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]]

Buffer action

The property of a buffer solution to resist change in its pH value even when small amounts of the acid or the base are added to it is called buffer action.
Consider the acidic buffer solution containing acetic acid and sodium acetate. They dissociate as
CH3COONa <=======> CH3COO_- + H₊
CH3COONa <=======> CH3COO_- + Na₊
When a few drops of an acid, HCl is added to this buffer solution, the H₊ ions combine with CH3COO_- ions to form weakly ionized molecules of CH3COOH.
CH3COO_- + H₊ <=======> CH3COOH
Thus H₊ ion concentration does not change and hence the pH of the solution remains constant.
When a few drops of base, NaOH is added to the buffer solution, hydroxyl ions of the base neutralize the acid, forming salt and water.
Similarly, in a basic buffer solution of NH4OH and NH4Cl, they dissociates as
NH4OH <======> NH4₊ + OH_-
NH4Cl ----------> NH4₊ + Cl_-
When a few drops of a base added, the OH_- ions given by it combine with NH4₊ ions to form the weakly ionized NH4OH.
NH4₊ + OH_- ---------> NH4OH
Thus the OH_- ion concentration or the pH of the solution remains unaffected.
When a small amount of an acid is added, the H₊ ions given by it combines with the OH_- ions already produced by NH4OH.
H₊ + OH_- --------> H2O
Therefore the H₊ ions concentration or the pH of the solution remains unaffected.
The buffer capacity of a buffer solution is defined as the number of moles of acid or base added per liter of the solution to change the pH by one unit.

Buffer solutions

Maintenance of PH in blood and in intracellular fluids is absolutely crucial to the processes that occur in living organisms. This is primarily because the functioning of enzymes is sharply pH dependent. The normal pH value of blood plasma is 7.4 and several illness or death can result from sustained variations of a few tenths of pH unit. Also many medical and cosmetic formulations require that these must be kept and administered at a particular pH. There are solutions which resist the change in pH on addition of small amount of acid or alkali and are called Buffer solution. For example a mixture of H₂CO₃ and HCO₃- is a natural buffer system which maintains the pH of blood. A buffer that is widely used in clinical laboratory and in biochemical studies in the physiological pH range is prepared from tris amino methane (hydroxy methyl) (THAM) [(HOCH₂)3CNH₂].
In order for a solution to act as a buffer it must have two components, one of which is able to neutralize acid and the other able to neutralize the base. Common buffer solutions are mixtures containing a
Weak acid and its conjugate base (one of its salt) called acidic buffer
eg:- CH₃OOH/CH₃COONa, H₂CO₃/Na₂CO₃, Boric acid/borax
Weak base and its conjugate acid (one of its salt) called basic buffer
Eg:- NH4OH/NH4Cl, Zinc hydroxide/ zinc chloride, Glycine/ glycine hydrochloride.