So far we have seen how the addition of acids or bases to water modifies its pH. However, salts can also change the pH of water by hydrolyzing. Hydrolysis is a process by which the cation or anion of a salt, provided it comes from a weak base or weak acid, reacts with water to generate its conjugate base or acid.

Let's see some examples:

  • Sodium acetate dissociates in water, generating acetate anions and sodium cations. Sodium cations coming from a strong base (NaOH) do not hydrolyze in water, but acetate anions coming from a weak acid ($CH_3COOH$) undergo the following transformation:

$CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-$

When hydrolyzed in the aqueous medium, the acetate generates hydroxide ions, which give rise to a basic medium. We can say that sodium acetate is a basic salt.

  • Ammonium chloride dissociates in the aqueous medium, forming chloride anions and ammonium cations. Chloride anions from a strong acid (HCl) do not undergo a hydrolysis process, however, ammonium cations react with water through the following hydrolysis reaction:

$NH_4^++H_2O\rightleftharpoons NH_3 + H_3O^+$

The hydrolysis of the ammonium cation produces protons, which give rise to an acidic medium. We can say that ammonium chloride is an acid salt.

  • Sodium chloride dissociates in water to form chloride anions and sodium cations, both coming from strong acids and bases, therefore stable in the aqueous medium. This type of solution does not alter the pH of pure water. It is a neutral salt.

Possibly the concept of hydrolysis has not yet been clear, so I will add an exercise.

Indicate whether the following solutions are acidic, basic, or neutral:

(a) Aqueous sodium hypochlorite (NaClO).

Sodium hypochlorite dissociates in water generating sodium cations and hypochlorite anions. Sodium cations are stable in water because they come from a strong base (NaOH). On the contrary, hypochlorite anions come from a weak acid, hypochlorous acid (HClO), thus hydrolyzing. We write the hydrolysis reaction of hypochlorite.

$ClO^-+H_2O\rightleftharpoons HClO + OH^-$

Hydrolysis of hypochlorite generates a basic solution.

(b) Aqueous potassium cyanide (kCN)

Potassium cyanide dissociates in water, generating potassium cations and cyanide anions. The potassium cation is stable in water because it comes from a strong base (KOH), while the cyanide is hydrolyzed (it comes from a weak acid, HCN). We write the cyanide hydrolysis reaction.

$CN^-+H_2O\rightleftharpoons HCN + OH^-$

Once we understand the concept of hydrolysis and the reactions involved in it, we will go on to detail the calculation of pH in salt solutions. Although we will do it in the next section of the topic.