Effect of density in Saturation Index

[DFhidro]

Hi!

I'm calculating the saturation index for brine and noticed that I get very different values depending on whether I input the measured density or not when defining the solution.

These are my data:

Solution 1: 
- Units: mg/l 
- Density: 1.225 
- Cl: 217968 
- Na: 87431 

Solution 2: 
- Units: mg/l 
- Cl: 217968 
- Na: 87431 

When checking my output, the calculated density for Solution 2 is very similar to the density I use in Solution 1: 

Solution 2
- pH: 7.000 
- pe: 4.000 
- Specific Conductance (?S/cm, 25?C): 316867 
- Density (g/cm?): 1.20933 

But I get the following saturation indices:

Solution 1: 
- H2O(g): SI = -1.60, log IAP = -0.10, log K(298 K, 1 atm) = 1.50 
- Halite: SI = -0.21, log IAP = 1.37, log K(298 K, 1 atm) = 1.58 

Solution 2: 
- H2O(g): SI = -1.65, log IAP = -0.15, log K(298 K, 1 atm) = 1.50 
- Halite: SI = 0.24, log IAP = 1.82, log K(298 K, 1 atm) = 1.58 

Why are the saturation indices so different when I enter or do not enter the density?

I?m using Pitzer Database.

Thank you very much!

[dlparkhurst]

Look at the concentrations (mol/kgw) that are calculated.

For mg/L, first the density is used to calculate mol/kgs; then the mass of solutes is subtracted to determine the mass of water; and finally, mol/kgw are calculated.

When a density in not defined, a density of 1.0 is used. In your case, this makes a big difference in concentrations.

Probably best is to let PHREEQC adjust the density to that which, when entered in input would produce the calculated density in output. The advantage of this is that when you use the Basic functions RHO and SOLN_VOL, you will get back your original concentrations; otherwise there will be a difference between your input concentration (in mg/L) and if you calculate them in Basic (totmol("Cl")*gfw("Cl")*1000/soln_vol).

Code: [Select]

Solution 1
-Units mg/l
-Density 1.225
Cl 217968
Na 87431
END

Solution 1
-Units mg/l
#-Density 1.225
Cl 217968
Na 87431
END

Solution 1
-Units mg/l
-Density 1.225 calc
Cl 217968
Na 87431
END