SELECTED_OUTPUT

[ehsan]

Hi
How to get activity coefficient carbon dioxide(aq) and water of the user_punch in SELECTED_OUTPUT?

[dlparkhurst]

USER_PUNCH.

[ehsan]

Code: [Select]

     SELECTED_OUTPUT ; -file  output; -reset  false; -ionic_strength  false
     USER_PUNCH
     10 s$ = s$  + STR$(GAMMA("HCO3-")) + STR$(ACT("H2O"))
     20 PUNCH s$I used of ACT for activity coefficient of water and GAMMA for CO2 activity coefficient.
because GAMMA don't work for H2O
 

[dlparkhurst]

Yes, the activity of water does not use the same approach of gamma * molality as all other aqueous species. Solvent water activity is essentially the mole fraction of water in the solution and the gamma is meaningless and set to 1 for ion association models.

For Pitzer and SIT databases, the osmotic coefficient is calculated and the activity of water is related to it by the relation, where mi is molality of the ith aqueous species and Mw is 18 g/mol:

ln(aw) = -(Σ mi · ϕ· Mw /1000)

[ehsan]

Thanks David
I use the Phreeqc database to model a mixture of CO2 brine and I need to get the solution(brine) activity coefficient and the CO2 activity coefficient from the Phreeqc output.
And I don't know how to do that.
Do i use GAMMA(HCO3-) or GAMMA(CO2) for CO2 activity coefficient, GAMMA(H2O) or GAMMA(H+) for solution?

[dlparkhurst]

As usual, I don't understand your question. GAMMA("H2O") is not a meaningful quantity in PHREEQC. The activity of water that is used in all mass-action equations, is simply ACT("H2O") or log activity LA("H2O"). If you want the solution activity coefficient of CO2(aq), it is GAMMA("CO2") or LG("CO2") for the log10 quantity.

The following relations exists among the Basic functions, where the second involves log10 quantities.

Code: [Select]

ACT("CO2") = GAMMA("CO2")*MOL("CO2")
LA("CO2") = LG("CO2") + LM("CO2")

The following fugacity relation for CO2(g) [fugacity is equal to SI("CO2(g)")] exists for the solution using Basic functions.

Code: [Select]

SI("CO2(g)") = LA("CO2") - LK_PHASE("CO2(g)")

At equilibrium, the fugacity of CO2(g) calculated from the solution is equal to the fugacity of CO2(g) in the GAS_PHASE, which is partial pressure of CO2(g) times the fugacity coefficient:

Code: [Select]

LK_PHASE("CO2(g)") = LA("CO2") - LOG10[PR_P("CO2(g)") * PR_PHI("CO2(g)")]

When defining a GAS_PHASE, the quantity defined is the pressure of CO2(g), here 100 atm.

Code: [Select]

GAS_PHASE
CO2(g) 100

Similarly with EQUILIBRIUM_PHASES, the quantity defined is log10 of the pressure of  CO2(g), 10^2 or 100 atm in this example. So, you are setting the pressure rather than the fugacity. With Peng-Robinson calculations (phreeqc.dat, Amm.dat, and pitzer.dat), pressure is not equal to fugacity; with all other databases, fugacity equals partial pressure.

Code: [Select]

EQUILIBRIUM_PHASES
CO2(g) 2


[ehsan]

Which keyword determines the water activity coefficient in USER_PUNCH?

[dlparkhurst]

Here are two previous posts in this thread:

"The activity coefficient GAMMA("H2O") is not a meaningful quantity in PHREEQC. The activity of water that is used in all mass-action equations, is simply ACT("H2O") or log activity LA("H2O"). "
---------

"Yes, the activity of water does not use the same approach of gamma * molality as all other aqueous species. Solvent water activity is essentially the mole fraction of water in the solution and the gamma is meaningless and set to 1 for ion association models.

"For Pitzer and SIT databases, the osmotic coefficient is calculated and the activity of water is related to it by the relation, where mi is molality of the ith aqueous species and Mw is 18 g/mol:

ln(aw) = -(Σ mi · ϕ· Mw /1000) "

[ehsan]

Ok thank
Because I saw log10 (h20) = 0, I thought gamma h20 = 1.