The quantitative relationship among PH,buffering action of 
amixture of weak acid and its conjugate base and the PKa of 
the weak acid is given by a simple expression called 
HENDERON-HASSELBALCH EQUATION.
The shape of titration curve of any weak acid is expressed 
by Henderson-Hasselbalch equation.This is auseful way of 
restating the expression for the dessociation constant of 
an acid.
For the dessociation of aweak acid HA into H+ and A-,this 
equation can be derived as follows.


Ka=[H+][A-]/[HA]
by rearraning the equation for solving H+

[H+]=Ka[HA]/[A-]

by converting to logarthmic functions

log [H+]=LOG Ka+log[HA]/[A-]

making expression negative

-log[H+]=-logKa-log[HA]/[A-]

SUBSTITUTING ph for -log[H=] and PKa for -logKa
PH=PKa-log[HA]/[A-]
NOW TO REMOVE MINUS SIGN INVERT LAST TERM I.E -LOG[HA]/[A-] 
TO OBTAINHENDERSON-HASSELBALCH EQUATION.
PH=PKa-LOG[A-]/[HA];