Reef Chemistry Question of the Day #104 Alkalinity

Randy Holmes-Farley · Feb 2, 2015

Reef Chemistry Question of the Day #104

The total alkalinity of seawater at pH 8.0 is closest to the concentration of which of the following in the water?

A. Carbonate
B. Chloride
C. Carbonic acid
D. Bicarbonate

Good luck!

.

Skydvr · Feb 2, 2015

I know it is not carbonate, as I dose carbonate to raise my alkalinity and pH from 7dkH and 7.8pH to approximately 9.5dkH and 8.2pH.

Carbonic acid is from dissolved CO2, which can pull a tank's pH down below 7.8 overnight.

I had it down to bicarbonate or chloride, leaning more towards the chloride just because I know that when bicarbonate it added as an alkalinity supplement, it temporarily pulls the pH down.

I just performed a little experiment adding some baking soda to some RO. It brought the pH up from 6 to 7.8-7.9 on my Red Sea test kit. I tried it with my old API freshwater kit and was getting something it that range as well. I am going with d, bicarbonate.

I would have sworn that bicarbonate had the capacity to pull pH lower than that, but I guess not. I didn't think chloride was correct either though. If it was, wouldn't we have a serious issue trying to change pH from 8.0 as chloride is the dominant ion in sea water? Although I then pH would be pretty stable at 8.0 and that would be our goal as that would be what the ocean would be at?

Steven@ · Feb 2, 2015

Based my previous research I believe the answer to be D.

Randy Holmes-Farley · Feb 11, 2015

And the answer is...D. Bicarbonate

Total alkalinity (as all but a few specialized tests measure) is defined in seawater as:

TA = [HCO₃^-] + 2[CO₃^--] + [B(OH)4^-] + [OH^-] + [Si(OH)₃O^-] + [MgOH⁺] + [HPO₄^--] + 2[PO₄^---] - [H⁺]
this article gives more info on this equation:
http://www.advancedaquarist.com/2002/2/chemistry

Bicarbonate (HCO₃^-) is the largest of those contributors unless the pH is above about 8.6 or 8.7 in seawater, where carbonate can become a bigger contributor (since its contribution is doubled).

Borate is much lower, and all of the others are trivially small contributors in seawater at reasonable pH.