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^-
K_a = [H⁺] [A^-] / [HA]
[H⁺] = K_a [HA]/[A^-]
[H⁺] = K_a [acid]/[salt]
There fore pH = -log[H⁺]
pH = pK_a + log [salt]/[acid]
when, [salt]/[acid] = 1 ,
pH = pK_a
Since pK_a 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^-
K_b = [B⁺][OH^-]/[BOH]
[OH^-] = K_b [BOH]/[B⁺]
pOH = pK_b + log [salt]/[base]
pH = 14 – pOH
= 14 – [pK_b + log [salt]/[base]]