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Geochemist's Workbench Support Forum

Some problems about Eh-pH Diagram


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Hi Kelvin:


I'm not sure why the sharp boundary exists at the temperature interval you give; it's difficult to troubleshoot given the large number of components in the diagram. The trouble is that you have 7 additional species (all speciating) in addition to your diagram species and axes, rendering the diagram nearly useless. When working with that many components, it's a better idea to move to speciation and reaction path models. Try building this system in React, and then sliding your pH and Eh values to understand the behavior of your system better.




Tom Meuzelaar

RockWare, Inc.

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There is something strange about your diagram. In the medium pH-range, moving from reduced to oxidized conditions, you calculate the predominance of Uranite (UO2, U+IV), then UO2.25 and UO2.333 (both of which are questionable in aqueous equilibrium systems anyway and which represent more oxidized forms of uranium), and then again Soddyite, which is (UO2)2(SiO4)•2 H2O(cr): that is again U+IV phase! This seems to me a hint for a serious bug in your database.


Apart from that you should avoid calculating with uranium phases which contain mole numbers of oxygen which cannot be summed to whole numbers. Depending on the intial boundary conditions issued to GWB, you force the program to introduce charge inbalances by violating the mass balance. Thus, UO2.333 should be transformed to U3O7 (while three times UO2.333 gives U3O6.999 !).





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