By using the same chemical symbol, As, you need a reaction that connects As(3) to As(5). This is the reaction from wateq4f.dat. # H3AsO3 = H3AsO3 # log_k 0. H3AsO4 + 2H+ +2e- = H3AsO3 + H2O logk 18.897 delta_H -30.015 kcal However, the consequence of doing this is that As(3) and As(5) will always react to redox equilibrium. If you want As(3) independent of As(5), it is possible to disconnect the redox states, but As(3) would have to be renamed and all the reactions redefined. The following example separates As(3) from As(5) and As(5) will not react to As(3) or vice versa. If you want minerals or other reactions, you would have to rewrite them to use "Arsenite" as in the following example. One final possibility, probably much more than you want, is that, given the separation defined below, it would be possible to write a KINETIC reaction that would transfer Arsenite to As(5) according to some rate expression. David SOLUTION_MASTER_SPECIES Arsenite H3ArseniteO3 0.0 74.9216 74.9216 SOLUTION_SPECIES #Arsenite3 master species H3ArseniteO3 = H3ArseniteO3 log_k 0 #H2ArseniteO3- 478 H3ArseniteO3 = H2ArseniteO3- + H+ log_k -9.228 delta_h 6.56 kcal #HArseniteO3-2 479 H3ArseniteO3 = HArseniteO3-2 + 2H+ log_k -21.33 delta_h 14.2 kcal #ArseniteO3-3 480 H3ArseniteO3 = ArseniteO3-3 + 3H+ log_k -34.744 delta_h 20.25 kcal #H4ArseniteO3+ 481 H3ArseniteO3 + H+ = H4ArseniteO3+ log_k -0.305 SURFACE_SPECIES # Arsenite Hfo_wOH + H3ArseniteO3 = Hfo_wH2ArseniteO3 + H2O log_k 5.41 David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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