Chemistry - Effect of dilution in buffer solutions?

Question

I'm asking how, exactly, does dilution affect the pH of a buffer solution, to be more precise, NH3/NH4Cl. In theory, the quotient between the concentration of NH3 and NH4 should remain a constant, therefore making the pOH equal to pKb plus log([NH3]/[NH4+]). But this isn't true in practice, why?

Answer 1

The only thing I can think of is that if you lower the concentration of the ammonia too much, then the dominant equilibrium will be that of water. Water's equilibrium will push things towards a more neutral state.

Answer 2

Keep in mind that pH is just an easy way of saying the concentration of H+ and OH- ions that are in a solution. So say you originally have 4 mols of H+ ions in one L of solution. If you add another liter in volume, you now have 4 mols in 2 L of solution. So the concentration went from 4 M to 2 M. Therefore, even though the quotient of NH3/NH4Cl is the same, it is within a larger volume thus affecting the pH of a buffer.