[What's the deal with charge balance?]

Can you attach the script in it's current state so I can take a look?

 

Regards,

 

Tom

 

Absolutely, thank you Tom!

9-9-09Run.rea

[What's the deal with charge balance?]

Yes- any component you titrate into the Basis system must also be present in the Basis. If you don't have a lab value, just specify a very small Ba++ concentration in the Basis.

 

Regards,

 

Tom

 

Hi Tom,

 

Okay, I think I got it - but for the mineral view I want to make sure I'm interpreting these predictions right. So basically for the barium minerals, witherite and barite will both form in the solution in small amounts (up to 10-4M), then where these curves level off it is precipitaing out of solution at that point?

 

Thank you!

[What's the deal with charge balance?]

One more question, how can I tell what will be precipitated? (Can I tell what will be precipitated?) Essentially I want to know if it will be BaSO4 and see how much.

[What's the deal with charge balance?]

Also - I guess I need Ba++ concentration in my starting solution?

[What's the deal with charge balance?]

Sure- keep your original solution in the Basis tab; in the Reactants tab, choose Add- Simple Reactant - Aqueous - BaCl2; specify the mass to be titrated, and click Run.

 

You'll have a lot of output data, so I recommend moving to plotted output (Gtplot) for analysis.

 

Hope that helps,

 

Tom

 

Under simple reactant - aqueous I only have BaCl+. Under minerals I have BaCl2*H2O and BaCl2*2H20. Do I have to do something to get just BaCl2 as an option? Thanks Tom!

[What's the deal with charge balance?]

Hi Tom,

 

Thanks again for all you help! Now can you walk me through the steps of a BaCl2 titration into this system?

 

Best,

 

 

Hi:

 

Gas fugacities in GWB are expressed in atmosphere units - 1 atm = 1.01325 bar (or 1013.25 millibar). The value for CO2 reflects the composition of CO2 in the atmosphere, about .035%, or .00035 atm (~.355 millibar).

 

I hope that helps,

 

Tom

[What's the deal with charge balance?]

If I have a partial pressure of 8mbar CO2, how do I translate that to the "log of a fugacity"? Is that an equation? How did you get -3.5?

 

 

Glad to be of help.

 

One way to constrain HCO3- is to set a CO2 fugacity (partial pressure of CO2 in your system)- if your fluid is equilibrated with the atmosphere, for instance, add HCO3- to your Basis, swap in CO2(g), and set a log fugacity value at -3.5.

 

Best regards,

 

Tom

[What's the deal with charge balance?]

Hi again Tom,

 

Thanks for your patience with my many question! So I just talked with my supervisor - as for cations both Ca++ and Mg++ can be varied, as they are supersaturated in our solution. We don't know the HCO3-, can it calculate it?

 

 

Great point - I think part of the problem is that these are all the set concentrations of things we could measure. There's probably something else in the system, but we don't know what...

[What's the deal with charge balance?]

Great point - I think part of the problem is that these are all the set concentrations of things we could measure. There's probably something else in the system, but we don't know what...

 

Hi:

 

I think the question to ask- if all the concentrations are set, then why the big discrepancy in charge balance? I'm not sure what to suggest without knowing more about your water.

 

Regards,

 

Tom

[What's the deal with charge balance?]

Hi Tom,

 

Thanks for getting back to me. So the problem is that all those concentrations must be pretty much set. I can't allow say Na+ to change. What do you suggest I use that's not already in the picture?

 

 

Hi:

 

In this case, it looks like you have a shortage of cations, not anions, which is why React has difficulty balancing on either HCO3- or Cl-. If you set charge balance to Na+, React converges without trouble. Similarly, you can speciate the water without charge balance and there are no convergence issues.

 

I hope that helps,

 

Tom

[What's the deal with charge balance?]

Thanks Tom! So I've set HCO3- to charge balance, now I have this error:

 

 

Residuals too large, 663-th interation

Largest residual(s):

Resid Resid/Totmol Cbasis

--------------------------------------------------------

HCO3- 0.0002174 1.723e+200 4.703e-203

--------------------------------------------------------

 

8-31-09 Run.rea

[What's the deal with charge balance?]

For some reason I'm getting this error message anyways:

 

 

Caution: molality of Cl- component forced negative by charge balance: -0.0002024

*N-R didn't converge after 999 its., maximum residual = 1, Xi = 0.0000

Largest residual(s):

Resid Resid/Totmol Cbasis

--------------------------------------------------------

ClO4- 2.072e+006 1 3.293e-009

--------------------------------------------------------

[Closed vs. Open System]

Okay, I understand the difference btw an open and closed system. But I have somewhat of an interesting case - where the solution is exposed to and interacting with external atmosphere which has circulated into the headspace of the beaker. But it's not an open system in a traditional sense... Would this still be considered an open system or closed?

[What's the deal with charge balance?]

Hi,

 

So Cl- usually comes up as the thing to be used to charge balance. But my Cl- concentration is more or less fixed, so I don't want to use that. But what else can I use that won't change my other fixed concentrations?

 

Thanks!

[Back calculate pH in GWB Standard]

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).

[Back calculate pH in GWB Standard]

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?

[Back calculate pH in GWB Standard]

This is all really helpful!! Thanks so much!!

[Back calculate pH in GWB Standard]

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!

[Back calculate pH in GWB Standard]

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!!!