Determining Solution composition from mix

[MartiN]

Hi all,

I would like to determine the compositon of a unknown solution from a known mix. I know the compositon of the initial solution and the final mix. I would like to get the solution that is added to the initial soultion to get the mix.
Does anybody have an idea how I can use PHREEQC to solve that problem?
Thanks for your suggetions.

MartiN

[dlparkhurst]

I have never tried it before, but I think you can do it by defining a solution with sufficiently large concentrations, and set the uncertainties for the solution at 1. However, the problem may be ill posed unless you know one of the concentrations in the unknown solution.

Here is a rough example that mixes seawater with carbonate water (1:1) to make a final solution. A new solution with 1 mol of each major element is defined and the uncertainties are set to 1, except chloride which is set to the seawater concentration (0.567 mol/kgw) and 0.05 uncertainty. The seawater solution and mixing fractions are recalculated at a first glance.

[MartiN]

Ok, thanks a lot for your suggestion. I´ll try it out.

Cheers,

Maritn

[MichaelZ20]

Hi David!
I tried the approach that you have proposed for estimation of the composition of the unknown solution in a mix by setting in inverse modeling large uncertainties and fixing a concentration of chloride. At the considered site fresh groundwater is salinized by a brine arriving from the depth. I tried to set different Cl concentrations of this brine and received ion concentrations of other ions. In a wide range of Cl concentrations of the brine its ion concentrations were obtained proportional to the fixed Cl- (all ion ratios stayed constant). Can this be interpreted in any way, or, as you have mentioned, the problem is merely ill posed?
Thank you in advance for your explanation.
Sincerely,
Michael

[dlparkhurst]

As you change the Cl concentration, the mixing fraction for solution 3 changes proportionately. Without more constraints, you only know the product of the chloride concentration and the mixing fraction is fixed.

[MichaelZ20]

David, what about ion ratios? They stay constant at differing Cl concentrations. May they be considered as found correctly?

[dlparkhurst]

I think you have found a consistent mixing fraction times concentration for each of the elements.