Buffer Solutions

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Buffer Solutions

Buffer is a solution that does not allow variation in pH on the addition of acid or alkali. A buffer may be prepared by taking a weak acid or a base and adding its conjugate salt to it, so that the conjugate salt ensures a common ion composition. For example, a mixture of acetic acid and sodium acetate in certain molar ratios acts as a buffer, which behaves as follows:

Lionization of a weak acid follows the mass law which is expressed in terms of Henderson – Hasselbalch equation. If HA is a weak acid, then the dissociation equation is:

According to the law of mass action, K_a, the dissociation constant of an acid is defined as:

Solving for [H⁺] we get

Taking logarithms of both sides we get

log₁₀ [H⁺] = log₁₀K_a + log₁₀

(as log ab = log a +log b) multiplying throughout by negative sign ,

- log₁₀ [H⁺] = log₁₀K_a – log₁₀

- log₁₀ [H⁺] = pH
- log₁₀ K_a = pK_a

And

-log 10

= -log 10

. . pH = pK_a + log₁₀

In general

pH = pK_a + log₁₀

Conjugate acid and base represent the corresponding molar concentration, moles/1. Ionization of weak acids or weak bases in aqueous solutions is a reversible process.

Its equilibrium constant K

The concentration of water in aqueous solution is constant, hence its concentration (55.5moles/I) may be combined with K to give the ionization constant K_a:

H₃O may be represented as H, hence the expression for K will be

The numerical value of K_a for acetic acid is 1.795 x 10^-5. For dilute aqueous solution of NH₃, the equilibrium reaction is

The ionization constant

The numerical value of K_b for ammonia solution is 1.8 x 10^-5 , which is almost identical to K_a.Hence weak acids and bases are ionized to the same extent in aqueous solutions.

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