# Buffer solutions

Buffer weak base - conjugate acid

The buffer consisting of a weak base and its conjugate acid is prepared by mixing relatively large amounts of a weak base and a salt thereof. For example, you can get a tampon of this type by mixing a solution of ammonia (NH 3) with an ammonium chloride (NH 4 Cl).

The reasoning that we practice is identical to the previous one. Based on the equilibrium of the ammonia hydrolysis in H 2 O, containing both the kind that the NH 3 NH 4 +

$\bg_white&space;NH_3&space;+&space;H_2O&space;\rightleftharpoons&space;NH_4^+&space;+&space;OH^-$$\bg_white&space;K_b&space;=\frac{&space;[NH_4^+][OH^-]}{[NH_3]}&space;=&space;1,8&space;\cdot&space;10^{-5}$

It is obtained by an expression very similar to the previous one that allow to go back to the pH of this solution:

$\bg_white&space;[OH^-]&space;=&space;\frac{[NH_3]}{[NH_4^+]}\cdot&space;K_b&space;\cong&space;\frac{C_b}{C_s}\cdot&space;K_b$

From which:

That is the equation of Henderson-Hasselbach for tampons constituted by a weak base and a salt of its conjugate acid.

Even in this case are subject to the consideration previously. The buffer to be effective it must be made ​​of relatively large amounts of weak base and conjugate acid (salt).