Charged water turning into pure water

[Jicé]

Through Phreeqc 3.1.7.9213, I tried running a speciation using attached phreeqc file, and my water is turning into pure water, making some species disappear.

Does anybody reproduce this bug? Is there any explanation (high concentrations, etc...)?

Thanks!

[dlparkhurst]

The issue is in the conversion from /L or /kgs to /kgw. PHREEQC sums the mass of solutes and subtracts from 1 to determine the mass of water. With your extreme concentrations, this difference is negative, and the molalities are all set to negative numbers, which are then censored to zero. In the next version, I will check for the negative mass of water and write an error message.

The fundamental problem is that PHREEQC will probably fail to converge with these really high concentrations.

More specifically, to avoid the negative mass of water calculation, the density must be greater. A density of 3 causes the solution to converge, albeit with concentrations of 27 molal.

You can avoid some of the conversion issues if you do not use /L concentrations. if you define the concentrations as /kgw there is no conversion involved except for a simple factor from mass to moles. However, I think you may easily be outside the range of applicability for the aqueous model, in addition to having numerical issues trying to solve the equations.