My tank's alkalinity (and consequently PH) is constantly decreasing. It got to 7.3 at one point. I have been adding sodium bicarbonate based on this calculator. While it never gets to my desired target (10), it also drops very quickly the next day, from 9.1, to 8.6 or so. I have let one of the glass lids open for air circulation. Any tips on what could be the cause or what else I should do?
Help with Low Alkalinity
- Thread starter Europa
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What is in the tank that’s consuming it? It’s your job to make sure it’s replenished as it needs it. Don’t know what air circulation has to do with it, unless you think pH is the same as Alkalinity.
I think @Randy Holmes-Farley has covered this before;
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.
I can't give you the chemistry of why but sodium bicarbonate will lower the pH slightly. Soda ash will raise pH a lot. I mix 2 parts sodium bicarbonate to 1 part soda ash and dose that. It affects it much, much less, if at all.
HuduVudu
2500 Club Member
Because it is Bicarbonate (HCO3). It still has a hydrogen ion and one more hydrogen will push it to carbonic acid. Carbonate (CO3) however needs two hydrogen ions to push it to carbonic acid. It isn't that it is lowering the PH it is that it isn't buffering it as much.
HuduVudu
2500 Club Member
During the increased presence of the CO2 the PH will be lowered because of the presence of two hydrogen ions. When the CO2 is removed from the solution the hydrogen ions will also be removed and the PH will go back to it's original.
When the CO2 is in solution from source elevation, testing the solution will reveal the carbonate from the carbonic acid. When the CO2 is dropped back to it's previous levels then the CO2 comes out of solution then the carbonate will not longer be a part of the solution and will no longer be testable. Thereby pushing the solution back to the original carbonate levels.
The problem is when the CO2 levels remain elevated. The carbonic acid remains in solution and lowers the PH and tests as carbonate. If this is done indefinitely then it creates a new normal. The way to "correct" this situation is to remove the excess CO2.
