Calculation the pH of salt waters (KCl, NaCl, MgCl2, and CaCl2)

[Mohamadreza]

I want to set the pH value of different salt waters in the SOLUTION part of phreeqc. For example:

Code: [Select]

SOLUTION 0 formation water
pH ?
pe 4
redox pe
units   mol/kgW
density 1.023
C  1e-2
Ca+2  1.2
Cl- 1.2
water 1 # kg
According to the amount of salt, how much pH should it be?
or
How do I calculate the pH of different salt waters at different pressure, temperature, and molality? I was not found suitable reference (experimental data, correlation, or general formula) for calculation pH of salt waters.

Many thanks for your help.

[dlparkhurst]

If you are adding CaCl2 to your water, the Cl would be twice the concentration of Ca. But assuming you have given careful thought to your solution composition, you could calculate pH through charge balance. That would give a pH near 14.

Code: [Select]

SOLUTION 0 formation water
pH 7 charge
pe 4
redox pe
units   mol/kgW
density 1.023
C   1e-2
Ca  1.2
Cl  1.2
water 1 # kg
END


[Mohamadreza]

Thanks. But I want to calculate the pH before running phreeqc because I need to set the concentration of H+ and OH- in the CFD part of the software. So, I want to calculate the pH of different salt waters then get the H+ concentration from the pH. So, could you please refer me to a suitable reference (experimental data, correlation or general formula) to calculate the pH of salt water.

I truly appreciate your time.

[dlparkhurst]

Sorry, but I think you have to run PHREEQC first to get the values for your CFD. PhreeqcRM does not use H+ or OH- as primary input, rather it uses the total moles of H, O, and charge, plus the number of moles of each element  in a solution.

The strategy of PhreeqcRM is to run an input file with one of the IPhreeqc instances in PhreeqcRM. After the input file is run, the definitions of all of the solutions and reactants needed for the reactive-transport simulation are known, including the total moles of H, O, charge, and solutes for each solution. These solutions and other reactants (EQUILIBRIUM_PHASES, EXCHANGE, SURFACE, etc) are then transferred from the IPhreeqc instance to the cells of the model with a PhreeqcRM method.

If your reactive-transport model does not use a PHREEQC input file to define the initial and boundary conditions, I can't really help you. You will have to figure out how to generate the input that you need for your model on your own.

Hopefully, the pH of your saltwater is measured. If not I don't think you could do better than to use the capabilities of PHREEQC to calculate a pH.

[Mohamadreza]

I understand how phreeqcRM works, thank you.

What the formula does phreeqc use to calculate pH of saltwaters?

[dlparkhurst]

Then why are you not using the approach intended for PhreeqcRM? It is expected that definition or calculation of pH would be done in the Initial Phreeqc instance and the resulting solutions would be used for intitial an boundary conditions. I do not see another way to accurately determine the needed quantities, for instance, the concentration (often mol/L) of H or O in each cell that needs to be transported.

There is no formula for pH. a(H+) is one of many unknowns calculated by solving a set of non-linear equations. The unknowns include a(H+), a(e-), a(H2O), activities of master species of elements in the chemical system, activities of non-master species, ionic strength, activity coefficients, and possibly other unknowns related to mineral equilibrium, exchange, surface complexation, solid solutions, and gas equilibrium.

The equations include mole balance on elements, charge balance, ionic strength, activity of water, mass action equations for aqueous species, activity coefficient equations, and other equations related to mineral equilibrium, exchange, surface complexation, solid solutions, and gas equilibrium.

The nonlinear set of equations is solved by linearization with the Newton-Raphson approach and sequential adjustment of the unknowns until a solution to the nonlinear equations is achieved. The solution includes values for all of the unknowns, including a(H+).

Often pH is associated with the charge-balance equation, but that is misleading. The pH is that which, along with the values of all the other unknowns, solves the set of nonlinear equations. pH cannot be calculated independently of all the other unknowns and equations.

[Mohamadreza]

Thank you.

The input file for CO2 injection in CaCl2 solution (salt water) as below:

Code: [Select]

SOLUTION 0 formation water
pH 7
pe 4
redox pe
units   mol/kgW
density 1.023
C  1e-4
Ca 1.2
Cl 1.2
water 1 # kg


REACTION_TEMPERATURE
60

EQUILIBRIUM_PHASES
CO2(g) 2.064 10

END
Run the simulation then the pH started from 7.6 and decrease. So, How do I know that the value of pH is correct? I was change the value of C and pH is changed. I'm confused because I don't know how much the pH value of CaCl2 solution (salt water) should be?

I was change the pH to:

Code: [Select]

pH 7 charge

then

the pH value started from 12.5.

There is no way to know what the pH value of the salt water is before running the simulation?

[dlparkhurst]

No, there is no way to know the pH if you keep changing the conditions and reactions.

If you want a CaCl2 solution (not the one you specified) and you specify the partial pressure of CO2, then the pH can be known.

You have one more question, use it well.

Code: [Select]

SOLUTION 1
-pressure 100
pH 7 charge
END
USE solution 1
REACTION
CaCl2 1
1.2 moles
EQUILIBRIUM_PHASES
CO2(g) 2.06 10
END


[Mohamadreza]

I used this code but the phreeqcRM get error:

Code: [Select]

.
.
.
Chem cell 2370 = Grid cell(s): 2370
Chem cell 2371 = Grid cell(s): 2371
Chem cell 2372 = Grid cell(s): 2372
Chem cell 2373 = Grid cell(s): 2373
Chem cell 2374 = Grid cell(s): 2374
Chem cell 2375 = Grid cell(s): 2375
Chem cell 2376 = Grid cell(s): 2376
Chem cell 2377 = Grid cell(s): 2377
Chem cell 2378 = Grid cell(s): 2378
Chem cell 2379 = Grid cell(s): 2379

ERROR: PhreeqcRM failed.
ERROR: PhreeqcRM::RunCells


[dlparkhurst]

Sorry, I can't help with the code you are using. I don't know how it works or what you have tried to simulate.

I can only help with PHREEQC scripts or the reactive transport models PHAST and VS2DRT.

Good luck with your research.

[Mohamadreza]

Thank you very much.

[Mohamadreza]

Hi, Mr. Parkhurst.

Excuse me. I have a question. How do I calculate the concertation of Mg and Cl ion of 2 mol/kgw of MgCl2 for input file of phreeqc?

Code: [Select]

SOLUTION 0 formation water
pH 7
pe 4
redox pe
density 1.124
units   mol/kgW
C  1e-3
Mg ?
Cl ?
water 1 # kg
END
I truly appreciate your time.