Turn Off Redox?

[JD]

Hi,

I'm new to PHREEQC.  I'm trying to model the speciation, pH, and density of a salt mixture in a high ionic strength solution (~8 M).  Is there anyway I can turn off the redox reactions?

For example, the following input predicts an initial ~1M NaNO3 solution will have a pH of about 6.5.  But then if I react it with nothing (baseline sanity check), I generate a substantial amount of N2 gas and increase the pH to around 12.5.

Example Input:

SOLUTION 3 1M NaNO3
    temp      55
    pH        7 charge
    pe        4
    redox     pe
    units     mol/l
    density   1
    N(3)      1
    Na        1
    -water    1 # kg
REACTION 3
    1 moles in 1 steps

[dlparkhurst]

You can disable the production of N2(aq) by decreasing the log K for the species. Add the block below to your input file. However, there is something else that is getting oxidized, so the nitrogen is just half of the story, and there may be other redox reactions.

Also, with concentrated solutions, it would be better to use the Pitzer approach for solution thermodynamics. Unfortunately, nitrate is not available in pitzer.dat. If you have a limited system, just NaNO3, it should be possible to find interaction coefficients. Not sure how good the density calculation will be even if you go to the effort, because the volume parameters may need fitting.

SOLUTION_SPECIES
2 NO3- + 12 H+ + 10 e- = N2 + 6 H2O
   #-log_k   207.08
        -log_k 0
   -delta_h -312.130   kcal
   -dw    1.96e-9
   -Vm 7 # Pray et al., 1952, IEC 44. 1146

[MichaelZ20]

It should be N(5) instead of N(3) in your input file.