pH of Buffer solution

The pH of acidic and basic buffer can be calculated by Henderson – Hasselbalch equations. Consider an acidic buffer HA + A-
HA <=======> H+ + A-
Ka = [H+] [A-] / [HA]
[H+] = Ka [HA]/[A-]
[H+] = Ka [acid]/[salt]
There fore pH = -log[H+]
pH = pKa + log [salt]/[acid]
when, [salt]/[acid] = 1 ,
pH = pKa
Since pKa of an acid is a constant at constant temperature, the pH of the buffer is constant. Thus buffer capacity is maximum in a solution containing equivalent amount of acid and its salt.
The pH of basic buffer is also given by Henderson – Hasselbalch equation
BOH <=======> B+ + OH-
Kb = [B+][OH-]/[BOH]
[OH-] = Kb [BOH]/[B+]
pOH = pKb + log [salt]/[base]
pH = 14 – pOH
= 14 – [pKb + log [salt]/[base]]

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