Gas solubilities in liquid solutions

[micho_95]

Hello everyone!
I am glad to joing the PHREEQC users community.
I've decided to use this software to compare my experimental data to the ones simulated by it.
but i find that the manual is a bit hard to handle so, after many trials, i've decided to ask my question here because i wasn't able to solve it myself.

I would like to simulate the hydrogen solubility in water (pure or brine). First, i tried with pure water and i followed the example 22, page 446 in the phreeqc manual (i am not an expert and i honestly don't know how to write my own code):

SOLUTION 1 Pure water
pH 7.0
temp 25.0
GAS_PHASE 1
    -fixed_volume
    Hdg(g) 0
    H2O(g) 0
REACTION
 Hdg 1;
 0 27*1
INCREMENTAL_REACTIONS true

up to this point, i don't understand why there is the number (0), why is H2O in the gas phase even if i want it to be a liquid solution. i put the hdg instead of h2 or h2(0) because @dlparkhurst corrected it to another user. thank to him, his comment was very useful. i also do not understand what is o 27*1. sorry for that
i'll not post the rest of my code because it is a mess.
so to summarize, i would like to simulate the hydrogen solubility in liquid water or brine and have an output a value of the number of H2 moles/kg water vs pressure.
any help is appreciated and i thank you so much, in advance,
Best regards!!
M

[dlparkhurst]

The GAS_PHASE data block defines a gas with two compontents Hdg(g) and H2O(g). The initial composition of the gas phase has 0 partial pressure for each gas. By default the volume of the gas phase is 1 L.

The SOLUTION is defined to be pure water at a temperature of 25 C. To that solution the REACTION adds specified amounts of Hdg to the solution. First 0 moles of Hdg are added, then 27 moles of Hdg are added in 1 mole increments. Note that the 27*1 notation is interpreted in this way because of INCREMENTAL_REACTIONS true (if false, then the same 1 mole addition to pure water would be repeated 27 times). Hdg paritions between the aqueous phase and the gas phase. The graph below plots the pressure of Hdg as a function of Hdg aqueous concentration (molality).

H2O(g) refers to water vapor. Water, as well as Hdg, will partition between the solution and the gas phase. At 25 C the contribution of water vapor to the total pressure of the gas phase will be negligible compared to the pressure of Hdg(g) under these conditions, but at high temperature and lower Hdg concentrations, H2O(g) could be important.

Finally, the calculation uses the Peng-Robinson equation of state rather than the ideal gas law. Thus, the Hdg(g) pressure is not equal to fugacity. Fugacity = pressure * phi / 1 atm.

Code: [Select]

SOLUTION 1
-temp 25
-pH 7
END
USE solution 1
INCREMENTAL_REACTIONS true
REACTION 1
Hdg 1
0 27*1
GAS_PHASE 1
-fixed_volume
-volume 1
Hdg(g) 0
H2O(g) 0
USER_GRAPH 1
    -headings               Hdg P_Hdg(g) F_Hdg(g)
    -axis_titles            "Hdg, molality" "Hdg(g) Pressure, atm and fugacity, unitless"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
20 GRAPH_X TOT("Hdg")
30 GRAPH_Y PR_P("Hdg(g)"), 10^SI("Hdg(g)")
  -end
    -active                 true
END


[micho_95]

Hello again,
sorry for the late reply, I was forced logged out.
thank you very much @dlparkhurst, everything is working fine now.
You are so kind and gentle
Best regards,
MT