Carbonate System

lduncan

Member
Hi Randy, a quick question on the carbonate system.

Basically the question is, does alkalinity in our tanks buffer changes in pH from changing partial pressures of CO2?

For example: tank 1 runs an alkalinity of 8dkH. tank 2 runs at an alkalinity of 12dkH.

If the the partial pressure is changed by the same amount around both tanks, then will the change in pH in tank 2 be less than that in tank 1?

My gut feeling is that both tanks see a drop in pH of the same value. IE the change in pH is irrespective of the alkalinity (unless alkalinity is REALLY low). Is this correct? I understand that the pH value itself may be different. But the change in pH should be the same in both tanks shouldn't it?

(Of course this would be different for mineral acid addition etc)
 

Randy Holmes-Farley

Reef Chemist
Premium Member
The higher the alkalinity the more the tank is buffered against changes in pH due to the addition (or removal) of any acid, including CO2/H2CO3/carbonic acid.

However, there are other factors as well. The buffering provided by bicarbonate/carbonate is greater the higher pH is in the range of 7.5 to 9.

I discuss the mathematics of pH buffering in this article:

Boron in a Reef Tank (and its effect on pH buffering)
http://www.advancedaquarist.com/issues/dec2002/chem.htm
 

lduncan

Member
Yes, I can see Boron would have an effect. As well as how pKH and pH determine the optimum buffering point. Just looking at the carbonate system in isolation:

[HCO3(-)] [ H(+)] / [H2CO3] = 10^ -6.3

[CO3(2-)] [ H(+)] / [HCO3(-)] = 10^-10.3

[H2CO3] = 10^-1.5 P(CO2)


[H(+)] = SQRT( 10^-1.5 * 10^-6.3 * 10^-10.3 * P(CO2) / [CO3(2-)] )

pH = -Log( SQRT( 10^-1.5 * 10^-6.3 * 10^-10.3 * P(CO2) / [CO3(2-)] ) )

Is this calculation correct (logically at least, Ka values may be wrong)? If so, it's showing me that for two different alkalinity levels if you change the partial pressure of CO2, then the pH drops by the same amount in both cases?

Layton
 

Spectre2006

Reef Addict
Am I wrong in thinking that increased carbon fixation occurs at night versus daytime? The symbiotic algae create energy and produce O2 with CO2 converted to organic molecules. Mainly monosacchrides/polysacchrides. Cellulose on boreal forms. Glucose etc. At night since photosynthesis doesn't occur that extra CO2 must hang around for conversion to bicarbonate. Also those energy stores built up during the day get converted to CO2 at night. Most CaCO3 formation must occur at night but is dependent on day time CO2 fixers for the extra energy to do so.

So drastically upsetting the daytime/nighttime cycle may occur at the detriment of our artificial reef system. I stopped turning on the lights after 12 hrs of daytime. It must really upset the biochemical processes and equilibriums. Also algae expend alot of energy stores creating chlorophylls that get trashed during nightime and get remade during daytime. So constantly turning on the lights and turning off the lights must also upset the symbiotic algae in our coral.

Chris
 

lduncan

Member
I'd rather not complicate things by bringing biological processes at this stage. From a pure chemistry standpoint, it seems that when the carbonate system is used to provide buffering, that the change in pH from varying CO2 partial pressures is independent of the carbonate / bicarbonate contribution to alkalinity.
 

Randy Holmes-Farley

Reef Chemist
Premium Member
Is this calculation correct (logically at least, Ka values may be wrong)? If so, it's showing me that for two different alkalinity levels if you change the partial pressure of CO2, then the pH drops by the same amount in both cases?

No, they will not drop by the same amount. Take an extreme, with no alkalinity (no HCO3- or CO3--). The first little bit of CO2 will drop the pH way down, like you'd get in fresh water.

For a small amount of CO2 added, the pH drop is directly proportional to the amount of alkalinity. :)

I don't know how your equation shows anything about the effect of CO2 on pH. The level of carbonate is not a constant. The higher the alkalinity, the higher the amount of carbonate at any given pH.
 

Boomer

Bomb Technician (EOD)
Premium Member
That program will allow you to calculate what you want, where you can then plot CO2, Alk & pH graphs. It is not assumed knowledge in a pure CO2/carbonate system. If you know the pH and Alk you can calculate CO2 or CO2 & Alk calculate pH, etc.. with great accuracy. How it is working out ?? Chemical oceanographer use it all the time and is why it was developed, as it does work out. These are often called Buch-Park equations as I explained in the other link. These same equations are in your pdf's :) You will have to plug in things like Salinity, Temp, pK1 & pK2 etc., unless you use tables then only pK1, pK2, pH and CA ( carbonate alk minus borate alk)
 

lduncan

Member
That's part of the problem, I don't have a PC handy to run the program. I use Macs mainly. :D

But thanks Boomer, i'll keep reading, and find a PC to run the program on, and play with it a bit.

Seems to be a very important system in oceanography.
 

lduncan

Member
Thanks Randy, I should read more carefully :) And thanks Boomer for those interesting links, the one about phyto induced carbonate precipitation was pretty interesting.
 
Top