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Stefan

Charge balance problems

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Hello,

I am trying to model the dissolution of amorphous silica at high pH (addition of NaOH). To improve the dataset of Reardon 1992 I added some polymeric Si species from the literature. This was successful in terms of calculated Si concentrations compared to literature data. Unfortunately, with increasing reaction progress the charge balance gets progressively lost.

I have added an extract of the data file as well as an input script. It is running well as it is. But when unsuppressing the second polymeric species the described error occurs. It is getting worse with additional polymeric species (not included in the data file).

Thank you for any help.

Best regards,

Stefan

SiO2_test.dat

SiO2_test.rea

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Hi Stefan:

The charge imbalance message is not an error, but simply a warning that you are not forcing charge balance. A plot of charge imbalance error shows that your initial system, as you've defined, carries a charge imbalance error of 3.2e-4%. At the end of the simulation, this decreases to 3.2e-5%. Is this significant?

Regards,

Tom Meuzelaar

RockWare, Inc.

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Hi Tom,

thank you for your quick reply. I have not made a plot of charge imbalance errors up to now. (By the way: How can I create such a plot?) I just had a look on the listed molalities of the individual species in the output files. For case 1 (only the first polymeric species considered) all is fine. When unsuppressing the second polymere species it starts fine with output ā€œaā€. But when reaching the end of the calculation (last entry in output ā€œdā€) the listed molalities are far from summing up to charge balance.

Regards,

Stefan

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Hi Stefan:

In version 8, you can plot charge imbalance error under the System parameters variable type in Gtplot.

I wonder if we are looking at different output files- when I run the script you sent me, and look at the last reaction path script in the 'd' output file, charge balance adds up fine. What version are you using? (use the Help - About menu to determine version and sub-version).

Regards,

Tom

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Hi Tom

Sorry, I was a bit careless when writing down my questions. Furthermore, for clarity I should have added also the react file for the second case which shows the results I wonder about. This file is now added (SiO2_test2.rea). The only difference to the first file is the species Si2O3(OH)4-- that is now not suppressed. The problem occurs between the calculations c and d. Step #100 of calculation c does not coincide with step #0 of calculation d.

Regards,

Stefan

P.S. I am using React 7.0.6.

SiO2_test2.rea

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Hi Stefan:

Attached is an alternate way to set up your problem, which doesn't run into any of the charge balance issues of the first method.

Let me know if this is helpful.

Regards,

Tom Meuzelaar

RockWare, Inc.

SiO2_test2 - Copy.rea

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Hi Tom

Thank you for this clever trick. I thought about adding NaCl myself. But, I did not believe that 1 nmol would be enough for adjusting a charge imbalance that is ten times higher. It proofs how important it is to make use of the charge balance option of GWB. (However, it seems that the mismatch between step 3 and step 4 is worth a look by the programmer.)

Kind regards,

Stefan

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