Alkalinity is one of the primary things measured in water chemistry. But what is it? Total Alkalinity (TA) often gets confused with pH and words like alkali, alkaline, etc. I think this topic is one of the least understood in pool management. Many pool service pros know how to test for TA, but may not even know what it actually is, what it does, and why it's important.
What is alkalinity? What does it do for water chemistry? And how is alkalinity different from pH? And what is the ideal pH and alkalinity for a swimming pool? Either the subject hasn't been clearly explained, or... no wait, that's what it is. Hopefully my article will simplify the science and help you understand alkalinity better.
What is alkalinity?
Alkalinity is a measurement of dissolved alkaline substances in water (higher than 7.0 pH). It tells us the water's ability to neutralize acid. There are three primary types:
Pure water, at 7.0 pH has the perfect balance of hydrogen in it. Its molecules (H2O) dissociate into an equal amount of hydrogen and hydroxide ions. The chemistry looks like this:
H2O ⇌ H+ + OH−
Alkalinity can be calculated using this formula (it's okay, you don't need to memorize this):
Alk = [HCO3−] + 2[CO32-] + [OH-] – [H+]
pH always matters
The pH of the water determines which form of alkalinity is most prevalent in that moment. Here's a science-y chart that you may just give an academic nod to, and pretend it makes sense. At least, that's what I did. But don't worry my friend, I'll explain it.
So let's go left to right on the graph. Carbonic acid is dissolved carbon dioxide (CO2). 4.3 pH is the point where alkalinity begins to exist in water. Anything below 4.3 pH, there is zero alkalinity in the water; just dissolved CO2. Essentially, at 4.3 pH, carbonic acid begins to transition into bicarbonate ions, indicated by the red line in the graph. Pool chemistry should range between 7.2 and 7.8 pH, so it's almost all bicarbonate ions.
Now wait--the plot thickens--a new ion arrives. At 8.3 pH, carbonate ions show up, and the transition occurs from bicarbonate to carbonate ions. Dissolved hydroxides are not shown on the graph, unfortunately. They show up around 12 pH. The most common hydroxide in swimming pools is calcium hydroxide, which bleeds out of curing plaster, around a pH of 12.6. According to the National Plasterers Council, calcium hydroxide is the #1 reason pH and alkalinity rise during a pool startup.
So as you can see, pH controls the species (and percentage of each species) of alkalinity present in water.
What does alkalinity do in pool chemistry?
Alkalinity buffers (stabilizes) the pH of water by neutralizing acids. It makes it more difficult for pH to fluctuate or bounce, and is also a positive contributing factor to the Langelier Saturation Index (aka the LSI, or corrosion index).
Technically, alkalinity buffers pH by either providing or absorbing a Hydrogen ion (H+) as needed. So when acid is added, carbonate ions can absorb Hydrogen to create bicarbonate ions. This is an equilibrium, just like pH, so it can go the other way too. The equilibrium looks like this:
HCO3- + H+ ↔ H2CO3
We need these dissolved alkalinity in our water to keep the saturation index balanced and our pH stabilized. The ideal TA level depends on the type of sanitizer used, but generally ranges between 80 - 120ppm. What really matters, in our opinion, is the overall LSI balance of the water. It is feasible to have balanced water with 60 ppm TA, provided you are compensating for it with warmer water, higher pH or higher calcium levels.
In pool chemistry, sanitizers, salt systems, and of course pH adjusters (like acid and sodium bicarb) impact pH. But alkalinity needs to be raised with sodium bicarb or soda ash, and lowered with acid.
Several commercial pool operators tell us that injecting CO2 into a pool can also raise alkalinity–perhaps because it adds enough carbonic acid that eventually converts to bicarbonate–but we have not done enough research to know why that is. One thing is for sure: acid lowers both pH and alkalinity. In residential pools, misuse of acid leads to so many problems that its our number 1 bad habit to avoid. On commercial pools, acid is fed using automated (or timed) feed pumps. If the pool feeds acid without pre-diluting it, however, they are likely to struggle to maintain decent alkalinity levels.
For the LSI to be properly calculated, we have to find the carbonate alkalinity by removing the cyanurate alkalinity. To do this, we have to measure cyanuric acid (CYA), and know the pH. The higher the CYA, the more severe the impact to the LSI.
According to the IPSSA Basic Training manual, you can correct alkalinity by taking 1/3 of the CYA level and subtracting it from the TA level. To be exact, you will need to know the pH of the water, but since pools are generally between 7.2 and 7.8 pH, you can get close with using 1/3. But don't worry, the Orenda app's LSI calculator will automatically make the exact correction for you, and be precise. Here's what the equation looks like:
Corrected [carbonate] Alkalinity (CA) = TA ppm - (CYA ppm x [correction factor @ pH])
CA = 100 ppm - (90 ppm x [correction factor @ 7.6 pH])
CA = 100 ppm - (90 ppm x [0.33])
CA = 100 ppm - (30 ppm)
CA = 70 ppm
What is the difference between pH and alkalinity?
We have an article that explains this more in depth. In short, pH is the power of Hydrogen, or the potential of Hydrogen. It is a logarithmic scale from 0-14, where 7.0 is neutral. It measures the concentration of hydrogen to determine how acidic (below 7.0) or alkaline (above 7.0) a substance is.
We measure alkalinity in parts-per-million, and pH on its own scale (like LSI). In other words, pH does not tell us how much acid or alkali is dissolved in water, it just tells us where on the equilibrium it the overall pH is. The higher the alkalinity, the harder it is for pH to change, due to the water's ability to neutralize acid.
Related: How to Lower pH and Alkalinity
If you have a high pH in pool water, or are wondering why your pH keeps climbing, there are several possible reasons for it. Things can lower the pH too, of course. Whereas alkalinity is about dissolved alkali in the water that can either take or give a hydrogen, pH is about the concentration of that hydrogen. Believe it or not, it's mostly about the behavior of carbon dioxide (CO2).
To learn more in depth, I encourage you to do research beyond this article. My primary sources were:
- IPSSA Basic Training Manual, 2016 Revised Edition. By Robert W. Lowry
- The Importance of General Chemistry Relationships in Water Treatment. By Mealy & Bowman, Wisconsin DNR
- Study & Interpretation of the Chemical Characteristics of Natural Water. US Geological Survey
- Water Dissociation and pH. By Martin Chaplin
- Alkalinity and pH Relationships. By James McDonald, PE, CWT
- Startup Certification Course, National Plasterers Council. By Greg Garrett