I am a novice in this field and wonder if there's more satisfying answers to explain the low conductivity of Tris in pH7-8 aqueous solution. The equation of pH = pKa + log(conc of base/conc. of acid form) shows ~87% of ionization at pH7.2.  

If low ion mobility is a factor, is there some guidelines to how molecular properties (e.g. M.W.) affect ion mobility?  

Thank you all in advance for your input.