report Watact

[p.m.berger]

Hello,

I am trying to use SpecE8 to calculate chemical potential and so need the activities of everything in solution, including water. Can someone clarify if the command "report Watact" reports the activity of water as it specifies in the GWB reference or the activity coefficient? The moles of water in 1 kg is roughly 55.5 but the report command for a dilute system returns 0.999... That would imply the 55 moles have an effective concentration of less than one. Or are solvents treated differently? Assumed to have a base activity of one, or scaled to one mole?

Thanks,

Peter

 

[Jia Wang]

Hello Peter,

 

The command Watact reports the activity of water. The activity of water is consistent activity at temperature and pressure of interest . The activity coefficient for water can be calculated by dividing the activity by the mole fraction of water.

 

Hope this helps,
Jia Wang

[p.m.berger]

Thanks for the reply,

So the water activity results are different from the species activity results? The activity reported for species are act_coef*molality and not act_coef*(mole fraction).

  Aqueous species       molality    mg/kg sol'n    act. coef.     log act.
 ---------------------------------------------------------------------------
   Na+                      2.265     4.598e+04      0.6761        0.1850

Peter

[Jia Wang]

Hello Peter, 

The activity for water is calculated differently than species activity. The water activity in the Bdot method is calculated using the mole number of water, stoichiometric or actual ionic strength, and osmotic coefficient. The equation to calculating the osmotic coefficient is given as power series in equation 8.8 in section 8.1.2 'B-dot model' of the Geochemical and Biogeochemical Reaction Modeling text. The water activity coefficients used in the calculation of the osmotic coefficient is given in the Tables section of the thermodynamic dataset. You can find the equation for the osmotic coefficient and water activity for the hmw activity model in table 8.3 of the GBRM text. 

Hope this helps,
Jia