Calcium Carbonate Precipitation Potential - Setup and Interpretation of Results

[EV_8221]

Hi, I am a new PHREEQC user, running PhreeqPython specifically. I have a simple problem to set up calculating the calcium precipitation potential of a solution after NaOH addition, although I'm not sure if I am using/interpreting PHREEQC correctly here.

Here is my input script based on documentation and examples I have seen, where Mix_01 is a solution I have defined:

Mix_01 = Mix_01.add('NaOH',1, 'mg')
initial = (Mix_01.total_element('Ca'))
Mix_01.equalize(['calcite'], [0])
delta = Mix_01.total_element('Ca')
ccpp = delta*2.5
final = initial - delta

I'm assuming this output is reporting my delta in mol/L, although are there any glaring errors here? I am getting a different result from a reference spreadsheet model.

[dlparkhurst]

I am not familiar with the coding for PhreeqPython. CCPP should be simply the amount of calcite that will precipitate from a solution. At its most basic, you simply equilbrate a water with Calcite, and the calcite delta in the output is the number of moles of calcite that precipitates (or dissolves). A positive number for Calcite in the Phase Assemblage represents precipitation and is equal to the CCPP. The Basic function EQUI_DELTA("Calcite") would also be equal to the CCPP.

Sometimes you do not have ~1 L of water, and then you would need to divide by EQUI_DELTA by the solution volume (Basic function SOLN_VOL).

I am not clear why you are adding NaOH. It seems you are calculating how much calcite would precipitate if you added base. The change in the amounts of calcite would be found in the same way as described above, or the value of "final" would also be equal to calcite precipitation in mol/kgw. Addition of base is not normally part of a CCPP calculation.