also called pool stabilizer, acts as a sunblock to protect the chlorine from the sun’s UV light, which will quickly destroy it if there’s no protectant.

Unstabilized chlorine comes in 3 types: sodium hypochlorite (liquid chlorine), lithium hypochlorite, and calcium hypochlorite.

Sodium hypochlorite is a liquid form of chlorine that can be directly added to your pool skimmer. It won’t raise the pool’s pH level, and at 10-12% available chlorine, it’s pre-diluted for a safe-to-use level.

Lithium hypochlorite has 35% available chlorine and is granular in form. This makes it a good candidate for super chlorinating your pool, although it’s a bit pricey and hard to find these.

Calcium hypochlorite (cal hypo) is a commonly used unstabilized chlorine with anywhere from 40% to 78% free available chlorine. It’s available as a slow-releasing tablet for residual sanitation. However, it will raise your calcium hardness levels, eventually leading to problems with scale.

Stabilized chlorine comes in 2 forms: sodium dichlor and lithium trichlor.

Dichlor has a near-neutral pH level, so you can use it without affecting the pool water chemistry. It’s also resistant to high water temperatures, making it ideal for hot tubs, and it’s very effective at killing black algae.

Trichlor comes in puck/tablet form and is the go-to choice for most pool owners as a residual chlorine. It has 90% available chlorine, and a low pH level of 3, making it ideal for use over long periods of time.

What Is Pool Chlorine Shock?

Pool chlorine shock is a high concentration of chlorine in a single dose.

The idea behind shocking your pool is to quickly elevate the level of free chlorine in the pool, effectively killing all germs and contaminants.

As stated earlier, a residual level of “day to day” chlorine is kept in the pool at all times, which gives you round the clock sanitization of the water.

But, because it’s constantly working to destroy pollutants, it’s also constantly losing effectiveness as more and more combined chlorine (aka chloramines) become present in the pool.

As a result of these pollutants building up, they need to be destroyed. This is where shock comes into play, flooding the pool with a massive amount of chlorine to reach breakpoint chlorination level.

Breakpoint chlorination is the level of free chlorine that’s required to destroy the combined chlorine in the water. Due to its higher percentage of free chlorine, shock can take care of all the pollutants that the residual chlorine no longer can handle.

Once this is done, the pool is essentially “reset” in terms of pollutants, and the cycle starts all over again. As this is a never ending cycle, most people need to shock their pool every 1 to 2 weeks to keep it fully sanitized.

Are There Different Types of Shock?

There are two types of shock you can use: chlorinated and non-chlorinated.

The most commonly used chlorinated shock is a granular version of calcium hypochlorite, which is a 2-in-1 oxidizing and sanitizing chlorine.

It’s affordable, high in free chlorine level, unstabilized (burns off quickly from the sun’s UV rays), and gets rid of all the undesirable elements in your pool water to restore clarity.

Non-chlorinated shock is potassium monopersulfate (aka MPS), which is only an oxidizer. It works by oxidizing organic compounds in the water. By focusing on them, it allows the residual free chlorine in the pool to focus strictly on sanitizing the water.

Chlorine Vs Shock: The Comparison

When comparing pool chlorine vs shock, there are certain characteristics they have in common. However, there are differences, which are especially apparent when using a non-chlorine shock.

How They’re Similar

Both kill bacteria: Whether its residual chlorine or chlorinated shock, bacteria doesn’t stand a chance at surviving when sufficient levels of chlorine are present in the water.

Both kill algae: Algae blooms in pools can be a problem, with mustard algae and black algae being the most common types. Normally controlled using an algaecide, some blooms can only be removed by shocking your pool.

Both keep the pool water clear: Only when the residual chlorine starts to break down will the water chemistry become unbalanced and lead to a cloudy pool. Shocking will restore the proper chlorine level and restore water clarity.

Both available in various forms: Chlorine comes in liquid, dissolvable granules, and granule pucks/tablets for slow-release. While shock is more fast-acting, it’s also available in both granule (most common) and liquid forms.

Both oxidize and sanitize: Sanitizing will get rid of bacteria, algae and hazardous microorganisms in the water, while the oxidation power in chlorine will take care of any organic pollutants.

How They’re Different

Chlorine is a residual cleaner: When you add chlorine tablets to your pool, they’re slow-releasing, giving your water a consistent sanitizer over the duration of a few days. Chlorinated shock is quick-releasing, emptying a highly concentrated dose of chlorine into the water for rapid sanitization.

Stable vs unstable: Most pool owners will use stabilized chlorine tablets. These contain cyanuric acid, which add a layer of protection to the chlorine, as the harmful rays of the sun break chlorine down fast. Chlorinated shock is unstabilized, meaning there is no CYA in it. This allows you to dose the pool, kill the bad stuff in the water, and then have the sun burn it off so your pool’s chlorine returns to a safe level.

Different cleaning agents: There are a few different types of chlorine – dichlor and trichlor being stabilized and residual chlorines, with sodium hypochlorite, lithium hypochlorite, and calcium hypochlorite being unstabilized shock chlorines. Most chlorinated shock uses cal-hypo, and non-chlorinated shock is potassium monopersulfate (MPS).

Chlorinated shock kills new pollutants: As residual chlorine is always working, it’s always breaking down. Once it reaches a certain point, it becomes ineffective at cleaning. It’s at this point that chlorinated shock can kill the newly formed pollutants in the water.

Non-chlorine shock: MPS shock does not contain chlorine and works differently from chlorinated shock. MPS shock focuses solely on oxidizing organic matter in the pool. As a result, the residual chlorine won’t need to oxidize the water, and can focus solely on disinfecting/sanitizing it.

They’re Allies, Not Rivals.

When it comes to pool chlorine vs shock as a method for keeping your pool clean, the main difference is in their strength.

They’re meant to be used together, and if done correctly, you’ll quickly find out why they’re a great one-two punch for restoring your water to a pristine state.

Categories: Pool Care, Pool Chemistry