Another way of decoupling in Example 9

[MichaelZ20]

I tried to decouple Fe(2) and Fe(3) by decreasing log_k of the reaction Fe+2 = Fe+3 + e-, but this doesn't work, and I don't understand why. I attach the input file and will be very grateful for the help.

[dlparkhurst]

No, it will not work.

First, I don't think you can use valence states in the -formula definition, so you cannot remove Fe(2) or add Fe(3).

Second, I think that removing the Fe+2 -> Fe+3 + e- reaction would not allow Fe(3) species to form under any conditions.

I think you mus either use the example 9 approach, or the following, where there is not oxygen in the system, and you titrate oxygen into the solution to oxidize Fe(2). I have extended the time because the rate seems too slow otherwise.

Code: [Select]

TITLE Example 9. Kinetically controlled oxidation of ferrous iron.
      Decoupled valence states of iron.

SOLUTION 1
    pH  7.0
    #pe 10.0  O2(g) -0.68 # 21% O2 in air; log(0.21) = -0.678
    Fe(+2)  0.1
    Na  10.
    Cl  10.  charge

#EQUILIBRIUM_PHASES 1
#    O2(g)    -0.68 10

RATES
Fe_di_ox
-start
10  Fe_di = TOT("Fe(+2)")
20  if (Fe_di <= 0) then goto 200
30  p_o2 = 10^(SI("O2(g)"))
40  moles = (2.91e-9 + 1.33e12 * (ACT("OH-"))^2 * p_o2) * Fe_di * TIME
200 SAVE moles
-end

KINETICS 1
Fe_di_ox
    -formula  O 0.5 #Fe(+2)  -1.0  Fe(+3)  1.0
    -m0  0.1
    -steps  864000000 in 60 steps  # 10 days in sec

INCREMENTAL_REACTIONS true

USER_GRAPH
-headings Time Fe(+2) Fe(+3) pH
-initial_solutions true
-axis_titles "Time, days" "Fe(+2), Fe(+3), umol/kgw" "pH"
-start
10 GRAPH_X SIM_TIME/3600/24
20 GRAPH_Y TOT("Fe(+2)")*1e6, TOT("Fe(+3)")*1e6
30 GRAPH_SY -LA("H+")
-end
END


[MichaelZ20]

David, thank you for your help!