How Does CO2 Affect pH In Water?


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The amount of carbon dioxide (CO2) in a solution is one of the many factors that determines the pH of water. As pH levels fluctuate during the day due to photosynthesis, respiration, and decomposition; the extremity of these changes depends on the alkalinity of the water which is influenced by CO2 levels. When CO2 levels increase in water, the pH level drops, which makes the water become more acidic. 

How Does CO2 Affect pH In Water?

Carbon dioxide is influential in regulating pH in water. Measuring the amount of CO2 in a solution is one way to determine the pH. For example, the more CO2 in water (H2O), the lower the pH will be. When CO2 is dissolved in water, a part of it reacts with water to become carbonic acid (H2CO3). It is the hydrogen ions present in carbonic acid that make water acidic, lowering the pH. 

We can look at how carbonic acid is formed by the following equation:

CO2 (aq) + H2O → H2CO3 (aq)

Carbonic acid can also break up or “dissociate” into hydrogen ions (H+) and carbonate ions (CO3–2) which can be seen in the following equation:

H2CO→ 2H+ + CO3–2

When this happens, two hydrogen ions are formed for every single carbonate ion. 

In water, it is the relationship between carbon dioxide and pH that influences pH levels. As mentioned, when carbon dioxide is dissolved in water, the release of hydrogen ions in carbonic acid is what lowers the pH. As CO2 levels increase around Earth, the amount of dissolved CO2 also increases, which increases the amount of carbonic acid, therefore decreasing the pH. 

Acid Rrain

An example of this can be seen in the pH of rainfall influenced by carbon dioxide. As raindrops fall through the air, CO2 molecules interact creating carbonic acid in the raindrops, which lowers the pH value. Even though a pH value of 5.65 is acidic, it is not considered acid rain. Normal and clean rain has a pH value between 5.0 and 5.5, while acid rain typically has a value of 4.0. 

Acid rain affects soils, which kills trees and other vegetation as it leaches aluminum from the soil and removes essential nutrients and minerals, which can also be harmful to animals. 

Increased anthropogenic activity can also cause pH levels to fluctuate due to extremely high levels of CO2 being released into the atmosphere. This further decreases the pH in raindrops, contributing to acid rain. 

Stratification in water usually comes from temperature differences within the same body of water that create layers, these are known as thermoclines or chemoclines.  Thermoclines are temperature divides, and chemoclines are chemistry gradients that are affected by oxygen, salinity, or CO2.  

When CO2 influences the pH of water, chemocline stratification can create different pH levels within the same body of water. The differences in pH come from increased CO2 levels from respiration and decomposition of organic material. 

Carbon dioxide also affects the pH in human blood, which contains 90% of water. As we know, when CO2 combines with water, it forms carbonic acid. As CO2 enters the bloodstream, it lowers the pH in the blood, making it more acidic.

Importance of pH in Water

pH is an important parameter in water stability, therefore regular testing using a pH kit and chemically altering pH levels is important in many industries such as swimming pools and spas, aquariums, and the food and beverage industry. 

Lowering pH In Water with CO2

It is possible to lower pH by increasing the amount of carbonic acid in water. This can be done by injecting CO2 directly into water to reduce the pH, however, this method does not reduce alkalinity. 

However, if acid is added to water, this lowers both pH and alkalinity as the acid must convert bicarbonate alkalinity into carbonic acid by adding hydrogen; when bicarbonate is converted into carbonic acid, alkalinity is reduced. 

Raising pH In Water with CO2

If CO2 is reduced in water, the pH will increase. The best method is to add high-pH additives such as non-stabilized chlorine, a pH adjuster (e.g. sodium carbonate or sodium bicarbonate), or a salt-chlorine generator. 

Another way you can raise the pH in water is to let it naturally increase by off-gassing. Off-gassing is the release of dissolved gas, in this case, CO2

Aeration features such as jets can be added in systems such as swimming pools to release CO2 from the water, naturally raising the pH. 

To understand how CO2  affects the pH of water further, particularly the loss of CO2, we must look at the law of physics called Henry’s Law.  

Henry’s Law

Henry’s Law

Henry’s Law states that the amount of gas dissolved in water will be proportional to the amount of the same gas in the surrounding air outside the water to reach equilibrium. Once equilibrium is achieved, the off-gassing process stops. Following Henry’s Law, we know that pH can only reach certain levels naturally because the atmospheric pressure will push CO2 back into the water.

The point of equilibrium signifies a limit or ceiling. For example, as CO2 leaves water in a swimming pool, the pH rises, which is known as the “pH ceiling”. The pH in water can rise above the ceiling, but not naturally, which means something must be added to the water such as a calcium-rich substance, to force the pH to increase above the natural pH ceiling. 

Another prime example of how CO2 affects pH in water, can be observed in ocean acidification. 

Ocean Acidification

Carbon dioxide, a naturally occurring gas in the atmosphere, can dissolve in seawater, changing the pH. When carbon dioxide and water combine to form carbonic acid, the weak acid breaks into hydrogen and bicarbonate ions.  Because of increased anthropogenic levels of CO2 in the atmosphere, more CO2 is being dissolved in the ocean. As seawater absorbs more CO2, the pH decreases and the ocean becomes more acidic, causing ocean acidification. 

When water is saturated by CO2, it depletes calcium carbonate sources in water. As we know, when CO2 is absorbed into ocean water, carbonic acid increases the acidity. This usually occurs at the surface of the water, inhibiting seaweed growth and creating reproductive disorders in some fish. 

The pH levels in the oceans have dramatically decreased by ~30% and if CO2 is continuously emitted at the current rate, it is estimated that by 2100, the ocean’s acidity will increase by 150%

As the oceans become more acidic, shell-building marine organisms such as mollusks are heavily impacted. When ocean acidification increases, free carbonate ions bond with free hydrogen, producing fewer carbonate ions for calcifying organisms to maintain their shells, skeletons, and other calcium-based structures. When water becomes too acidic, their shells and skeletons start to dissolve. 


Carbon dioxide is influential in regulating the pH in water; the amount of CO2 in a solution is one way to determine the pH. The more CO2 in water, the lower the pH. 

When pH levels drop due to increased levels of CO2, it creates unstable aquatic environments, produces acid rain, and drives ocean acidification. 

If you have any questions regarding pH/CO2, or the water testing kits we have to offer, please feel free to contact our world-class team at Atlas Scientific, we are always happy to help!

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