Re: PHREEQC Question

["David L Parkhurst" <dlpark@xxxxxxxx>]

·     Can you please confirm whether PHREEQC always assumes a 1 liter
solution, or if the volume of solution is determined from the density and
mass of solution entered.

Phreeqc takes the input concentrations and converts them to molality,
mol/kg water. Normally, the initial solution, after conversion, contains 1
kg of water (volume is not known). If an amount of water other than 1 kg is
specified, it then multiplies by the mass of water specified to geth the
total moles of elements, including H and O that are in the specified
solution.

·     Does the mass of water refer to jus the mass of H2O (meaning that 1
liter of any solution would have a mass of 1 kg) or the mass of solution?

The mass of water is the mass of water in the solution, normally 1 kg, not
counting any solutes. The H and O associated with SO4-2, HCO3-, are also
not counted as part of the mass of water.

·     What information does PHREEQC use to calculate molality?  Does
PHREEQC calculate density independent of the estimated input value?

Phreeqc takes the input concentrations in mg/L. It divides by density (kg
solution/L solution) to get mg/kg solution, which is ppm. PHREEQC then sums
the mg of solutes in solution and subtracts that from 1 kg of solution to
arrive at a mass of water in the solution. The ppm values are converted to
moles and divided by the mass of water to get molality and normally, the
moles of solute and mass of water are adjusted so that the solution
contains 1 kg water (or it is adjusted to give the mass of water specified
with -water).

David



David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled