Total inorganic carbon vs pH of seawater calculation or figure

[UK_Pete]

Hi Randy,

I was trying to calculate the amount of CO2 required to change the pH of a litre of ASW and realised how difficult it is, and I wondered if you could either show us a simplified equation, or a graph, or point towards a calculator which is simple enough for laypersons to use etc. The fact that the CO2 changes from bicarb to carb to CO2 etc is just too difficult to get my head round the calculations.

I know it depends on alk, salinity, temp etc, but if we assume normal values (or whatever you might have handy), could you help? Reason is I am trying to calculate the flow of water required for a CO2 remover. I can see that the water flow requirements are far lower than the gas flow so a trickle filter gas exchange design would seem best, I can calculate that to remove say 5g a day of CO2 at air effluent CO2 of 300ppm and influent of 0 ppm would require about 200 cc of air per second. But working out the water flow has turned out too complex for me.

What I need to know is the mass proportion (or mole proportion etc) of total inorganic carbon in seawater at pH 8.5 and 8.4, at reef type alk, salinity, temp etc. I'm only after a ballpark figure as I know the gas exchange efficiency won't be 100%. I've found a figure for pH 8.1/8.2 at a TIC delta of about 2.6mg co2 / kg water (@2.3 meq alk, 35ppm salinity & 26 degs C) but I am not sure if its accurate either. According to that figure it would only need a flow of 25cc of water per second or so to remove 5g a day of CO2 from a 160L tank with influent @ pH 8.2 and effluent @ pH 8.1 (but I want the TIC delta for pH 8.4 / 8.5 ideally).

You've written a lot about the CO2 seawater dynamics so I wondered if you would have any relevant numbers handy.

Thanks, Pete

 

[Randy Holmes-Farley]

I do, but not on my phone.

I'll get back to you later.

 

[UK_Pete]

Ah thought you might great thanks!

 

[Randy Holmes-Farley]

As I show in this question of the day, there is about 2 mmole (90 mg) of carbon dioxide per liter of seawater.

https://www.reef2reef.com/forums/re...emistry-question-day-60-gas-exchange-1-a.html

If you remove 10% of that (0.2 mmole/L; 9 mg/L), you will convert that much bicarbonate into carbonate.

You can see in a graph like this one that if you remove 10% of the bicarbonate, you will increae pH by a few tenths of a pH unit:

https://www.e-education.psu.edu/earth103/node/694

 

[UK_Pete]

Great thanks Randy. That does mean that only quite a small flow of water would be adequate which is quite surprising., but good, as it does seem to make the trickle stripper possible.

 

[Randy Holmes-Farley]

I'm sorry, I don't get the flow rate of water question.

 

[UK_Pete]

Well, I was trying to work out how much water flow would be needed in a CO2 stripper column, if you wanted to remove 5g per day of CO2. So with a flow of 25ml / second, with inflow of (say) 90mg/l, and outflow of 2mg less (88mg / l), thats 4.3 g per day CO2. Although now I think about it, I am wondering if thats useful to know - I was originally shooting for a pH difference of 0.1 unit between inflow and outflow, but the pH of the water returned from a CO2 stripper to the tank would not matter would it. As long as theres enough CO2 in the water which flows to the CO2 stripper to enable removal of my figure of 5g a day, the pH difference between the inflow and outflow is irrelevant. And theres far more than enough if seawater contains 90mg / l. Sorry Randy I think I made a mistake in my reasoning with this.

 

[Randy Holmes-Farley]

The pH in the stripped water will only become critical if it is high enough that you begin to precipitate calcium carbonate. 88 from 90 is not significant in that way.

But in terms of calculations, I'm not sure how you know how much CO2 is entering the tank in other ways, like the tank top, offsetting the stripper.

 

[UK_Pete]

No well I don't - although as for the tank top, the plan would be to use a sealed hood, and circulate the same air through the hood as in the stripper. So it would look something like a trickle filter, with a fan blowing a large volume of air through the trickle filter and recirculating it through a tube of soda lime granules, with the tank hood air being in the air loop somehow. Since I would need a small flow of outside air to provide oxygen, it could be that the stripper / soda lime unit has an air pump blowing a small flow of outside air into it, and it vents the excess pressure (created by the air pump) to the hood, through which it escapes to the room, something like that.

But, with a unit like that, if I did manage to make it quite efficient, it might be too efficient - and even if it was about the right efficiency to balance the CO2 produced in the tank (by respiration etc), like perhaps I could tune the fan speed to get the pH right, but, if something died, it would suddenly be vastly undersized - it would not remove the CO2 remotely fast enough under the die off condition.

If I used a pH probe to control the fan speed, IE the rate at which air is circulated through the trickle scrubber and media, I might as well go the simple route and use the pH probe instead to control NaOH additions (ie forget the scrubber). But using a pH probe to control such an important thing as NaOH dosing is something that worries me. However if I did do that, it could also deal with the 'die off' scenario, and up the quantity of NaOH dosed to the tank if it detected the low pH that the die off created, to keep the tank CO2 down (and fishes happy).

Ideally I was hoping for a passive type system that could both control pH / CO2 under normal conditions, and also under the condition that a large fish died, for instance. But as I learn about it (here), I am realising there is probably no passive system that can do it (apart from lots of outside air). So maybe a pH probe (or 3) would be the way to go.

Going back to my original problems, evaporation and CO2, either the NaOH or the scrubber would seem to be able to fix the problem, but the NaOH is simpler. If they would both need to be controlled by a pH probe, the only problem with NaOH would be alk rising. But I am now wondering if I can use controlled abiotic calcium carbonate precip to keep the alk down until the biotic precip (corraline, SPS) picks up speed and takes over. The high tank pH should keep the alk down I am hoping.