Page 26 - Pharmaceutical analytical chemistry |

of these salts with water does not proceed to completion. It reaches an

equilibrium point and thus may be represented by an equilibrium constant

expression with an equilibrium constant known as hydrolysis constant

Kh.
a. Salts of weak acids and strong bases

An example of this salt is sodium acetate which is a strong electrolyte and

is completely ionized. In addition, the anion of the salt of a weak acid is a

Brönsted base which will accept protons. It partially hydrolyses in water

to form OH- and the corresponding undissociated acid.
H2O + CH3COO- ↔ CH3COOH + OH-

acid1 base2 acid2 base1

This ionization is known as hydrolysis of the salt ion. The ionization

constant for the above equation is equal to the basicity constant of the

salt. The weaker the conjugate acid, the stronger is the conjugate base.

The hydrolysis constant can be calculated from two constants Kw and Ka.

For the salt of concentration Cs, the pH can be calculated as follows

pOH = ½ (pKb + pCs)
pH = 14 - ½ (pKb + pCs)
The degree of hydrolysis (h) of a salt is a fraction of that salt hydrolyzed
when equilibrium is established, thus: h= x/Cs
b. Salts of weak bases and strong acids

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