Why Re is different?

[Zhaoyang]

Dear all,

Solution 1 contains 0.01 moles of dissolved calcite. I want to how much HCL is need to bring the solution to a PH 6. There are two methods to do that.
Method 1:

Code: [Select]

SOLUTION 1
   pH 7 charge
EQUILIBRIUM_PHASES 1
   Calcite 10 0.01
SAVE SOLUTION 1

END
PHASES     
   Fix_H+ 
   H+ = H+
   log_k 0
USE SOLUTION 1

EQUILIBRIUM_PHASES 2
   Fix_H+   -6   HCl    0.02
   -force_equality
ENDMethod 2:

Code: [Select]

SOLUTION 1
   pH 6
   unit mol/kgw
   Ca 0.01 as CaCO3
   C 0.01 as CaCO3
   Cl 1 charge # as HCl charge

 REACTION 1
However, I got different Re for these two methods. Do you know what is going on? Any help will be grateful.
 
Best regards,
Zhaoyang

[dlparkhurst]

It's a little funky, but method 1 is probably correct. By setting the target SI to 10, you ensured that all of the calcite in EQUILIBRIUM_PHASES dissolved. The second step added HCl to a pH of 6. Note that the resulting solution has an SI of -0.67, and so, is undersaturated with calcite.

The solution composition for the second method is the same: Ca and C molality 0.01, Cl molality 0.01642. There is a slight difference because method 1 starts with 1 kg water and equilibrates with equilibrium phases followed by addition of HCl; so, the mass of water may vary because of the addition of CaCO3 and HCl. In the second case, the solution is adjusted to have exactly 1 kgw in the SOLUTION calculation. The difference is quite small.

Note that "as CaCO3" is not correct, but is ignored because the units are given as mol/kgw, which needs no conversion by PHREEQC.

[Zhaoyang]

For method 2, the value of pe is initially 4. When implementing the REACTION, pe doesn't change and is still 4. However, implementing the REACTION usually leads to a different pe. I was wondering whether it is a coincidence (4 is the equilibrium value). Thanks in advance.
Best regards,
Zhaoyang

[dlparkhurst]

When you use SOLUTION, the pe is fixed at the specified value. That means that the solution will not be at redox equilibrium in many cases. Redox equilibrium requires that every redox couple is consistent with the pe. If, if for example, you define pe, Fe(2), and Fe(3) in SOLUTION, you will almost certainly have a different pe from the Fe(3)/Fe(2) couple than the pe specified.

When you run a reaction (SOLUTION + EQUILIBRIUM_PHASES in your case) then the all redox couples will  react so that the pe calculated by any redox couple will be the same as the calculated pe.

In your case, there are really no redox reactions; there are no elements changing redox state. So, the calculated pe makes little difference. The pe is not well poised, in that any pe (I haven't checked) from say 0 to 11 will make not change the concentration of any aqueous species more than say 1e-12. In other words, don't worry about the pe for this calculation.