Some results from sodium hydroxide dosing

[Gigajoulz]

There's no issue with dosing hydroxide and lack of carbonate/bicarbonate unless you drive the pH too high (higher than you'd go).

This is a cut and paste from my limewater/kalkwasser article which has the same "concern":


...and the hydroxide ions supply alkalinity. Hydroxide itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when hydroxide is used in a reef aquarium, it quickly combines with atmospheric and dissolved carbon dioxide and bicarbonate to form bicarbonate and carbonate:

4. OH- + CO2 ---> HCO3-

5. OH- + HCO3- ---> CO3-- + H2O

In an aquarium with an acceptable pH, there is no concern that the alkalinity provided by hydroxide is any different from any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in aquarium water is really a function of only pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 meq/L). Consequently, the fact that alkalinity is initially supplied as hydroxide is not to be viewed as problematic, except as it impacts pH.

I think this is what I have been missing, what is the HO- doing that adds alkalinity if "alkalinity" is some form of carbonate... I found the question poised much better than I've attempted. The whole time I've mistaken the Hydrogen in carbonic acid as coming from another source when its really just the water from the tank dissolving the CO2; like H2(CO2)O -> which then falls apart into (H+) + HCO3- at a given pH, and THAT (H+) is whats tied up with the HO-.... add another HO- and the HCO3- -> CO3 + H2O at a higher pH.

did I mention I was BAD at stoichiometry?

Its also the equilibrium with the atmosphere I'm still a little fuzzy on, specifically how this contributes to the carbonate system in general. What's confusing me there is I've read (misread?) about hydroxide dosing allowing for the uptake of CO2, but I guess that makes sense now because the tank it out of equilibrium, but the concentration is already "determined" by the pH and what was dosed. does that all sorta add up better?

 

[sean151]

For the CO2; An extreme example would be tank and air are at 500ppm. You add calcium hydroxide (or any other hydroxide) which drops your tank CO2 down to 450, thus raising Ph. As your skimmer works to equalize the CO2 to the air value the Ph drops back down. The Alk effect stays, but the CO2 drop is temporary. This is the same reason that your Ph will fluctuate during the day and night cycle, CO2 rising and falling, but potentially no change to Alk.

 

[Randy Holmes-Farley]

I think this is what I have been missing, what is the HO- doing that adds alkalinity if "alkalinity" is some form of carbonate... I found the question poised much better than I've attempted. The whole time I've mistaken the Hydrogen in carbonic acid as coming from another source when its really just the water from the tank dissolving the CO2; like H2(CO2)O -> which then falls apart into (H+) + HCO3- at a given pH, and THAT (H+) is whats tied up with the HO-.... add another HO- and the HCO3- -> CO3 + H2O at a higher pH.

did I mention I was BAD at stoichiometry?

Its also the equilibrium with the atmosphere I'm still a little fuzzy on, specifically how this contributes to the carbonate system in general. What's confusing me there is I've read (misread?) about hydroxide dosing allowing for the uptake of CO2, but I guess that makes sense now because the tank it out of equilibrium, but the concentration is already "determined" by the pH and what was dosed. does that all sorta add up better?

Total alkalinity (as measured by any normal kits we use) is not just a form of carbonate. It is anything that takes up an H+ as the pH is dropped to about 4.3. Here's the full equation (TA = total alkalinity), showing that there are lots more contributors, including hydroxide. Sodium hydroxide solution has alk that can be measured with an ordinary alk titration test. Other than that, what you write is correct about what happens to hydroxide after adding it, although there are lots of other things that happen too. The article below has a lot more.

TA = [HCO3–] + 2[CO3—] + [B(OH)4–] + [OH–] + [Si(OH)3O–] + [MgOH+] + [HPO4—] + 2[PO4—] – [H+]

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.

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