Pools that inject carbon dioxide (CO2) instead of acid for reducing pH tend to see a rise in total alkalinity (TA). As a result, many pool operators think the CO2 system is the cause of the rising alkalinity. But that's not what's really going on. Alkalinity rises because of the chlorine being used.
Pop quiz: what is the pH of dry cal hypo chlorine? Or calcium chloride? Or Trichlor? Or sodium bicarbonate?
Answer: They don't have a pH (yet).
You read that correctly. Dry chemicals do not have a pH...until they dissolve in water. You see, pH is the concentration of Hydrogen ions (H+). The more Hydrogen, the lower the pH, and the more acidic the substance. This is all relative to the water itself, thanks to the process of dissolving substances, called hydrolysis.
Source: Orenda Technologies. Used with Permission.
When acid is added to the pool, it introduces Hydrogen ions. These hydrogen ions neutralize against dissolved alkali (high pH, basic substances) in the water. The sum of all titratable bases is called Total Alkalinity.1
All titratable bases in the water that can neutralize hydrogen are part of total alkalinity. The primary buffering system is called carbonate alkalinity, which is made up of both bicarbonate (HCO3-) and carbonate (CO32-) ions. Cyanurate alkalinity (from CYA) and boric alkalinity (from borate) are also buffers, if used.
For example: an indoor pool that does not use borate or cyanuric acid will not have these buffers in the water. All TA in this water would be carbonate alkalinity.
Carbonate alkalinity in particular is important to this topic. Both bicarbonate and carbonate ions contain carbon dioxide, albeit in a different form. When acid neutralizes against these substances in water, they form carbonic acid (H2CO3), which lowers the pH. Carbonic acid is in equilibrium with dissolved carbon dioxide. In fact, a very very small percentage of CO2 in the water stays in the form of carbonic acid. The equilibrium looks like this:
CO2(g)+ H2O(l) ⥃ H2CO3(aq) ⇌ HCO3-(aq)+ H+(aq) ⇌ CO32-(aq) + 2H+(aq)
Carbon dioxide + water ⥃ carbonic acid ⇌ bicarbonate + hydrogen ⇌ carbonate + 2 hydrogen
Notice the first set of arrows have a stronger arrow going left (⥃), indicating more CO2 and water will be present compared to dissolved carbonic acid. The rest of the equilibria are equivalent (⇌) and all of these are based on pH. As the pH rises, move to the right. As the pH is lowered, move to the left through the equation above.
You can see this more clearly in this diagram:
Source: Orenda Technologies. Used with Permission.
If you increase the amount of dissolved carbon dioxide, the pH decreases. Conversely, reducing (or more accurately, off-gassing) carbon dioxide raises the pH:
↑CO2 = ↓pH
↓CO2 = ↑pH
Source: Watershape University Service 2211: Essential Water Chemistry. Used with Permission.
This is why commercial pools (and some high-end residential pools) inject CO2 to manage pH instead of acid. CO2 is generally safer and more affordable at scale. On a smaller scale, CO2 systems tend to be more expensive, which is a big reason why they are rarely used on small residential pools. But for a large lap pool or waterpark with lots of turbulence and splashing? CO2 is a great way to suppress pH.
This is the major distinction between using CO2 vs. using acid.
Acid lowers pH by neutralizing bicarbonate alkalinity, converting it into carbonic acid, and re-carbonating the water, which lowers the pH.
Here is a Rule Your Pool® Podcast episode explaining this in more detail:
In commercial pools, the most commonly-used chlorine products are hypochlorites:
These chlorine products are made with a strong base, which intentionally raises the pH of the product (to ~11 for cal hypo, and ~13 for liquid chlorine). This high pH limits the chlorine from prematurely decomposing in storage, thereby extending its shelf life.2
These chlorines are literally made by chlorinating a strong base.Liquid chlorine is made by blending the ratio of 1 mole of chlorine gas (Cl2) per 2 moles of sodium hydroxide (NaOH).3 The equation looks like this:
Cl2(g) + 2NaOH(aq) → NaOCl(aq) + NaCl(aq) + H2O(l)
Chlorine (gas) + Dissolved Sodium Hydroxide → Sodium Hypochlorite + Salt + Water
Later in the process, hydrochloric acid (HCl, aka muriatic acid) is also a byproduct of this chlor-alkali process. But that's another article for another day.
Cal hypo can be made in a similar way, though it produces a dry chlorine product instead of a liquid. There are generally two methods of manufacturing cal hypo chlorine.4
1. Chlorine gas is passed through a slurry of calcium hydroxide:
2Cl2(g) + 2Ca(OH)2(aq) → Ca(OCl)2(s) + CaCl2(aq) + 2H2O(l)
Chlorine (gas) + Dissolved Calcium Hydroxide → Calcium Hypochlorite + Calcium Chloride + Water
Notice that liquid chlorine makes salt as a byproduct (sodium chloride, NaCl), and cal hypo also makes a calcium salt (calcium chloride) as a byproduct.
