Processes > Oxidation and reduction equilibria

Redox calculation example

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Charlie:
#Simulation of redox reactions with the stepwise addition of organic carbon. (Based on example in Appelo and Postma, 2005)

TITLE Redox calculations

SOLUTION 1
        units mmol/kgw
   pH   6.5
   Na   1.2
   Cl   1.5
   O(0)   0.12
   N(5)   0.06
   Mg   0.12
   S(6)   0.09
   Ca   0.07
   Alkalinity   0.3

EQUILIBRIUM_PHASES 1
   Goethite   0   2.5e-3
   FeS(ppt)   0   0
   Pyrolusite   0   4e-5

REACTION 1
   C
   0.6E-3 in 25 steps
INCREMENTAL_REACTIONS true

USER_GRAPH
   headings C   O2   NO3   Mn   Fe   SO4   S(-2)   C(-4)
   axis_titles   "Carbon added (mmol/L)" "Concentration (mol/L)"
   -start
10 graph_x step_no * 0.6/25       #converted from mol/L to mmol/L
20 graph_y tot("O(0)")/2, tot("N(5)"), tot("Mn(2)"), tot("Fe(2)"), tot("S(6)"), tot("S(-2)"), tot("C(-4)")
   -end

END

JG:
Hi. I ran this.

(there is a missing parenthesis in Basic line 20: => ... tot("N(5)"), ... )

Not much is happening. Not enough C added?

dlparkhurst:
Your solution is defined as mol/kgw. So, you have 112 mmol of O(0) and add only a fraction of a millimole of C, which makes only a small dent in the O2(aq) concentration. You probably mean mmol/kgw for your solution; then, mmol of C will make a difference.

Charlie:
Thanks - have amended the script above

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