Open CO2 calculation

[Tom Meuzelaar]

[admin note: this post represents recovered forum content after a critical hard drive failure - it is not in it's original form]

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Jul 8 2007, 10:32 PM

Hi there

I am trying to do a (very simple) open CO2 calculation in SpecE8 using CO2(g) and H+ as components (cut down basis file attached). The calculated pH is 8.63, compared to that obtained by graphical means of approx 6.45. The problem appears to be due to SpecE8 resetting HCO3- as the component... What am I doing wrong?

Also. It would be useful if SpecE8 reported true alkalinity (i.e. -Total H) as opposed to carbonate alkalinity. This would be more relevant in low pH & low alkalinity systems, or in natural waters where other components contribute to pH buffering.

Ewen

Attached File(s)

open_CO2.sp8 ( 218bytes ) Number of downloads: 5

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Jul 9 2007, 08:24 AM

Post #2

Hi there

I am trying to do a (very simple) open CO2 calculation in SpecE8 using CO2(g) and H+ as components (cut down basis file attached). The calculated pH is 8.63, compared to that obtained by graphical means of approx 6.45. The problem appears to be due to SpecE8 resetting HCO3- as the component... What am I doing wrong?

Also. It would be useful if SpecE8 reported true alkalinity (i.e. -Total H) as opposed to carbonate alkalinity. This would be more relevant in low pH & low alkalinity systems, or in natural waters where other components contribute to pH buffering.

Ewen

Hi Ewen:

Can you give me a little more information? A pH of ~8 is not necessarily unreasonable for a system in contact with a CO2 buffer (ie. seawater). Please tell me the assumptions and procedure going into your graphical method.

Also- your script appears to be using a modified database that I do not have access to- are the modifications pertinent to this calculation?

Regards,

Tom Meuzelaar

RockWare, Inc.

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Jul 10 2007, 05:01 PM

Hi Tom

The system is a ground water feed to an alpine peatbog, hence the extremely low alkalinity. I have no problem simulating the solution chemistry prior to contact with the atmosphere (basis file attached) using carbonic acid as a component in a closed system (i.e. fixed Total H2CO3). The problem I have is that when I try to simulate the pH of the water after equilibration to atmospheric pressure (PCO2 = 10-3.5 atm) it comes unstuck. I have noticed that even when I perform the closed system calculation, GWB resets the carbonate component to HCO3- rather than the H2CO3 in the basis, and modifies the Total H accordingly. This is fine in a closed system, but I can't see how it could work in an open system.

As for the graphical method. On a log C vs. pH plot, H2CO3 is set constant at 10-4.7 M (note: temperature 5 C). HCO3- is a line with slope +1 crossing the H2CO3 line at the pKa (6.5). As I say, it is a trivial problem because the solution is [HCO3-] = -Total H (=1.8x10-5 M) which is at pH 6.48

The only modifications I have made to the database concern temperature dependence data for mackinawite (FeS), and do not affect this calculation.

Cheers, Ewen

Attached File(s)

watchbed_feed.sp8 ( 321bytes ) Number of downloads: 3

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Jul 15 2007, 06:13 AM

Hi Ewen,

The procedure in SpecE8 to calculate pH in equilibrium with atmospheric CO2 is

temperature = 5

swap CO2(g) for HCO3-

fugacity CO2(g) = 10^-3.5

balance on H+

This gives a pH of 5.6, which is correct. If you impose mass balance on carbonate in your hand calculation, I think you will get this value, not 6.5, which is too high.

A good way to carry an alkaline water to equilibrium with atmospheric CO2 is to use a sliding fugacity path, as discussed in the User's Guide, and Chapter 12 of the Geochemical Reaction Modeling text.

The procedure to calculate total alkalinity, as opposed to carbonate alkalinity, is given in Chapter 13 of GRM, on page 190.

HTH,

Craig

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Jul 15 2007, 07:38 PM

Hi Craig

Yes the pH should be 5.6 if the Total H is zero - although when I try this in GWB it doesn't converge!! But that aside, the basis input that I have been asking about was for Total H = -1.8e-5 M, so the equilibrium pH will be higher. The pH value that I obtain graphically of 6.48 is consistent with that which I calculate using other code, so I can only assume that it is a problem with the basis set up, but is it such a simple calculation that I can barely see where the problem would be.

The suggestion of imposing mass (I assume you mean mole) balance on carbonate in the graphical calculation is wrong (I believe) for an open system calculation - Total H2CO3 is not constrained.

The sliding fugacity scale sounds like just the thing - but I need this to work for one condition first.

Thanks for the info about the "true" alkalinity.

Cheers, Ewen

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