The second method introduces sodium hydroxide to the mix so that instead of calcium chloride as a byproduct, it also creates sodium chloride, which is an easier byproduct to manage (and sell):
2. Chlorine gas is passed through a slurry of both calcium hydroxide and sodium hydroxide:
2Cl2(g) + Ca(OH)2(aq) + 2NaOH(aq) → Ca(OCl)2(s) + 2NaCl(aq) + 2H2O(l)
Chlorine (gas) + Dissolved Calcium Hydroxide + Dissolved Sodium Hydroxide → Calcium Hypochlorite + Salt + Water
For our purposes in this article, notes that both of these chlorines are made with slightly more hydroxide than needed. As mentioned before, this is to increase the pH of the product and slow the decomposition rate of the product in storage, extending its shelf life. Keep these hydroxides in mind as we move deeper into this topic.
When adding hypochlorites to the water, they cause a temporary pH rise in the water. This is only temporary, because after it either kills, oxidizes, OR decomposes due to sunlight exposure (UV degradation via photolysis), chlorine releases hydrochloric acid to bring the pH back down.
Source: Bob Lowry, Pool Chemistry Training Institute. Used with Permission.
Both cal hypo and liquid chlorine are almost net-neutral pH, after they are used up.
The "almost" in that previous sentence is the reason for this article. The HCl acid created by chlorine doing it's thing (or sunlight destroying it) almost neutralizes the chlorine's pH entirely. But those excess hydroxides slightly outnumber the acid.
Most pool operators never notice this slight accumulation of hydroxides thanks to regular use of acid to maintain a desired pH. But what if your pool isn't using acid regularly, and instead, using a CO2 injection system?
Normally, a pool operator needs to add sodium bicarbonate (baking soda) to increase alkalinity in the water. But as discussed in the previous section, hypochlorite chlorines leave behind excess hydroxides (OH-) that accumulate over time. These hydroxides are alkaline, and are powerful acid-neutralizers. They interact directly with dissolved CO2 in the water to convert it into bicarbonate ions. This process increases carbonate alkalinity, which subsequently increases total alkalinity (TA).
OH- + CO2(aq) ⇌ HCO3-
Hydroxide ion + dissolved Carbon Dioxide ⇌ Bicarbonate ion
And if the pH climbs high enough, even if it's just locally (where the chlorine was added, for instance), these hydroxides can also convert bicarbonate ions into carbonate ions:
OH- + HCO3- ⇌ CO32- + H2O
Hydroxide ion + bicarbonate ion ⇌ carbonate ion + water
These reactions create new bicarbonate and carbonate alkalinity in the water, which directly increase TA. The carbon dioxide itself does not increase TA directly, because it is not alkaline, and has no ability to neutralize acids.
Hypochlorite chlorines (liquid chlorine and cal hypo) are manufactured with a slight excess of hydroxide to elevate the pH of the product and extend its shelf life. These excess hydroxides left behind by chlorine are the cause of rising alkalinity in CO2 fed pools.
Most pool operators try to maintain a desired pH using acid. Acid neutralizes the excess hydroxides in the hypochlorite chlorines, so there is no visible rise in alkalinity.
Pool operators managing pH using carbon dioxide injection, on the other hand, are not neutralizing the hydroxides. So they accumulate in the water, converting dissolved CO2 back into bicarbonate ions (HCO3-). Bicarbonates and carbonates directly contribute to alkalinity, and that's where the rise in TA comes from.
To combat the rising TA, operators of pools using CO2 feeders should manually add dissolved muriatic acid on occasion to address the TA. This may need to be done once a week, but more likely, every few weeks when operating smoothly.
1 "Titratable" means a titration test can be used to quantify the concentration of a dissolved substance. In swimming pools, liquid reagent test kits use titration to measure both calcium hardness and total alkalinity. The total alkalinity test titrates using sulfuric acid drops, gradually neutralizing alkali in the water until a desired color change is achieved. Each drop represents 10 ppm (mg/L) of alkalinity. That means if it takes eight (8) drops to achieve the desired color change, the water has approximately 80 ppm of total alkalinity (8 drops x 10 ppm = 80 ppm).
2 Su, Y.S., Morrison, D., Ogle, R. (2009). Chemical Kinetics of Calcium Hypochlorite Decomposition in Aqueous Solutions. Journal of Chemical Health and Safety. 16(3), 21-25. https://doi.org/10.1016/j.jchas.2008.07.002
3 Powell Solutions (2026). Sodium Hypochlorite - FAQ. Powell Industrial Chemical Resources. https://powellsolutions.com/resources/faq/sodium-hypochlorite-faq/
4 Turito Tutoring (2022). Calcium Hypochlorite - Structure, Properties and its Uses. Turito.com Tutoring and Test Prep. https://www.turito.com/blog/chemistry/calcium-hypochlorite