database with thiosulfate and specific conductance calculation

[mgsephton]

I would like to use PHREEQC to calculate specific conductance for some water analyses with measured thiosulfate (S2O3--) concentrations.  It seems that none of the phreeqc databases that allow specific conductance calculations, also recognise thiosulfate.  Is there a database I am overlooking?

If not, what lines would I need to add to phreeqc.dat, so that phreeqc can correctly use thiosulfate concentrations in the geochemical model?

[dlparkhurst]

You could copy all of the definitions for thiosulfates (SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES)  from llnl.dat. You would have to add diffusion coefficients (-dw in SOLUTION_SPECIES) for all of the thiosulfate species.

However, I suspect that thiosulfates are metastable species relative to sulfate and sulfide, which means that, if you do as outlined above, the thiosulfate species would only exist in the initial solution speciation calculation when you define S(2), S(3), etc. After that, in any redox-equilibrium reaction calculation, the thiosulfate species will not exist in appreciable concentrations. So if you want to do reaction calculations, it will require a more complicated definition of species that somehow isolates the the thiosulfate reactions, although, I am not sure how you would pose the reactions.

[mgsephton]

An example of the solution I want to calculate specific conductance for is this:

   SOLUTION 706
   pH 7.520000
   Temp 21.200000
   pe 6.923597
   units ppm
   Alkalinity 27.800000
   Ca 578.297164
   Si 8.639984
   S(6) 1333.000000
   S(2) 322.000000 as S2O3
   Zn 0.005164

If I use this description as phreeqc input with the llnl.dat database, there are no sulphur species outside of the S(2) and S(6) groups in the output.

So why is it necessary to include data for sulphur species in the S(3), S(4), S(5) and (S7) groups the in the database?

Thank you

[dlparkhurst]

If  all you want is to speciate the two redox states, then you do not need the rest. If you use that solution in any other reaction (EQUILIBRIUM_PHASES, SURFACE, REACTION_TEMPERATURE, etc), then the other species will be included and redox equilibrium will be calculated. (And you probably end up with no thiosulfate species.)