Example 1

[Ricardo Borba]

Dear David,

In this example, as in many other simulations, we observe cases where log γ > 0, which means that the activity of a species becomes greater than its concentration. Here, the ionic strength of the solution is 6.745 ? 10⁻?, so it was not necessary to use pitzer.dat. Could you explain why this happens (log γ > 0)?

Thanks

Ricardo

[dlparkhurst]

The formulas for activity coefficients in ion-association models can be found in the manual in the SOLUTION_SPECIES section. The form of the equations for charged species is generally to decrease from 1.0 and at higher ionic strengths to increase after a minimum to values that can be greater than 1.0.

For uncharged species, the default is for the log activity coefficient to increase from 1.0 proportionately to ionic strength:

Code: [Select]

log(gamma) = 0.1I

Here is a script that shows the change in activity coefficients for Na+, Cl-, and CO2(aq) as a function of ionic strength.

Code: [Select]

SOLUTION
pH 4 charge
C(4) 1
END
USE solution 1
REACTION 1
NaCl 1
6 in 60 steps
USER_GRAPH 1
    -headings               mu Na+ Cl- CO2
    -axis_titles            "Ionic strength" "Activity coefficient" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X MU
20 GRAPH_Y GAMMA("Na+"), GAMMA("Cl-"), GAMMA("CO2")
  -end
END


[Ricardo Borba]

Thank you!