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 it different from pH? 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.
1. 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 (but don't worry, you don't need to memorize this):
Alk = [HCO3−] + 2[CO32-] + [OH-] – [H+]
The pH of the water 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.
2. What does alkalinity do in water 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 corrosion index).
We need these dissolved alkaline products 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 70ppm TA, provided you are compensating for it with warmer water, higher pH or higher calcium levels.
Cyanuric Acid correction factor
For the saturation index (mineral saturation of water, determining how corrosive or scale forming the water is), we have to consider cyanuric acid (chlorine stabilizer). The LSI calls for a correction of TA by factoring in how much cyanuric acid is in the water. The higher the CYA, the more severe the impact. Using the CYA correction factor allows us--for LSI purposes--to adjust TA into corrected alkalinity, or carbonate alkalinity. But you may be thinking..."wait, carbonate ions don't show up until the pH is over 8.3"... and you are correct. That said, CYA corrections are still necessary for the saturation index.
If you do not have any CYA in your water, like an indoor pool, TA is the factor you use. But if you do have CYA, you need to adjust the alkalinity before plugging it into the LSI equation. 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. You really 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 if you want to be exact, the Orenda app's LSI calculator will automatically make the 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
3. 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 a 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 the concentration of it.
The higher the alkalinity, the harder it is for pH to change, due to the water's ability to neutralize acid.
For more information
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