[Big-Daddy]

If you have a solution containing a pure amino acid in water, is it correct that the pH is independent of the initial concentration of the acid?

My logic is this: if it is possible to alter pH by dilution, then perhaps it is possible to reach the isoelectric point by dilution, but then the charge balance reduces to [H⁺]=[OH^-] so the isoelectric point must be at pH=7 (pH = 1/2 pKw). So for any amino acid except those with an isoelectric point at pH=7, it must be impossible for dilution to bring the acid to isoelectric point. As the initial concentration of the acid tends to infinity and [H⁺] tends to [OH^-] (if pH is dependent on initial concentration for amino acids), then pH tends to 7, but regardless of any value of initial concentration it must be impossible for the pH to touch the isoelectric point, unless that point is at pH=7 itself, because for a pure amino acid solution to be at isoelectric point the solution must be neutral (wrt protons). If pH is not dependent on initial concentration here, then of course the pure amino acid cannot reach the isoelectric point.

Is this true or is there a flaw in my logic?