I am still having problems with my model. When I model the system in React, with sorbing surfaces included, it crashes as one iron solid phase is depleted and the second one approaches its maximum. When the model is run without sorbing surfaces, it runs to completion, but at the point where all of the ferrihydrite is gone there is a change in pH and Eh that the moeld with sorbing surfaces doesn't get through, i.e. it crashes at that point. I have attached the sorbing basis species and surface species I am trying to model as a .txt file. The general model is simple, I am trying to model adsorption of As on two iron surfaces under decreasing redox potential. The log Ks are from the literature and are consistent with the thermo.com.v8.r6+ database. When I model this in React, I start with 5 free grams of Fe(OH)3 and titrate 50 mmol acetic acid to drive the Eh to reducing conditions. My React script is include at the bottom of the attached .txt file. To get Fe(OH)3 as the metastable phase I have to suppress the other iron oxides. Do you have any suggestions that can help the model get through the point where it crashes?