Henderson hasselbach and ion pairing

[browncomputer]

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EK Gen Chem on page 104 says that "the henderson hasselbach equation in the form above does not allow for ion pairing."
They mention ion pairing when they say that adding water to a buffered solution will affect the pH of the buffer by increasing the pH in an acidic buffered solution and decrease the pH in a basic buffered solution

What does ion pairing have to do with the henderson hasselbach and why do they even mention it? i don't even understand its relation to the henderson hasselbach.

 

[DrknoSDN]

if you have an acidic buffer (pH lower than 7),,, and you add water which is almost equal to a neutral buffer at pH7, the acid buffer will be less acidic.

Water exists with an equilibrium of hydronium and hydroxide ions in solution to give the Kw = 1x10^-14
Mixing water with a different buffer will 'normalize' the acidity and make it more similar to the pH of water.
The HH equation only uses [A-] and [HA] but you can't forget that there is also H3O+ and OH- when water is added, and that alters the pH.

I think it's just warning you that the HH equation is a simplified version of the Ka espression and ignores H+ concentrations.