Reef Chemistry Question of the Day #137 Bicarbonate Concentration

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#137[/HASHTAG]

OK, we'll take a short break from the gas diffusion questions to ask a different sort of question.

Corals get carbonate to make their calcium carbonate skeletons in two ways. Some is obtained from CO2 that is present from metabolic processes, and the coral spits out a couple of protons to make carbonate:

CO2 + H2O --> H2CO3 --> 2H+ + CO3-- (carbonate)

The other process involves direct uptake of either carbonate or bicarbonate from the water, and bicarbonate is believed to be the form used.

Many aquarists are very focused on maintaining a particular alkalinity, but not many think of what this means in terms of what corals really use: bicarbonate.

Which of the following aquarium situations has the highest bicarbonate concentration?

A. pH 7.6; alkalinity 6.3 dKH
B. pH 7.9; alkalinity 6.8 dKH
C. pH 8.3; alkalinity 7.9 dKH
D. pH 8.5; alkalinity 9.0 dKH
E. pH 8.6; alkalinity 9.9 dKH

Good luck!





















.

 

[beaslbob]

f

 

[Randy Holmes-Farley]

Funny man

I thought you liked C.

 

[JimWelsh]

Answer is A. http://cdiac.ornl.gov/oceans/co2rprt.html

 

[kevinsmixed90]

B

 

[Randy Holmes-Farley]

Answer is A. http://cdiac.ornl.gov/oceans/co2rprt.html

While I didn't use that program ( I created my own spreadsheet), I suspect you used the "seawater pH scale" that the program has as a default rather than the NBS (National Bureau of Standards) pH scale that most reefers use when they calibrate their pH meters. Or perhaps you selected different K values.

I didn't specify in the question, but for folks who it matters to, use the NBS scale.

Perhaps the answer I got (not A) isn't perfect if the K values I used are from a scale different than the NBS scale.

However, later today I'll give the answer in more general terms and these differences will go away.

 

[beaslbob]

Funny man

I thought you liked C.

this is for the ones I really really don't know.

 

[JimWelsh]

Randy is correct regarding pH scale. Using NBS scale, I'm switching my answer to "B".

 

[mcarroll]

I wonder if people wasting energy and time baking their bicarbonates into carbonates would consider this?

The "pH benefit" of the baking is so transient that it seems to border on useless even if your tank is a "low pH tank". Also it seems that causing pH to go higher during dosing (as carbonates do) is a good way to facilitate more rather than less precipitation during dosing.

 

[Randy Holmes-Farley]

So in terms of the answer, my intent was to have them all be the same total bicarbonate. Tweaking different pH scales and published dissociation constants for bicarbonate and carbon acid moves the answer around a little, but all fairly close to each other.

I thought in particular that the folks who are so focused on keeping alkalinity in the 7-8 dKH range might be surprised that bicarbonate is so pH dependent, and so if bicarbonate is what is important to corals in terms of alkalinity, that they might want to consider whether the "optimal" alkalinity might be higher if the pH is higher.

Answer is: none of the above for most practical purposes, although I'm sure Jim ran the calculations correctly.

And, of course, beaslbob got it exactly right with choosing f.

 

[Randy Holmes-Farley]

I wonder if people wasting energy and time baking their bicarbonates into carbonates would consider this?

The "pH benefit" of the baking is so transient that it seems to border on useless even if your tank is a "low pH tank". Also it seems that causing pH to go higher during dosing (as carbonates do) is a good way to facilitate more rather than less precipitation during dosing.

It is transient, but you add it every day if not more frequently, so it keeps the average pH higher. The effect is obviously more extreme using limewater, but only by a factor of 2.

 

[JimWelsh]

Now that you mention it, the values using the NBS pH scale are all within about 1.7% of the average.

 

[beaslbob]

So in terms of the answer, my intent was to have them all be the same total bicarbonate. Tweaking different pH scales and published dissociation constants for bicarbonate and carbon acid moves the answer around a little, but all fairly close to each other.

I thought in particular that the folks who are so focused on keeping alkalinity in the 7-8 dKH range might be surprised that bicarbonate is so pH dependent, and so if bicarbonate is what is important to corals in terms of alkalinity, that they might want to consider whether the "optimal" alkalinity might be higher if the pH is higher.

Answer is: none of the above for most practical purposes, although I'm sure Jim ran the calculations correctly.

And, of course, beaslbob got it exactly right with choosing f.

shhhhhhhhhhhhhhhhh

don't tell anyone.

You do have a reputation to protect.