PhreeqcUsers Discussion Forum
Beginners => PHREEQC basics => Topic started by: Ucef on October 08, 2020, 12:12:20 PM

Hi
I am trying to simulate a laboratory experiment set up that uses 0.05 L of leaching solution (twice a week) and 67g of solid. I’ve taken into account the solution volume in the A/V term in KINETICS keyword but I am wondering if I should change the default mass of water from 1 kg to 0.05kg assuming a water density of 1. And I also intend to use TRANSPORT to add flux boundaries to the system so I am wondering what would be the appropriate solution volume in the SOLUTION keyword block given that the unit of my concentrations is mg/L. I read in one of the forum posts that is preferable to use the default value of 1 kg of water, however in my case I am only using 0.05L of the leaching solution for each rinse.
Thank you in advance for any help.

I think you just need to be consistent to use either moles of each reactant in 1 L of solution, or moles of each reactant per 0.05 L of solution (ignoring the difference between L and kgw). By reactant I mean moles of kinetic reactant, moles of equilibrium phases, moles of exchanger, etc.
For your kinetic reaction, you will be using something like SAVE moles in the RATES definition. This is exactly moles, not moles per liter. So it does make a difference whether you are adding that number of moles to 0.05 L or 1 L. The change in concentration in the solution will be 20times different.
If your reaction depends simply on the surface area of reactant, then the number of moles is calculated independently from the volume of solution. For homogeneous reactions occurring in solution, then rates are often mol/L/s, in which case you need to account for the time step (variable TIME) and the volume of solution (SOLN_VOL) to arrive at the number of moles to SAVE in the rates definition (moles = rate*TIME*SOLN_VOL).

Thank you very much; your answer is really helpful.
The reactions that I try to simulate are heterogeneous and mainly related to the surface area. Given that the number of moles is calculated independently from the volume of solution is it right to set the A/V term in the overall reaction rate to PARAM(1)=LOG10(A)? without including V in the expression given that the number of moles will be dissolved in the mass of water specified in the SOLUTION keyword block.
Thank you

As Mr. Mullen taught me in high school chemistry, "follow the units". In other words, make sure the rate definition ends up so that you SAVE units of moles.

Thank you very much :)