Need help with some chemistry math - CO2, HCO3, CO3, pH, kH

karimwassef

karimwassef

Active member
Ok.. for personal edification, I'd like to be able to calculate my CO2 components and after some research, found this paper...

http://www.soest.hawaii.edu/oceanography/courses/OCN623/Spring2012/CO2pH.pdf

AWESOME! right?

Ok... this is why I need help because after running through some math, I think I got an answer, but I need some validation that I'm doing this right.

Based on the paper (page 4) and using a pH of 8.3, salinity of 35, P at 1atm and T of 25C:

pK1 = 5.8401
pK2 = 8.9636

So K1 = 1.44e-6, K2 = 1.09e-9. and H+ = 5.01e-9

Using page 9, I get that

[CO2] = 0.0035 [HCO3-]
[CO3 2-] = 0.22 [HCO3-]

so that the

Sum (CO2) = 1.22 [HCO3-]

Assuming sum (CO2) = 2.3 mM, then

[HCO3-] = 1.88mM

with a molar weight of 61 g/mol, that is 0.115g/L or 115ppm for bicarbonate.

Is this correct?

If it is, then [CO2] = 0.4ppm, [CO3 2-] = 25ppm, and sum(CO2) = 140ppm

The big assumption here is that sum(CO2) = 2.3mM. I used this from page 9 again, but I don't know how this compares to my system.

full paper:
https://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/ZeebeWolfEnclp07.pdf

and another reference (Exam with answered questions :D ):
http://www.ocean.washington.edu/courses/oc520/MT1_09_key.pdf
 
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By the way, my Alkalinity is 8.5dKH

The freshwater tank calculators that use pH and Alk to calculate CO2 give an answer of 1.3ppm. I realize this is freshwater and doesn't apply directly, but I couldn't find a simple formula for salt water.
 
One more question - I would also like to be able to relate my room CO2 (~600ppm) to my tank CO2. Maybe that's a different set of formulae, but ultimately, I'd like to connect all those CO2 dots.
 
They got CO2 ppm = 1.1 in the paper, right?

Where you get Sum(CO2) = 2.3mM ??

Your math works out fine if all your assumptions are correct... I just don't understand a few of the assumed values.... and again, they get CO2 ppm = 1.1, which is close to what the other paper you mention gets for freshwater.
 
Found this online

With CO2 atmospheric of 387 PPM, dissolved CO2 is 1.32 x 10-5 M. Assuming simple Henrys Law, guess value at 600 PPM is 2.046 x 10-5 M


START OF TEXT FOUND ONLINE
Carbon Dioxide in the Ocean The levels of carbon dioxide in the atmosphere has been increasing since the beginning of the industrial revolution due to increased combustion of fossil fuels. Much of the carbon dioxide that has been released dissolves in the ocean. We can use Henry's Law to calculate the concentration of dissolved carbon dioxide in an aqueous solution. The current, average concentration of CO2 is 387 ppm, that is 387 x 10-6 atm. [CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M
 
I used their assumption for sum CO2. That's the big unconnected dot for me.

I dont expect CO2, I do expect HCO3-.. that's why CO2 came out at 0.4ppm or 6.5e-6 M. That's half of what you get at 387ppm atmospheric CO2.

At least we're in the ballpark ( x2).. :D

my atmosphere CO2 is actually 450ppm.
 
I don't understand this :

[CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M

where does 29.41 atm/M come from?

If I use 450ppm, does that convert to 1.53e-5 M?? If that's true, I can use that and work backwards to calculate the constitutive elements and the correct sum (CO2).
 
ok.. if it is, then:

atm CO2 = 450ppm
=> 0.015mM [CO2] = 0.9ppm

that works is sum(CO2) = 5.4 mM

following through:

atm(CO2) = 450ppm
[CO2] = 0.9mm
[CO3 2-] = 59ppm
[HCO3 -] = 270ppm
sum(CO2) = 330ppm

for a pH of 8.3

Is there a way to relate this to dKH?
 
karimwassef said:
ok.. if it is, then:

atm CO2 = 450ppm
=> 0.015mM [CO2] = 0.9ppm

that works is sum(CO2) = 5.4 mM

following through:

atm(CO2) = 450ppm
[CO2] = 0.9mm
[CO3 2-] = 59ppm
[HCO3 -] = 270ppm
sum(CO2) = 330ppm

for a pH of 8.3

Is there a way to relate this to dKH?
Click to expand...

[CO3]*2 + [HCO3] is the total bicarbonate alkalinity = 270 + 118 ppm HCO3

388 ppm HCO3 is 388 mg/kg of water. 388 mg/Kg / 61 mg/meq HCO3 is about 6.5 meq/Kg. I don't use dKH, so, I will let you do the conversion.
 
karimwassef said:
I don't understand this :

[CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M

where does 29.41 atm/M come from?

If I use 450ppm, does that convert to 1.53e-5 M?? If that's true, I can use that and work backwards to calculate the constitutive elements and the correct sum (CO2).
Click to expand...

1.53e-5 * 44 gm/mole = 67.3e-5 gm/L = 673 ppm. Right?. Not 900
 
ok. I'm much closer. I can calculate total alkalinity as ~ carbonate alkalinity = 2 [CO3 2-] + [HCO3 -]

but that returns a result in moles (M)

How do I convert that to meq/L or ppm or dKH?
 
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