Sodium Bicarbonate vs Sodium Carbonate

cypho · Dec 1, 2015

Carbonates are my Achilles heel. Just about everything about them seems to be counter-intuitive. My current puzzle is trying to understand the differences between Sodium Bicarbonate vs Sodium Carbonate for alkalinity supplement.

I just can't figure out the difference, I'm starting to wonder if there is none. Is the only difference the quantity of dissolved CO2 immediately after mixing the dry powder into water? Do the differences completely disappear once the dissolved CO2 is allowed to reach equilibrium with the atmosphere?

If there is some long term difference, can you explain the what and why?

Randy Holmes-Farley · Dec 2, 2015

There is not a "long term" difference in the aquarium, but there is a pH difference when added due to the CO2 level as you note. That pH boost can last for hours, or if dosing is spread out over 24 hours a day, could be effectively permanent (but smaller than an all at once dose).

Adding 0.5 meq/L (1.4 dKH) of sodium bicarbonate causes a slight (0.04 pH unit) drop in pH, while sodium carbonate causes about a 0.35 pH unit boost in pH (hydrioxide, as in limewater (kalkwasser) causes about a 0.6 pH unit boost for the same alkalinity).

Another difference is sodium carbonate is more soluble in fresh water, so one can make a more concentrated additive solution with it.

I discuss such issues here:

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/may2002/chem.htm

cypho · Dec 2, 2015

Thanks,

Are the CO2 differences short term in a distilled water solution just like they are in saltwater? If you mix bottles carbonate and bicarbonate solutions and do not store them in airtight containers, will they also become identical after a while. Or is the process different (or much slower) in freshwater?

Randy Holmes-Farley · Dec 2, 2015

The process is the same in fresh water, but it all depends on the degree of aeration. An open bottle might take a very long time to equilibrate. Run an airstone into it and it might equilibrate in a few hours or less. The freshwater solutions used for dosing generally stay quite a bit different for as long as you'd keep them.