About buffer solutions

[Myeou]

Well there is one thing that i dont get it on buffer solution...

 

Lets say that we will make buffer solution from Na2HPO4 and KH2PO4.

 

This salt will dissolve like this in water :

Na2HPO4 => 2Na + HPO4

KH2PO4 => K + H2PO4

 

So our buffer solution is going on this equilibrium ;

 

H2PO4 <=> H + HPO4

 

in that case , the pH of the solution is depends on H+ ions that forms in equilibrium.

But ;

doesnt HPO4 also dissolves like this in water ?

HPO4 <=> H + PO4

 

shouldn't the pH also depends on the last equilibrium too ?

Edited May 31, 2012 by Myeou

[John Cuthber]

Good point.

HPO4 -- does indeed dissociate into PO4--- and H+

But, if there are a significant number of protons about then the reaction will be pushed back to form HPO4--

So at the pH where you have H2PO4- and HPO4-- in significant quantities, the amount of PO4--- is tiny so you can neglect it.