What’s the Difference Between pH and Alkalinity in a Pool or Hot Tub?

Hot tub water chemicals can be confusing and I’ve often wondered what’s the difference between pH and alkalinity in a pool or hot tub?

I decided to put the question to rest and find out. Here’s what I learned:

pH is a measure of acidity in water, whereas alkalinity measures the ability of the water to neutralize acid. So while they interact with each other, they are also very different. Too low a pH and it can burn eyes and corrode metal parts. Too high though, and it severely limits the effectiveness of chlorine or bromine.

But there’s more to know about hot tub or pool pH and alkalinity. So we’ll get into the differences and similarities. But we’ll also look at what to do when you need to adjust one up or down but not the other.

So let’s keep reading!

There’s nothing about a pool or hot tub as important as getting the water chemistry right. For those of us with sensitive skin, that’s even more important.

If you struggle to get the water balanced correctly or if you or anyone in your family struggles with the dreaded hot tub or pool rash, definitely take a moment to check out a recent article I wrote where I break everything down in plain talk.

I was most surprised by what the #1 thing you can add to a hot tub that can cause the most damage (and it’s probably in your house right now).

Are pH and alkalinity the same thing?

No is the short answer.

pH and alkalinity, while similar, are altogether different. Here’s the breakdown of the 2:

  • Alkalinity is a measure of the ability of the water to neutralize acidity
  • pH is a measure of the acidity

So because they are different, but related, it’s possible to have a high pH and low acidity, or vice versa.

Where the confusion really comes in is with the chemicals we use to balance our pools and hot tubs. After all, most (but not all) say pH and Alkalinity up or down, implying that one chemical will raise or lower both.

From a technical standpoint, pH measures any water-based solution on a scale for how acidic it is. Totally neutral water, at room temperature, has  pH of 7.0. Despite what you might think, highly acidic water has a low pH, whereas highly alkaline water has a high pH.

As I mentioned above, alkalinity is the capacity of water to fight against pH levels that might make the water too acidic. More specifically, it’s a measurement of dissolved alkaline substances which would drive the pH above neutral (7.0).

When we get the alkalinity right, it makes the pH far less likely to fluctuate or bounce around too high or too low. If you’ve ever struggled to get the pH just right, the actual problem is probably your alkalinity.

Ultimately getting this right is important, as while chlorine can cause burning in the eyes, too low of a pH will cause that also.

What is more important pH or alkalinity?

Ultimately, pH is more important than alkalinity, at least by a small margin.

After all, as I mentioned, if the pH is too low, that can cause your eyes to feel that burning sensation. But too high a pH can lead to scaling and calcium deposits in the filter system and equipment; also not good!

Too low of a pH, not only can it burn your eyes, but the water will get so acidic that it can corrode metal parts in particular in the equipment.

But, ironically, if your alkalinity is off, that can make the pH hard to get to the right level. So we need to figure out how to dial in both.

The other way pH affects your pool or hot tub is by how it interacts with the chlorine or bromine.

Too high of a pH will result in the chlorine or bromine being far less effective at killing bacteria and viruses, ultimately defeating the purpose of using it. a pH of 8.0, for instance, reduces the effectiveness of the chlorine/bromine down to 20%.

The ideal pH for a pool or hot tub will be a reading between 7.2 and 7.6.

Is pH increaser the same as alkalinity increaser?

No, while very similar from a chemical composition standpoint, pH increaser and alkalinity increaser are 2 different things.

Alkalinity increaser is sodium bicarbonate (yes, normal baking soda like you have in your pantry).

And to save money, you can easily buy regular baking soda for when you need to increase the alkalinity to the recommended levels of  80-120 ppm.

pH increaser, by comparison, is sodium carbonate, sometimes called soda ash.  But this usually will increase both pH and total alkalinity.

There are, however, products which just raise pH and not the alkalinity, and sometimes you’ll find yourself needing that. These products are made from magnesium oxide but can be a little hard to find at your local pool supply store.

But there’s a great one on Amazon I mention in the next section.

How do I raise the pH in my pool but not alkalinity?

As I mentioned, most products designed to raise pH are made from sodium carbonate. Unfortunately, sodium carbonate also tends to raise total alkalinity quite a bit.

The only product I know of that raises total pH without significantly impacting total alkalinity is magnesium oxide powder. Now to be fair, it will raise alkalinity some, but it doesn’t raise it to the same degree as sodium carbonate.

Magnesium oxide powder is concentrated compared to sodium carbonate. So if you buy it, just know you won’t need to use as much.

As with much of pool and hot tub water chemistry, it’s not always an exact science. But start with using magnesium oxide by reducing the amount you would normally use by about 2/3.

While there are some brands of magnesium oxide powder you can find labeled for pools and hot tubs, since the only ingredient is magnesium oxide, there’s no need to overpay.

So if you find yourself needing to raise pH without significantly impacting total alkalinity, check out the Bulk Supplements brand of Magnesium Oxide powder on Amazon Prime.

Excellent reviews, a great price, and given how little you need, 1 bag will last you a while. Over 2 lbs for just around 20 bucks, and, of course, free Prime shipping.

Check it out on Amazon.

Did I cover everything you wanted to know about the differences between pH and alkalinity in your hot tub water?

In this article, we took an in-depth look at the world of pool and hot tub chemicals; pH and alkalinity in particular.

pH and alkalinity, while related, are not the same thing. But for hot tub owners, it’s a confusing thing. After all, there aren’t products that raise or lower 1 without affecting the other.

So specifically, we took a look at exactly how the two are related and how they differ, answering the question of what’s the difference between ph and alkalinity in a pool or hot tub?

That way you can treat your hot tub water knowing exactly what you’re doing and what products you need.

What’s your biggest challenge with treating your water?

If you struggle with wondering how often you should be changing the water in your hot tub, it’s not quite as simple as you might think, so take a moment and check out my recent article on that.

I go in-depth on water quality, how to tell when it’s time to change it, AND more importantly, the 1 super-easy way to change it that will streamline the whole process!

Just click the link to read it on my site!

Jeff Campbell

Jeff Campbell is a husband, father, martial artist, budget-master, Disney-addict, musician, hot tub lover, and recovering foodie having spent over 2 decades as a leader for Whole Foods Market.

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