How to consider water self-ionization in Comsol?

Hello,

I have a problem to consider the water self-ionization reaction. We know the water self-ionization H2O = H+ + OH- is an equilibrium reaction with a reaction constant Kw = [H+][OH-] = 1e-14 M^2

Either acid or base is added in water, the above equilibrium should be always obeyed. When I use Transport of Diluted Species in Porous Media model, I find two ways to consider chemical reactions:

1. Add Chemistry and add reactions, then add Reactions in Transport of Diluted Species in Porous Media model

2. In Transport of Diluted Species in Porous Media model, directly add Equilibrium Reaction

However, I tried these two ways, but the calculated concentration of H+ and OH- does not follow the equilibrium constant. I am wondering what the correct way is to consider the water self-ionization. Attached is the Comsol file for the test.

Best regards, Zhidong