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pH vs Alkalinity in Pools: What’s the Difference?

pH and alkalinity are said to be the most important factors when it comes to pool chemistry, but what are they, and how exactly are they different?

In this article, we’re going to break down what pH and alkalinity mean in the context of pool care, what their differences are, and how they work in harmony to keep your water balanced.

Quick answer: pH tells you how acidic or basic the water is, while alkalinity tells you how likely the pH is to change. Low alkalinity leads to wild fluctuations in pH, high alkalinity keeps it stable.

What is pH?

pH is a measure of how acidic or basic your water is.

The pH scale goes from 1 to 14, with 7 being neutral. Anything under 7 is increasingly acidic, while anything over 7 is increasingly basic.

To give some real-world examples:

  • Lemon juice has a pH level of 2 (very acidic)
  • Urine has a pH level of 6 (acidic)
  • Water has a pH level of around 7 (neutral)
  • Seawater has a pH level of 8 (basic)
  • Bleach has a pH level of 13 (very basic)

You can measure pH in a pool using test strips or a liquid testing kit.

Note: Either of these testing methods will also allow you to measure other chemical markers important for maintaining a pool, such as total alkalinity, calcium hardness, and free chlorine.

Why is pH Important in Pools?

pH is important for a number of reasons.

The most obvious is that extremely low or high pH levels can be harsh on the human body (such as skin, eyes, and hair), making the water very unpleasant for swimmers.

It’s also important for maintaining your pool surfaces and equipment. Highly acidic water is corrosive (especially to metal components), while highly alkaline water can deposit calcium scale or cause metal staining.

Finally, pH has a direct impact on how effective sanitizer is. For instance, adding chlorine to a pool with a high pH will put most of your chlorine in a weakened state, leaving contaminants to roam free.

You should know: The ideal range for pH in swimming pools is somewhere between 7.2 to 7.8.

What is Alkalinity?

Alkalinity, or total alkalinity, tells you how resistant the water is to a reduction in pH, and it’s measured in parts per million (ppm).

This is essentially the concentration of alkaline substances in the water, which are mostly carbonates and bicarbonates. The more you have, the more insulated the pH level is against negative changes in the water.

As with pH, you can measure total alkalinity in a pool using most testing strips or liquid testing kits.

You should know: The ideal range for total alkalinity swimming pools is somewhere between 80ppm and 120ppm.

Why is Alkalinity Important in Pools?

Alkalinity is important because it stabilizes your pH level.

If you have low total alkalinity, there’s very little alkaline substance in the water to absorb changes, resulting in large swings in pH caused by the smallest of changes in water conditions.

Conversely, if your total alkalinity is too high, it becomes very difficult to lower the pH level because the high concentration of alkaline substances absorbs more acid in the water.

The Relationship Between pH and Alkalinity

pH and total alkalinity are very closely related.

In fact, any changes to pH or alkalinity will always influence the other in the same direction, even if the knock-on effect can sometimes be slow.

To explain why this happens, we need to get a little technical:

How Does pH Influence Alkalinity?

All pool water contains some amount of dissolved carbon dioxide (called ‘carbonic acid’).

As your pH rises, it facilitates a chemical process that essentially converts carbonic acid into bicarbonate, which increases the total alkalinity since bicarbonate is an alkaline substance.

As your pH falls, however, the reverse begins to happen. Bicarbonate is converted back into carbonic acid, which reduces total alkalinity since there is now less bicarbonate in the water.

How Does Alkalinity Influence pH?

If you have low total alkalinity, there’s very little alkaline substance in the water to absorb changes, so you will see large swings in pH caused by the smallest of changes in water conditions.

On the other hand, if your total alkalinity is too high, it becomes very difficult to lower the pH level because the high concentration of alkaline substances absorbs more acid in the water.

But total alkalinity is also a factor for rising pH in itself.

When your total alkalinity is high, it causes dissolved carbon dioxide (carbonic acid) to outgas at a faster rate. Since carbonic acid is acidic, removing more of it from the water causes pH to rise.

Summary: pH vs Alkalinity

pH and total alkalinity are closely related, but they’re not the same.

The idea is to use pH to keep your pool water slightly basic, and alkalinity to keep your pH level from straying too far from that reading.

Overall, maintaining both of these levels keeps the rest of your pool chemistry in check, ensures the water isn’t harsh on swimmers, and protects pool surfaces and equipment from corrosion and scale.

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