about example 17

[Mercury]

Hi,

I am a new beginner of PHREEQC, I have a question about example 17 – Inverse Modeling With Evaporation.

   In this example, the “water mass” in both solution 1 and solution 2 are set to be 1, and the result shows that solution fraction of solution 1 is 88, which means 88-fold concentration. While I set the “water mass” to be 2 (4 or 88) and keep that of solution 2 to be 1, the solution fraction of solution 1 change to be 44 (22 or 1) correspondingly. I have tried several more values of “water mass” for solution 1 and found that the product of “water mass” and solution fraction of solution 1 is unchanged, keeping 88.

   I am wondering the meaning of solution fraction; and if I want to fix the concentrated multiple (for example, the concentrated multiple from solution 1 to solution 2 is predicated in advance), can I obtain the expected solution fraction by adjusting the water mass?
   
   Your timely reply would be much appreciated.

Sincerely,
Qiuli.

[dlparkhurst]

The inverse modeling capability of PHREEQC is solving a set of mole balance equations (Parkhurst, Water Resources Research,1997; and version 2 of the manual, Parkhurst and Appelo, 1999).

One of the mole balance equations is on water, where the moles of water in the initial solution times the solution fraction plus and minus water consumed in reactions is equal to the moles of water in the final solution times its solution fraction (normally 1). Usually, the amount of water produced or consumed in reactions is small other than the water removed by evaporation.

So the the mole balance is approximately f*moles initial - moles evaporated = moles final = 55.5.

Adjusting the mass of water in the initial solution simply changes the solution fraction.

Frequently, the solution fraction and moles evaporated adjust to account for a conservative constituent, such as Cl. To force a given solution fraction, you could specify a conservative species to have concentrations that produce the solution fraction that you want. Normally, the solution chemistry determines the solution fractions.

[Mercury]

Hi,

Thanks very much for your timely reply.

According to your interpretation on the water mass balance, it is the moles evaporated rather than the solution fraction means the concentration multiple. In the example 17, the 88-fold concentration is not influenced by the water mass or solution fraction, it is the chemical compositions of solution 1 and solution 2 determine the concentrated multiple, did I understand it correctly?
 
Sincerely,
Qiuli.

[dlparkhurst]

The interpretation is, ignoring differences between kgw and L, 88 liters of water is evaporated to 1 liter of water.