Help Understanding CO2’s Relationship With Alk

[BudgetR33f]

I understand that when CO2 binds with water through surface agitation it produces carbonic acid which the dissociates into carbonate ions, releasing H+ ions into the water and decreasing pH. But wouldn’t this chemical reaction also result in more carbonate ions to accept H+ ions and increase alkalinity? @Randy Holmes-Farley

 

[BudgetR33f]

After thinking about it more, wouldn't the CO2 ultimately decrease the alkalinity and thus pH because it dissociates H+ ions from water that otherwise wouldn't, and although they could take them back, they might not always be able to? Is there any flaws in this?

 

[anthonymckay]

This processes of creating carbonic acid and carbonates/bicarbonates goes back and forth as co2 is introduced. But ultimately reaches a level of equilibrium. In the end you still end up with additional H+ ions in theater as a result and lower the pH.

 

[Miami Reef]

CO2 dissolves in water to form carbonic acid, which converts to bicarbonate and H+.

pH is a measure of H+ concentration. More H+ = lower pH.

It’s basically taking some hydrogen from the water and releasing it as a hydrogen ion, which reduces pH.

 

[mook1178]

CO2 dissolves in water to form carbonic acid, which converts to bicarbonate and H+.

pH is a measure of H+ concentration. More H+ = lower pH.

It’s basically taking some hydrogen from the water and releasing it as a hydrogen ion, which reduces pH.

I would just like to add that the bicarbonate created is balanced by the carbonate in the equilibrium equations. Therefore the only change is pH with CO2 gas exchange. This is why when people chase pH, if your alkalinity is where it should be, a CO2 scrubber is suggested.

 

[Miami Reef]

I would just like to add that the bicarbonate created is balanced by the carbonate in the equilibrium equations. Therefore the only change is pH with CO2 gas exchange. This is why when people chase pH, if your alkalinity is where it should be, a CO2 scrubber is suggested.

Yes. CO2 has no effect on alkalinity.

This graph shows that higher alkalinity will have more buffer against CO2. In 2 tanks exposed to the same CO2 and aeration, the higher alk will have a higher pH by shifting the carbonate equilibrium.

2.5meq/L = 7dKH

 

[Randy Holmes-Farley Verified Community Expert]

As folks have suggested, it is actually a mathematical fact that no amount of added or removed CO2 can impact alkalinity, only pH. I discuss that here:

https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/
From it:

Alkalinity Facts​

There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts”, 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

 

[MiZuboov]

As folks have suggested, it is actually a mathematical fact that no amount of added or removed CO2 can impact alkalinity, only pH. I discuss that here:

https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/
From it:

Alkalinity Facts​

There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts”, 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

Hi! Why aren't protons neutralized by the formed carbonates and bicarbonates? Where do they go after carbonic acid dissociation?

 

[Randy Holmes-Farley Verified Community Expert]

Hi! Why aren't protons neutralized by the formed carbonates and bicarbonates? Where do they go after carbonic acid dissociation?

I’m not sure I understand the question. The H+ stays in the water and lowers pH. The bicarbonate also stays in the water. They are constantly combining and dissociating again as I described in the previous post.

 

[Curiousbranching]

As folks have suggested, it is actually a mathematical fact that no amount of added or removed CO2 can impact alkalinity, only pH. I discuss that here:

https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/
From it:

Alkalinity Facts​

There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts”, 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

If that’s the case, I really don’t understand why a device called Aquawiz is considered by some people to accurately measure KH. Or they just idealizing it too much?

 

[Randy Holmes-Farley Verified Community Expert]

If that’s the case, I really don’t understand why a device called Aquawiz is considered by some people to accurately measure KH. Or they just idealizing it too much?

I’ve followed it a bit. Why does the lack of CO2 effect on alk impact that device?

 

[MiZuboov]

I’m not sure I understand the question. The H+ stays in the water and lowers pH. The bicarbonate also stays in the water. They are constantly combining and dissociating again as I described in the previous post.

Usually, when we add bicarbonates, they neutralize protons and therefore the pH of the water rises. And here we get both bicarbonate and hydrogen at the same time, but the pH still decreases. This point is unclear to me..

 

[Randy Holmes-Farley Verified Community Expert]

Usually, when we add bicarbonates, they neutralize protons and therefore the pH of the water rises. And here we get both bicarbonate and hydrogen at the same time, but the pH still decreases. This point is unclear to me..

No, that is actually not the case. Adding bicarbonate to seawater releases some H + and pH drops.

 

[MiZuboov]

No, that is actually not the case. Adding bicarbonate to seawater releases some H + and pH drops.

Is this rule working for freshwater as well?

 

[Randy Holmes-Farley Verified Community Expert]

Is this rule working for freshwater as well?

No, unless the pH is fairly high. Bicarbonate is a stronger acid in seawater.

 

[MiZuboov]

No, unless the pH is fairly high. Bicarbonate is a stronger acid in seawater.

How is this acid? Isn't bicarbonate a base that neutralizes acid??

 

[IceNein]

If that’s the case, I really don’t understand why a device called Aquawiz is considered by some people to accurately measure KH. Or they just idealizing it too much?

The Aquawiz isn't changing KH. The Aquawiz is equalizing CO2 dissolved into the water by aerating it. This changes the pH of the system based on that chart shown previously. If you take two samples with different carbonate hardness, and then make sure that the dissolved CO2 in both samples is identical, then the pH in the sample with a higher dKH will be higher.

The Aquawiz is measuring pH. It is not changing the hardness of the sample water by aeration.

 

[Randy Holmes-Farley Verified Community Expert]

How is this acid? Isn't bicarbonate a base that neutralizes acid??

It is both.


Bicarbonate acting as an acid:

HCO3– ==> H+ + CO3—

Bicarbonate acting as a base:

HCO3– + H+ —> H2CO3

 

[MiZuboov]

As folks have suggested, it is actually a mathematical fact that no amount of added or removed CO2 can impact alkalinity, only pH. I discuss that here:

https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/
From it:

Alkalinity Facts​

There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts”, 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

Hi!
Can I say that (HCO3-) + (H+) is the form of being of Carbonic acid in the water at pH 4,5-8,4?

 

[Randy Holmes-Farley Verified Community Expert]

Hi!
Can I say that (HCO3-) + (H+) is the form of being of Carbonic acid in the water at pH 4,5-8,4?

When carbon dioxide dissolves in water, it combines with water to form carbonic acid, and that carbonic acid will dissociate into H+, HCO3-, and CO3- -. The relative amount of carbonic acid, bicarbonate, and carbonate depends on several factors, including pH, salinity, and temperature.