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How Does pH Affect Chlorine in a Pool? (Simply Explained)

Wondering how pH affects chlorine in a swimming pool? 

Not sure if it’s better to have a higher or lower pH when adding chlorine to your pool? Or if it even matters?

This article will explain the ideal pH level for chlorine, how pH affects the effectiveness of chlorine, and what happens to chlorine at varying pH levels.

Quick answer: Chlorine becomes less efficient at higher pH levels, but it’s still plenty effective inside the ideal pH range for pools, even more so if you’re using cyanuric acid to stabilize the chlorine.

What is the Ideal pH Level for Chlorine?

Chlorine is more effective at lower pH levels.

As you move up in pH, it diminishes the sanitizing power of free chlorine in the water, making it weaker the further up in pH you go.

But acidic water doesn’t make for a very pleasant swimming experience, which is a big part of why the ideal maintenance pH level for a pool is between 7.2 to 7.8.

Staying within this range keeps the water from harming us fragile humans, while also being optimal enough for the chlorine to work without having to add an unreasonable amount of sanitizer to your water.

Note: This range is also optimal for maintaining the right balance of active and reserve chlorine, which we’ll explain in the following sections.

How Does pH Affect Chlorine Effectiveness?

The simple answer is that pH helps convert your chlorine into an active sanitizing agent once it enters the water.

When pH is low, this process is very efficient, and most of your chlorine is converted into active chlorine. As pH gets higher, however, the process slows down, and more of your chlorine is put into reserve.

Here’s the more technical answer:

When you add chlorine to your pool, it immediately breaks down into two chemical compounds that make up ‘free chlorine’:

  1. Hypochlorous acid (the active part)
  2. Hypochlorite ion (the reserve part)

The ratio of active to reserve chlorine you end up with depends on the pH level of the water. The reason for this is, for reserve chlorine to become active, it needs to combine with hydrogen ions.

Since low pH represents a high number of hydrogen ions in the water, and high pH represents a low number of hydrogen ions, you can start to understand the relationship between pH and chlorine.

To put this in perspective:

  • At a pH level of 7, chlorine is roughly 75% active.
  • At a pH level of 7.5, chlorine is roughly 50% active.
  • At a pH level of 8 chlorine is roughly 25% active.

Note: A good ratio of active and reserve chlorine is important to avoid chlorine being used up very quickly, leaving your water vulnerable.

What Happens to Chlorine in Low pH Water?

Low pH water has a high number of hydrogen ions.

This means more of your chlorine will be converted into active chlorine, and less into reserve chlorine.

As a result, most of your chlorine is used up very fast as it seeks to neutralize contaminants in the water as quickly as possible.

This is great at first, but with little chlorine in reserve, you’ll soon find yourself without any remaining free chlorine in the water, and nothing to protect you from the endless onslaught of contaminants.

What Happens to Chlorine in High pH Water?

High pH water has a low number of hydrogen ions.

This means less of your chlorine will be converted into active chorine, and more into reserve chlorine.

As a result, very little of your chlorine will be actively sanitizing the water at any given time, which leaves you vulnerable.

While you’ll have plenty in reserve, there won’t be quite enough active chlorine to adequately deal with contaminants in the water without shoveling in an unhealthy amount of additional chlorine.

How Does Cyanuric Acid Affect Chlorine?

Cyanuric acid is also called “chlorine stabilizer”.

Chlorine is quickly burned off by sunlight, so many pool owners combine their chlorine with cyanuric acid to protect it from the sun’s UV rays. You can almost think of it as sunscreen for your chlorine.

Stabilized chlorine isn’t active or reserve; it’s its own category.

When cyanuric acid is present in the water, it ties up most of your chlorine before it has a chance to convert into either active or reserve chlorine. This chemical process is effectively blocked from taking place.

The amount of cyanuric acid dictates how much of your chlorine is allowed to convert, almost like a drip-feed system. Too much stabilizer and you prevent any chlorine from being converted, which is called “chlorine lock”.

In short, if you’re using cyanuric acid in your pool, the chlorine is much less susceptible to changes in pH because the chemical reaction it’s responsible for is greatly diminished.

Does Adding Chlorine Raise or Lower pH?

Depending on the type of chlorine you’re using, it will either raise or lower the pH in your pool.

Chlorine tablets are typically trichlor with a low pH level of around 3.

Being acidic, adding chlorine tablets to your water will lower the pH of your water, though this effect may be insignificant if your pH level tends to increase over time – which is common in swimming pools.

Powder or granular chlorine can be more varied. You can find it as trichlor, dichlor, or calcium hypochlorite.

While dichlor is slightly acidic, it’s reasonably close to neutral with a pH level of 6.5, which means it’s going to have a much lesser effect on your pH compared to trichlor.

Calcium hypochlorite, however, is highly alkaline with a pH of around 12, so this is going to raise the pH level of the water (as well as increase the calcium hardness level, for that matter).

Finally, liquid chlorine is sodium hypochlorite with a pH level of around 13, so it’s also going to increase the pH of your water. However, liquid chlorine only temporarily raises pH, effectively making it pH neutral in the long term.

Bottom Line: It’s Nothing to Worry About

The pH level of your pool water does have a significant impact on chlorine’s ability to effectively sanitize and clean your water.

With that being said, in reality, this is never going to be an issue when keeping your water in the ideal pH range (7.2 to 7.8), especially if you’re using a chlorine stabilizer in combination.

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