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Back calculate pH in GWB Standard

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Hi,

I'm new to the forum and GWB. I'm working with the Standard edition.

After we did all our initial wet chem measurements (all in-situ), we realized the pH meter was not properly calibrated. Is there a way I can enter all of the other species concentrations into GWB, and have it calculate what the pH must have been?

This is all the information that I have:

temp: 7 deg C

pCO2: 8.4 mbar

Na+: 1.3E-3 M

K+: 3.0E-4 M

Ca2+: 4.0E-4 M

Mg2+: 2.5E-3 M

ClO4-: 3.0E-3 M

Cl-: 4.0E-4 M

HCO3-: 2.0E-3 M

We suspect that the sample was super saturated w/ CaCO3.

Thanks in advance for your help!!!

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Hi,

I'm new to the forum and GWB. I'm working with the Standard edition.

After we did all our initial wet chem measurements (all in-situ), we realized the pH meter was not properly calibrated. Is there a way I can enter all of the other species concentrations into GWB, and have it calculate what the pH must have been?

This is all the information that I have:

temp: 7 deg C

pCO2: 8.4 mbar

Na+: 1.3E-3 M

K+: 3.0E-4 M

Ca2+: 4.0E-4 M

Mg2+: 2.5E-3 M

ClO4-: 3.0E-3 M

Cl-: 4.0E-4 M

HCO3-: 2.0E-3 M

We suspect that the sample was super saturated w/ CaCO3.

Thanks in advance for your help!!!

Hello:

Attached is one way to do this. It assumes that pH is constrained by equilibrium with pCO2. If you want to model ClO4- speciation, you'll need to include a constraint for oxygen (ie. Eh, D.O. etc.) in the model.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

calc_pH.rea

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Hello:

Attached is one way to do this. It assumes that pH is constrained by equilibrium with pCO2. If you want to model ClO4- speciation, you'll need to include a constraint for oxygen (ie. Eh, D.O. etc.) in the model.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

Thanks for your help! This is probably a silly question also, but at the top it says that pH = 9.386, but then about halfway through the data it says pH = 7.768. Which pH is it? They both say step 0 so what is the difference?

Also, can you explain in words what this line means/does?

swap "CO2(g)" "H+" log fugacity "CO2(g)" = -4.5

Also, I don't even see where I can input ClO4- here. Could you give me more specific instructions for adding it to the system?

Thanks again!

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Thanks for your help! This is probably a silly question also, but at the top it says that pH = 9.386, but then about halfway through the data it says pH = 7.768. Which pH is it? They both say step 0 so what is the difference?

In the first 'step 0', React calculates mineral saturation states but does not allow any supersaturated minerals to precipitate. In the second 'step 0', React iteratively precipitates those mineral phases that are thermodynamically favorable. In your case, a number of carbonate phases have positive log Q/K values (ie. are supersaturated), so React precipitates Dolomite until there are no supersaturated phases. The removal of solutes tied to Dolomite precipitation lowers the system pH.

Also, can you explain in words what this line means/does?

swap "CO2(g)" "H+" log fugacity "CO2(g)" = -4.5

In order to calculate pH, the H+ component must be defined in the system. Since you don't trust your calculated pH, we 'swap' in CO2(g) at 8 mbar (express as log fugacity) which allows React to calculate pH based on atmospheric CO2.

The whole business of species vs. components and 'swapping' can be rather confusing and unintuitive when you first start working with GWB. I highly recommend that you read chapter 3 in Craig's book, as this sheds light on the concept of Basis swapping, and the difference between components and species.

Also, I don't even see where I can input ClO4- here. Could you give me more specific instructions for adding it to the system?

You can swap ClO4- in as an aqueous species for any of the basis species that make up it's database reaction. In the thermodynamic database, the reaction for ClO4- is written as follows:

ClO4- = 1 Cl- + 2 O2(aq)

So, you can swap ClO4- in for the chlorine or oxygen component. I've attached a revised example where ClO4- is used to constrain oxygen in the system. However, in this simulation React predicts that it mostly dissociates to free Cl- ion.

Have a look at chapter 3- I think it will help you make a bit more sense of all of this!

Hope that helps,

Tom Meuzelaar

RockWare, Inc.

calc_pH_revised.rea

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So now I'm also trying to add ammonium (NH4+). I tried to add N, or H, or even NH3, and have it work the same way perchlorate did. But these are also not on the list of things you can add. Why is this? How can I get around it and add NH4+ anyways?

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So now I'm also trying to add ammonium (NH4+). I tried to add N, or H, or even NH3, and have it work the same way perchlorate did. But these are also not on the list of things you can add. Why is this? How can I get around it and add NH4+ anyways?

Hello:

By default, NH4+ is coupled to NO3- as a redox couple- this means that GWB will calculate the concentration of NH4+ in the system based on equilibrium with both NO3- and Oxygen (O2(aq) is the 'master redox species' in GWB). If you want to constrain NH4+ independently, you'll need to decouple it from NO3- via the Config - Redox Couples menu option. I'd recommend reading the information in the user's guides and modeling textbook about redox equilibrium and disequilibrium to help you decide whether it's appropriate to decouple or not.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

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Thanks!

So finally I want to try the model using the pH we measured. The pCO2 is still the same (8.3mbars). How can I input this, so that it stops yelling at me that I need HCO3-? (I was guessing at HCO3- previously so I want to take this value out). <_<

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Thanks!

So finally I want to try the model using the pH we measured. The pCO2 is still the same (8.3mbars). How can I input this, so that it stops yelling at me that I need HCO3-? (I was guessing at HCO3- previously so I want to take this value out). <_<

Hi:

In this case, just enter a direct value for the H+ component as pH, and then swap in your pCO2 value, as CO2(g) for the HCO3- component. You are now constraining HCO3- based on partial pressure of CO2.

Let me know if that makes sense,

Tom Meuzelaar

RockWare, Inc.

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