CO2 and pH: Understanding Henry's Law
This topic is fascinating to us. It is also an incredibly helpful lesson that explains why pH naturally rises. If you maintain pools on a weekly basis, you probably notice the pH is almost always higher when you come back after a week. That's not your fault. It's natural. This article explains why.
Covered in this article:
- Carbon Dioxide determines the pH of water
- Lowering pH means increasing CO2
- Raising pH means reducing CO2
- Henry's Law of the Solubility of Gases
- How to contain pH (without fighting it)
- LSI-first approach
- Measure and dose acid correctly
Carbon Dioxide determines the pH of water
First things first: technically speaking, the concentration of Hydrogen (H+) ions determines the pH. But in practice, there's something easier to conceptualize: the amount of carbon dioxide (CO2) in solution also determines the pH of the water. The most common source of acidity in water is dissolved CO2, so the more CO2 in the water, the lower the pH. This is because when CO2 comes aqueous in water, a small portion of it becomes carbonic acid (H2CO3). The reaction looks like this:
CO2 (aq) + H2O ⇌ H2CO3 (aq)
Carbon dioxide + Water ⇌ Carbonic Acid
Carbonic acid brings the water's pH down. And carbonic acid is formed when CO2 binds with water...so the question becomes how do we get more CO2 in the water to lower the pH? Well, let's think about how we lower pH in a swimming pool.
Chemically speaking, we can lower pH in a few ways by simply increasing the amount of carbonic acid in our water. First, we can inject CO2 directly into the water, which reduces pH but does not reduce alkalinity. Adding acid, on the other hand, lowers pH and alkalinity, because in order for it to create carbonic acid, acid has to convert bicarbonate alkalinity down into carbonic acid by adding Hydrogen to it.1 In the chart below, acid converts bicarbonate (HCO3) into carbonic acid (H2CO3).
When bicarbonate is converted into carbonic acid, total and carbonate alkalinity are reduced. CO2 injection, however, bypasses alkalinity and directly adds carbonic acid to the pool. See? As mentioned above, this still involves the concentration of Hydrogen ions, but it is how those Hydrogen ions fluctuate the percentage of carbonic acid vs. bicarbonate alkalinity.
The opposite is also true. The pH will rise as CO2 is reduced in water. One way to do this is to add high-pH additives like non-stabilized chlorine, a salt chlorine generator, or a pH adjuster like soda ash (sodium carbonate) or sodium bicarbonate.2 Another way to raise pH is to just let it rise naturally via off-gassing. There is natural aeration, and there is accelerated aeration, such as spa jets, spillways, vanishing edges (infinity pools), and other water features. As CO2 leaves the water–via aeration or algae consumption–pH will naturally rise. And the loss of CO2 is where Henry's Law of physics comes into play.
Related: Total Alkalinity vs. pH and their roles in water chemistry
Henry's Law of the Solubility of Gases
Henry's Law is a law of physics formulated by William Henry in 1803. It states:
"At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid." An equivalent way of stating the law is that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid." (emphasis added)3
As it pertains to swimming pools, we care most about CO2. Henry's law basically states that the amount of a gas dissolved in water will strive to be directly proportional to the amount of that same gas in the air above the water. In the case of CO2, the atmosphere above a swimming pool has a small percentage of CO2 in it, so CO2 leaves the pool over time trying to equalize with the air. This video explains it well. Feel free to watch the entire video, but as it pertains to this article, we really only care about the first two minutes before he gives the soda example:
Once equilibrium is reached, off-gassing stops
The inverse of Henry's Law equation is also true. Gases leaving water, once directly proportional to the air above the water, will stop off-gassing. This is because equilibrium has been reached. We will explain why this is significant in the next section (or you can just take a shortcut to that section here). Sure, aeration can temporarily force CO2 out of the water, but thanks to Henry's law, it should be pushed back into the water and redissolve. To cite our earlier source:
"Carbon dioxide enters the water through equilibrium with the atmosphere. CO2 (aq) << CO2 (g)"1
Because of Henry's Law, we know that pH can only go so high naturally, because atmospheric pressure will push CO2 back into the pool at a certain point. If the pH is going to rise above that natural limit, it has to be forced.
The pH ceiling
Thanks to Henry's Law, we know that CO2 will off-gas until it reaches equilibrium with the air above the pool. That point of equilibrium is basically a limit, or a ceiling. Since pH rises as CO2 leaves the water, we call this the pH ceiling of a swimming pool. And yes, pH can absolutely rise above this ceiling, but not naturally. Something must force the pH above the natural pH ceiling, such as etching a calcium-rich plaster surface, which leeches a high pH into the water, or someone adding soda ash to a pool incorrectly.
