Adding to the original query...It would be preferable to have suggestions on free softwares for this modeling - an option being PHREEQC

CO2 solubility in water is almost independent of pH and there is a general relation that describes pH, (HCO3) minus and (CO3) 2minus

I do not know what your goal is with it? If you want to work out a computer model for ARDS (COVID-19) (quasi permissive hypercapnia), you should consider pCO2 levels to an exaggerated extent, because other acids cannot go into the cytosol for hours! (H + can cross membranes very slowly).

Dear Paul Milham : The objective is to calculate the amount of CO2 and mineral acid needed to reduce the pH of water. This reference provides equations for scenarios in which alkalinity can be ignored:

http://www.geo.utexas.edu/courses/376m/LectureNotes/acidbase.pdf

To reduce pH from 8 to 7 : CO2 dosage of ~ 0.02 mg/l is needed

To reduce pH from 7 to 6 : CO2 dosage is ~ 2 mg/l

(Difference in CO2 dosage probably due to CO2-carbonate-bicarbonate equilibrium)

I am looking for software options which can do similar calculations taking alkalinity into account - Higher the alkalinity, greater is the amount of CO2 or acid needed.

Dear Andras Sikter , the application is for water treatment (drinking water and effluent water treatment). I am looking for online tool to verify the accuracy of my calculations

For the pH of CO2 in water, at equilibrium with air, the two following equilibria must be fulfilled:

(1) equilibrium of CO2 gas between air and water:

CO2(g) + H2O(vap) CO2(aq) + H2O(liq)

(this is governed by Henry's law)

(2) Dissociation of the dissolved gas to bicarbonate and carbonate

CO2(aq) + H2O HCO3- + H+

HCO3- + H2O CO32- + H+

(this is determined using the ionization constants)

There is a nice and free program for calculations with drinking water and other low concentration dilutions with water as the solute. It is called Visual Minteq. It has acid/base reactions, henry's law reactions, solid dissolution, and complexation all included.

Thank you Amie Mcelroy . Visual Minteq was the best choice. Thank you for the suggestion.