Multiple Fix_Ca in EQUILIBRIUM_PHASES produced by USER_PUNCH

[MichaelZ20]

Can somebody help to modify my code?
In its present form it does not take Ca for Fix_Ca from the SOLUTION_SPREAD.
Thank you in advance!

[dlparkhurst]

If you use your Fix_Ca, then you are fixing the activity of the Ca+2 ion. So, it is inconsistent to use TOT("Ca"), which is the total molality of calcium; you are setting the activity of calcium ion to the original total concentration of calcium.

LA("Ca+2") would give the original log activity of Ca+2. Here is a modification of your code to use this value:

Code: [Select]

  40 PUNCH "Fix_Ca  ", LA("Ca+2"), "  CO2(g)" + EOL$

The attached file Phrqc1.pqi removes the loop on solutions (one simulation is written for each initial solution) and uses log activity of Ca+2.

My guess is that you would prefer to keep the total molality of calcium unchanged, and simply add or remove CO2 to get to calcite equilibrium. The attached file Phrqc3.pqi does this calculation. There are often two solutions to this problem, one that adds CO2 to reduce the pH to calcite equilibrium, and one that removes CO2 to produce a relatively high pH and low carbon concentration. The input file adds CO2 (pH decrease) if SI calcite > 0, otherwise CO2 will be removed (pH increase).

[MichaelZ20]

Dear David, thank you for your help!
The second file solves my problem.