Control and Understand Charge Balance Element for SOLUTION_SPREAD inputs

[swhit]

I would like to be able to control which species is being used for charge balance when setting up initial solutions with the SOLUTION_SPREAD keyword.

I know that with the SOLUTION keyword, you can just put "-charge" next to the element you wish to be used for charge balance, but I'm not sure how to achieve the same with SOLUTION_SPREAD.

The code seems to use S(6) (Sulfate) when results are available, but otherwise "charge balance" isn't listed alongside any elements under the "solution composition" section of the results.

When I put in "dummy" results for S(6) so that all solutions have a result, the code automatically uses S(6) to adjust all solutions to CBE =0.

In a more general sense, how does PHREEQC choose which element to use (is there an order of hierarchy?) when nothing is specified?

It would be great to be able to understand and control the behavior a little more here.

Thanks,

[dlparkhurst]

There is no default, automatic, or default sequence for charge balance. You must choose an element and specify "charge". With SOLUTION, you can select "charge" for an element in each SOLUTION definition. With SOLUTION_SPREAD, you can only choose "charge" for one element that will be used in all solutions. It appears in the line below the element headings. Charge balance is applied to initial solution calculations. pH is adjusted for charge balance (or imbalance) in all other "reaction" calculations.

Unless I know the source of the charge imbalance, I tend not to change the imbalance of field samples.
 
Tabs may be lost in the following, so that it may not run correctly, but here is what it looks like. The "charge" is in the Cl column. The attached file should run correctly.

Code: [Select]

SOLUTION_SPREAD
 Na      Cl
  charge
  1       2
  2       1