Defining the amount of moles in "EQUILIBRIUM_PHASES"

[seanshi40@gmail.com]

Hello All,

I have a question about the "EQUILIBRIUM_PHASES" and how I should define the amount of moles.

I have a simple system such as:
SOLUTION 1
    temp      60
    pH        7
    pe        4
    redox     pe
    units     mol/l
    density   1
    Ca        0.01
    S(6)      0.01
    -water    1 # kg
REACTION 1
    H2O(g)     -1
    43.2 moles in 1 steps
USE solution 1
EQUILIBRIUM_PHASES 1
    Anhydrite 0 0
    Gypsum    0 0
END

When I define the followings I get different answers in terms of solution volume, density, etc.:

EQUILIBRIUM_PHASES 1
         Anhydrite 0 0
         Gypsum    0 0
or

         Anhydrite 0 10
         Gypsum    0 10

I know that the first number represents the saturation index and the second number is the amount of moles. But, I was wondering what is the difference when I define two different amount of the moles? And, how should I define the amount of moles.

Thank you in advance.
M

[dlparkhurst]

The key is that Gypsum has two waters of hydration.

At low ionic strength, Gypsum is the stable phase. If you have 10 moles of each mineral, all of the anhydrite will dissolve and reprecipitate as gypsum. The gypsum will remove 20 moles of water, but that only increases concentrations by about 60 percent. In the first reaction calculation, the Na concentration increases fro 1 to 1.56.

At high ionic strength, the activity of water decreases, meaning that the saturation ratio will decrease for gypsum will decrease because of the a(H2O)^2 in the saturation ratio. In the second calculation, gypsum starts out undersaturated and dissolving, but by adding H2O to the solution, the activity of water increases to the point that both gypsum and anhydrite are in equilibrium.

The number of moles you should use in EQUILIBRIUM_PHASES depends on your conceptual model of the system you are simulating.

Code: [Select]

SOLUTION
-units mol/kgw
Na 0.001
Cl 0.001
EQUILIBRIUM_PHASES
Gypsum 0 10
Anhydrite 0 10
END
USE solution 1
USE equilibrium_phases 1
REACTION
NaCl 1
10
END

[seanshi40@gmail.com]

Thanks a bunch for the explanation.
Regarding the last line in the reply, I was wondering how I should define the number of moles. Are there any guidelines or a general rule? Or, it will be safe if we always choose 0 for saturations and 10 for number of moles?

Best,
SeanShi

[dlparkhurst]

In many cases, if a sparingly soluble mineral is present, it is present in sufficient quantity relative to water that it will persist for many many, many pore volumes. In this case, 10 moles indicates an unlimited supply.

If there is a limited amount of mineral relative to its solubility, then your choice determines how quickly or how many pore volumes are necessary to remove the mineral from the system.

If the mineral is not present initially, then it should receive zero moles and will only precipitate if the saturation index increases to zero.

[seanshi40@gmail.com]

Thanks a bunch!