Does excess CO2 depress alkalinity?

[fishyjoes]

Just trying to understand how things influence each other.
I know that having excess dissolved CO2 in the aquarium will lower your PH.
Does CO2 lower alkalinity as well?

Somewhere I saw a test explained where you take a sample of water, measure the PH, then go aerate it outside to gas off any excess CO2 and measure the PH again. PH increase after aeration means you had excess CO2 holding your PH down. I did that and my PH went from 7.8 to 8.1 after ~15 minutes of aeration outside.

If I did the same test but measuring alkalinity, would I expect to see any change?

I could just do the test, but if somebody knows I'd rather save the hassle. I tried reading some papers that discuss the relation between CO2, alkalinity, and PH, but I don't know enough chemistry to make sense of it.

 

[CasperOe]

Okay, almost sure I got this one right but the experts might be able to correct me if I understood the science wrong!

Increased CO2 lowers alkalinity because it forms carbonic acid in water. This acid then consumes carbonate ions, reducing alkalinity. Carbonate ions are essential for stabilizing pH and supporting the growth of marine organisms, like corals. Therefore, excess CO2 disrupts this balance and can harm reef tank ecosystems by lowering alkalinity

BRS has a good description here- both in the article and in the video.

Also this one by Reefs.com is worth a read!

 

[Randy Holmes-Farley]

CO2 added or subtracted from seawater impacts only pH, and cannot have any impact on alkalinity.

 

[Randy Holmes-Farley]

Okay, almost sure I got this one right but the experts might be able to correct me if I understood the science wrong!

Increased CO2 lowers alkalinity because it forms carbonic acid in water. This acid then consumes carbonate ions, reducing alkalinity. Carbonate ions are essential for stabilizing pH and supporting the growth of marine organisms, like corals. Therefore, excess CO2 disrupts this balance and can harm reef tank ecosystems by lowering alkalinity

BRS has a good description here- both in the article and in the video.

Also this one by Reefs.com is worth a read!

This analysis fails for the reason I describe below:

Chemistry and the Aquarium: What is Alkalinity?

Randy provides an overview of alkalinity as to why it's important, how it's measured, and how can it be tested.

reefs.com

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.

 

[sc50964]

CO2 added or subtracted from seawater impacts only pH, and cannot have any impact on alkalinity.

Then what lowers alk? Waste from fish and corals?

 

[RedReefer]

Coral growth or coraline algae growth.

 

[Randy Holmes-Farley]

Okay, almost sure I got this one right but the experts might be able to correct me if I understood the science wrong!

Increased CO2 lowers alkalinity because it forms carbonic acid in water. This acid then consumes carbonate ions, reducing alkalinity. Carbonate ions are essential for stabilizing pH and supporting the growth of marine organisms, like corals. Therefore, excess CO2 disrupts this balance and can harm reef tank ecosystems by lowering alkalinity

BRS has a good description here- both in the article and in the video.

Also this one by Reefs.com is worth a read!

Sadly, BRS folks are not scientists and folks should not ever rely on their assertions about science.

 

[Randy Holmes-Farley]

Then what lowers alk? Waste from fish and corals?

Lots of things can add or deplete alk.

Deposition of calcium carbonate and rising nitrate levels from conversion of ammonia to nitrate deplete alk.

 

[sc50964]

Lots of things can add or deplete alk.

Deposition of calcium carbonate and rising nitrate levels from conversion of ammonia to nitrate deplete alk.

Thanks. How about magnesium level depletion? Is that also influenced by fish and coral waste?

 

[Randy Holmes-Farley]

Thanks. How about magnesium level depletion? Is that also influenced by fish and coral waste?

Magnesium consumption only happens along with calcium carbonate deposition. Magnesium is incorporated in place of some of the calcium. Depletion varies a bit, but is not more than about 1/10th of the calcium depletion rate.