GPS-X Technical Reference

Tools Object 340
The dissociation constants used in these equations can also be input on the control form
shown in Figure 12-2. The default dissociation constants are taken from the literature.
The ammonium dissociation constant is corrected for temperature.
To properly reflect the presence/absence of added alkalinity (lime addition, etc.) the
alkalinity state variable salk is assumed to be all carbonate alkalinity that is available (and
completely dissociated) for acid neutralization.
In the above equations, totalCO 2 is the total CO 2 available (converted to equivalent
charge). Because the alkalinity is assumed to be in the form of completely dissociated
CaCO 3, a 2+ charge cation is also assumed present in the equilibrium system.
The above equations are combined with the charge balance shown below:
Equation 12.7
An implicit nonlinear equation solver is used to solve this system of equations for [H+].
The calculated pH values are bounded by the low bound and high bound parameters
shown in Figure 12-1.
Once the solution is complete and the concentrations of the ionized components have
been calculated, the concentrations of the non-ionized components are calculated from
the speciation equation shown above.
The calculated pH and the concentrations of the individual components can be displayed
on an output form similar to that shown in Figure 12-3. This pH model is also used in
the HPO (High Purity Oxygen) object for pH estimation.
Figure 12-3 – Sample pH Outputs
GPS-X Technical Reference