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Residuals too large

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What can I do if the results pane show me 

Residuals too large, 702-th iteration

Cl-                                       Resid 0.0001415, Resid/totmol1.052E+200, Cbasis 5.618E-211

Im trying to model soprtion of heavy metals in Fe and Mn oxyhydroxides under reduced conditions (negative Eh).

Thank you

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Guest Melika Sharifi

Dear Abelo,

Please attach your script so we can take a look at it.

Thanks,

Melika Sharifi

Aqueous Solutions LLC

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Hello Twq,

I took a quick look at your script and noticed that all your concentrations besides AsO4--- are extremely low. If you are trying to ask the model to charge balance with such low concentration values concentrations, it might lead to stability issues and I think that is the case here. If you want to preserve charge balance, you might want to set a higher background electrolyte concentration, such as 1 or 0.1 molal for Na+. Alternatively, you can also turn off charge balance. To do so, right click on the unit of a basis species and select 'Do not balance'. You might also want to double check your chemical analysis to make sure your initial system concentrations are correct. 

I see that you swapped in AsO4--- for As(OH)4- and in doing so, the program calculates the species of other As oxidation states base on redox equilibrium. However, you can disable this by deoupling AsO4--- and adding it as a basis species. By decoupling the redox couple, you can constrained AsO4--- independently in your calculations. You might want to try this if you are interested in the speciation of AsO4--- only. Please refer to section 2.4 Redox Equilibrium for more information on decoupling. 

In the future, you can also check the equilibrium state of your initial fluid before any reaction path takes place by doing a 'Go initial' run ("Run" --> "Go initial"). This is a useful method to check your basis pane setup before adding in complexities of reactants. If the equilibrated fluid from the initial pane looks correct to you, then issues that occur will likely be due to the reactions from the reaction pathway. 

Hope this helps,

Jia Wang 

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