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Ion Size Parameter


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I am looking to add a number of basis species to thermo.com.V8.R6 and am confused by the ion size parameter. From what I gathered from reading the GWB reference docs, it is not the ionic radius? 

For example, bellow are a couple reactions I am trying to add, with the new species in bold:

Mo+3  +  2H2O <--> MoO2+  +  4H+  +  2e-       Log K = 9.96

MoO2+  +  2H2O <--> HMoO4-  +  3H+  +  e-      Log K = 15.97


I've attached my database and the source pdf for the reactions (table 3). How can I find the ion size?




Vlek and Lindsay (1977)_Molybdenum in Soils.pdf thermo.com.V8.R6+_Scotts dataset.tdat

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  • 1 month later...

Hello Scott,

I am sorry I missed your post from earlier. To answer your question, you are right that the ion size parameter does not reflect the ionic radius of the ion. In practice, this parameter is obtained by fitting the Debye-Huckel equation to experimental data to account for effects such as hydration sphere surrounding the ion of interest. These values may differ between datasets and whether or not the activity model is using the classic Debye Huckel equation or B-dot model (extension of the classic D-H equation). At moderate ionic strength, the affect of different ion size parameter is relatively small compared to ion charge and other uncertainties. If you would like to read about the theory, a good place to look would be Jacob Kielland's publication for ion activity coefficients. He reports the results for common ion size parameter calculated for the Debye-Huckel equation. 

By the way, the best place to post a new topic is on the GWB forum front page. We aren't notified when new topics are posted in the archive section. The archive section is a great resource for looking at previous user questions and corresponding answers. 

Best regards,
Jia Wang

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