I am collecting data from literature for CO2 solubility in aqueous solutions. These are usually synthetic brines and are reported as molalities or wt% of salts or salt mixtures (I have NaCl, KCl, MgCl2, CaCl2, Na2SO4, and NaHCO3). Since I am more interested in concentration of individual ions, I assumed a complete dissolution and added the ions from different salts in a straightforward manner to determine the final concentration of each species.
I tried using PHREEQC (phreeqc and llnl databases) to do speciation calculations, and noticed that it is different than the simple assumption of complete dissolution. At first, the initial salt amount dissolves completely, but at equilibrium there are other species besides Na+ and Cl-, such as undissolved NaCl (I was still way below solubility limit, the initial amount of Halite I added completely dissolved). For MgCl2 and CaCl2 solutions, I got other species such as MgCl+ and CaCl+, their molalities where not negligible (mainly when using llnl database where these species are actually included).
My questions are:
- Why do I have these species, even at low concentrations?
- Is my assumption of complete dissolution a valid one? or would it be more accurate to use PHREEQC to perform speciation calculations and consider all those species?
