What is Alkalinity in a Pool?
When we get a support question, we usually start with “where’s your pool’s Alkalinity level?” That’s because this often misunderstood pool parameter is the most important factor in your water chemistry.
What does pool Alkalinity measure?
Alkalinity is the measurement of pH buffers – specifically carbonates – in your water. pH buffers help the water resist changes in the acidity level. So while alkalinity and ph measure different things, they are very much connected.
How does Alkalinity resist changes in pH?
pH is a measurement of hydrogen activity in water. When you add a strong acid to water (like Muriatic Acid) it donates practically all its hydrogen ions into the water. pH buffers are weak acids that trap some of the hydrogen ions in water and reduce some of the activity.
In the case of carbonates, they absorb some of the hydrogen to become carbonic acid. Carbonic acid then breaks down into CO2 and is gassed off. The result is the pH stays the same (or increases slightly) and Alkalinity slowly drops overtime.
A Pool Naturally Trends to High pH and Low Alkalinity
A swimming pool’s chemistry is never static. Carbonates are continuously raising the pH and gassing off to some degree. This means in theory a pool’s pH will naturally increase as Alkalinity decreases.
This isn’t a perfect one-to-one ratio though. The pH scale is logarithmic. So every movement of 0.1 in pH levels is really ten times the difference.
What makes Alkalinity so important in swimming pools?
It’s always recommended you lower Total Alkalinity before attempting to lower your pool’s pH. That’s because whatever acid you add, Alkalinity will counteract some of it.
But alkalinity is important for another reason: stains and scale.
Alkalinity and Solubility
As we mentioned alkalinity measures carbonates. Carbonates play a major role in stains and scale. Metals (such as calcium, iron and copper) become insoluble when they react with carbonates. That means they fall out of the water and attach themselves to the surface of the pool, forming stains and scale.
So as you may guess, high alkalinity usually leads to stains and scale.
On the other hand, if you maintain zero alkalinity in the water, the pH will fluctuate greatly anytime acid is introduced. And for plaster pools, if the water becomes acidic it may try to dissolve some carbonates from the surface.
To balance Alkalinity levels you need to find a sweet spot. Keeping it low enough to prevent stains and scale, yet high enough to keep the pH stable. This dynamic is at the center of proper pool water chemistry.
How to Find the Right Swimming Pool Total Alkalinity
To determine the right alkalinity level, we will be referring to the Hamilton Index™. The Hamilton Index™ was developed specifically for pools. It was designed to replace the Langelier Saturation Index with more emphasis on Alkalinity.
Start by testing your hardness and find the value on the left hand of the chart. Moving to the right we see the maximum recommended level for Total Alkalinity.
- If you’re alkalinity is higher than the recommended value, then you’ll need to lower it. You can do this by adding an acid (sodium bisulfate will lower the Alkalinity while affecting pH much less).
- If your Alkalinity is far below this number (20% or more), then you’ll need an Alkalinity Increaser that adds carbonates to the water. This can be done by Baking Soda (sodium bicarbonate) or Soda Ash (sodium carbonate).
You can also fine tune Alkalinity based on how your pool “behaves”. If you notice some scaling or light staining, you can reduce your Total Alkalinity by 10ppm. If stains and scale aren’t a problem, but your pH dropping is, you can raise your Alkalinity by 10ppm. Over time you’ll be able to dial in the Alkalinity until it is perfect.
Everything in a Swimming Pool Depends on Alkalinity
To review, Alkalinity is the most important part of balancing because:
- Alkalinity controls how pH changes
- Alkalinity directly affects the likelihood of developing stains and scale
- It determines how your pool will overall behave