I would like to relate the pH of an aqueous phase diprotic acid solution to a value of the undissociated acid species concentration in the aqueous phase, and vice versa. I have Ka1 and Ka2 values for the acid. The overall dissociation equilibrium can be written: Ka1*Ka2 = ([A2-]*([H+]^2))/[H2A] The pH can be used to give a direct value for [H+], so my question is: from this how do I find the value of [H2A]? Additionally, for a given value of [H2A], how do I calculate the value of [H+]? This work is for the purpose of creating a mathematical model in Excel for the dissociation process. The pKa1 and pKa2 values of the acid are close together and I would like to try and avoid as many assumptions as possible, no matter how hard the maths becomes. Is anyone able to help?