(CO2)2

[nguyenp123]

Hello, I am a beginner at PHREEQC. I am trying to see how particular elements change at different pH. My input and output is below, I am wondering what the (CO2)2 species is? Thank you.

Input:

SOLUTION 1   
temp   25
pH   4
pe   4
redox   pe
units   mmol/kgw
density   1
C   1

output:
CO2   9.96E-04
HCO3-   4.47E-06
(CO2)2   1.82E-08
CO3-2   2.15E-12

[dlparkhurst]

It was added to fit CO2 solubilities at high CO2 pressures. Its concentrations should be small under most conditions. The Pitzer database uses activity coefficient parameters to account for the same effects.

[nguyenp123]

Thank you