The pH ceiling in swimming pools is normally around 8.2, which explains why almost all pools naturally face a rising pH each and every week. Carbon dioxide is naturally off-gassing thanks to physics, so you're not doing anything wrong! The exact pH ceiling depends on your carbonate alkalinity (or corrected alkalinity) level4:
Since most carbonate alkalinity levels in well-maintained swimming pools are between 50-80, you can see the numbers in the chart are bold. The closer your pH gets to the ceiling, the slower the pH rises. Conversely, the further you lower pH below its ceiling, the faster it will begin to rise. So correcting down to 7.2 instead of 7.5 usually leads to a faster pH rebound.
How to contain pH (without fighting it)
Ask any pool professional how easy it is to maintain a 7.4-7.6 pH for a week. They will laugh. Anyone who has managed pools for a while knows that without chemical automation, it's virtually impossible to maintain such a pH range for a week, except in some rare circumstances like a vinyl liner or fiberglass pools using trichlor. But that's another story entirely.
Trying to control pH is futile. Instead, contain it. We have Henry's Law on our side, so let's use it to our advantage! We know there is a pH ceiling, and we also know that pH rise slows as it approaches its natural ceiling. Over the course of a week, most pools will not reach their ceiling, but they will be about 8.0 or 8.1 pH.
Related: Containing pH vs. Controlling pH
Set your LSI parameters like calcium and alkalinity to allow you to lower the pH to 7.5 or 7.6 and have the LSI value yellow . In warm water temperatures, you will find this is easiest with a decent amount of calcium hardness and slightly lower-than-textbook alkalinity of 60 or 70 ppm. Remember from above, the carbonate alkalinity determines the pH ceiling. See the screenshot of the Orenda App.
Then, be sure that your high pH will stay in the green on the LSI calculator. In the screenshot, we use 8.0, based on the fact that our carbonate alkalinity has our pH ceiling of about 8.2. If done right, the pH will be rising so slowly by the end of the week, that it should not reach the ceiling in seven days.
In summary, use the Orenda App and lower pH down into the yellow LSI (not red), and let it rise up to 8.0 or 8.1 while staying in the green (not purple).
Measure and dose acid correctly
For this pH containment strategy to work, measuring and dosing acid correctly cannot be overstated. It is critically important. Lower alkalinity reduces the buffering capacity of your water, so measuring acid is imperative to avoid overcorrections, and it also matters that you pour acid correctly with dilution.
Related: Six Bad Habits that are Costing you Time and Money
Because each dose of acid will lower alkalinity–albeit slightly, when using this strategy correctly–every third or fourth week you will need to re-up the alkalinity using sodium bicarbonate. Again, we don't necessarily need 80-120 ppm alkalinity in the summer if we have enough calcium hardness to maintain LSI balance. Yes, this might be contrarian to pool industry dogma, but it works.
The amount of carbon dioxide in water determines the pH of the water. The more CO2, the lower the pH, and vice versa. A pool's pH will naturally rise over time, thanks to CO2 leaving the water. This is natural, and you are doing nothing wrong. Henry's Law of solubility of gases explains why CO2 leaves until it is in equilibrium with the atmosphere. When that equilibrium is reached, the water is at what we call its "pH ceiling". This ceiling is determined by the carbonate alkalinity in the water. And this is just one of many misunderstandings about pH.
To be proactive with your pool chemistry, you can adopt a pH containment strategy, rather than chasing it and trying to control it. Use calcium hardness and Henry's Law to your advantage, and you will be able to predict pH rise accurately and consistently. Doing so can save you a lot on wasted pool chemicals.
Note: We apologize for the inconsistent citation formats below, as some of these sources were difficult to find authors and publication dates. Use the hyperlinks to find the original sources.
1 Utah State University research paper (2006). Carbon Dioxide and Carbonic Acid.
2 It should be noted that the pH rise from non-stabilized chlorine like sodium hypochlorite (liquid chlorine) or calcium hypochlorite (cal hypo) is temporary, because when HOCl oxidizes or sanitizes, a byproduct of HCl offsets the high pH. Source: Robert Lowry.
3 Chemistry Libretexts. Henry's Law.
4 Carbonate alkalinity is confusing, because the term means two things. Carbonate alkalinity by itself is CO3, but for LSI and pH ceiling purposes, "carbonate alkalinity" refers to "corrected alkalinity", which includes bicarbonate, since you could simply remove a Hydrogen and still have a carbonate. Corrected alkalinity is Total Alkalinity minus cyanurate alkalinity, and if used, borates must be factored in too, because they also contribute to total alkalinity.