Note that CO2 appears in the H+ balance as a multiple of two because
it contributes two protons. Given values for the total concentration
of CO2, amine, protons and hydroxide and estimates for the equilibrium
constants the chemical speciation is then determined. This
is done by solving the system of nonlinear simultaneous equations
consisting of the equilibrium constants and mass balance equations
for the concentrations of all species using the Newton–Raphson
method (Maeder and Neuhold, 2007).
Once the chemical speciation has been determined the CO2 partial
pressure can be calculated from the value of cCO2 and the Henry
constant kH,CO2 of CO2 in water.