Carbonate vs. Bicarbonate

[mmcrae]

Hi Randy,
I have a question about the Homemade Two Part Calcium and Alkalinity article that you have recently written. Going back to a previous article (Calcium and Alkalinity), you state that natural seawater and balanced two part additives are 90% bicarbonate and 7% carbonate. Why do you suggest baking all of the baking soda, thus converting all of the bicarbonate to carbonate? It seems as though you would want to bake only a fraction of the total, keeping 90% of that part as bicarbonate. Can you elaborate on this? Thanks, Matt

[jfinch]

It doesn't matter if bicarb or carbonate is added to sea water. Given time they will equilibrate with atmospheric CO2. When that happens they will be distributed in an approximate 90% bicarb and 7% carbonate ratio. This happens whether or not you use 100% baking soda or 100% baked baking soda... the only noticeable difference is the initial pH when mixed up.

[mmcrae]

I understand that whichever you add, you will end up with bicarbonate ions in solution,
because, at the sort of pH that interests us, the ratio of HCO3- to CO3--
is large. If you use the bicarbonate, you will go directly to the new
HCO3- concentration, but if you use the carbonate, it must react with the CO2
in the water:

CO3-- + CO2 + H2O -> 2 HCO3-

If you add enough sodium carbonate to wipe out the CO2 in solution,
and do it quickly enough that more CO2 can't come in to make up, you will
get a huge pH spike. It would not be at all difficult to reach pH 10.
I would use the bicarbonate. I was wondering why Randy suggested baking the baking soda. What is the benefit of this vs. not baking it?

[mmcrae]

I guess if removing CO2 from the tank water was beneficial, then carbonate would be the desired form. However, with many users of limewater, CO2 in the system is necessary for for the converson of CO2 and OH into carbonate for balanced increases in calcium and alkalinity.

[jfinch]

I'll let Randy comment his reasons, but I like using sodium carbonate because it dissolves easier then bicarbonate and my tank tends to have a depressed pH anyway (the carbonate additions help to boost it up).

[Randy Holmes-Farley]

In the article gave the reasons for using one or the other of the two recipes. I didn't recommend one over the other for everyone.

Jon is correct that the only difference is the pH. One could not normally drive the pH to 10, but it could temporarily be driven above 8.5 if the pH started at pH 8.3+ and one added a lot of the baked version.

Many folks have excessively low pH, and those folks would benefit from the baked version.

Here's what I said in the article:

"Contrasting The Recipes
I actually provide two recipes in this article.

Recipe #1 is for use in aquaria where the pH is normal to low. It will have a pH raising effect due to the elevated pH of the alkalinity part, as do most of the commercial two-part additives. The rise that you get will depend on the alkalinity in your aquarium, and, of course, on how much you add. If you add on the order of 0.5 meq/l of alkalinity then the pH will rise about 0.15 to 0.35 pH units immediately upon addition (and higher locally before it has a chance to mix into the whole aquarium).

So if you are using limewater (kalkwasser) and the aquarium runs at pH 8.4 or above, this recipe is not the best choice. Otherwise, it is likely to be a good option.

Recipe #2 is for use in aquaria where the pH is on the high side (above 8.3 or so). It will have a very small pH lowering effect when initially added. The drop that you get will depend on the alkalinity in your aquarium, and, of course, on how much you add. If you add on the order of 0.5 meq/l of alkalinity then the pH will drop by about 0.04 pH units immediately upon addition. The pH may later rise if the aquarium is permitted to blow off excess CO2. This recipe is half as concentrated as Recipe #1.

So if you are using limewater (kalkwasser) and the aquarium runs at pH 8.4 or above, this recipe may be the best choice."