Jump to content
RockWare Support Forum
hredzak

What's the deal with charge balance?

Recommended Posts

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!

Share this post


Link to post
Share on other sites

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

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

Share this post


Link to post
Share on other sites

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!

Hi:

You have two choices:

1) Run the simulation without charge balance - in the units drop-down box, simply select the 'balance off' option.

2) If you have confidence in all of your fixed concentrations, but your overall solution has significant charge balance error, you can add the 'missing' component to your Basis without specifying a concentration. Set charge balance to this component, and SpecE8/React will add sufficient mass to achieve charge balance.

I hope that helps,

Tom

Share this post


Link to post
Share on other sites

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

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

Hello:

If you attach a copy of your script, I'd be glad to take a look.

Regards,

Tom Meuzelaar

RockWare, Inc.

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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:

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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...

Share this post


Link to post
Share on other sites

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?

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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?

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

Hi Tom,

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

Best,

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

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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!

There is an mineral called BaCl2© in the default database you are using (thermo.dat). I don't know much about BaCl- are you titrating this in as a solid?

You could check some of the other databases to see what minerals/species they have- have a look at section 2.3 Thermo datasets in the v8 GWB Essentials Guide.

Also- if you use the Config - Show option in React, you can quickly find all minerals or aqueous species in the database you've loaded for a given component.

Hope that helps,

Tom

Share this post


Link to post
Share on other sites

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

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

Share this post


Link to post
Share on other sites

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.

In Gtplot, just generate a plot of Minerals vs. Rxn progress. If you don't see BaSO4 precipitating, I'd plot Mineral Saturation vs. Rxn progress.

Regards,

Tom

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites

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!

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

Regards,

Tom

Share this post


Link to post
Share on other sites

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!

Almost. It looks like Barite and Witherite both precipitate along the course of the reaction path- Barite precipitates at the beginning of the reaction path, and continues to precipitate until there's .007 molal in the system. Witherite begins to precipitate about 20% into reaction path progress, and continues until there's 2e-4 molal in the system.

At 40% reaction path progress (Xi = .4), both minerals reach saturation, and there is no further precipitation. However, the mineral masses remain in the system.

I hope that helps,

Tom

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×