Ions as Acids and Bases

Acids, bases, and ionic compounds form anions and cations when they dissolve in water.

Anions that are the conjugate base of strong acids, like chloride formed by the dissociation of hydrochloric acid, are too weak to accept a proton from water. Therefore, chloride ions are pH neutral; that is, they are neither acidic nor basic.

In contrast, anions formed by weak acids, like acetate, the conjugate base of acetic acid, acts as a weak base as it can accept a proton from water.

Cations that are the conjugate acid of strong bases, like sodium ions formed by sodium hydroxide, cannot accept protons and, therefore, are also pH neutral.

In contrast, cations produced by weak bases, like ammonium, the conjugate acid of the ammonia, acts as a weak acid as it can donate protons to water.

When they ionize, salts can produce acidic and basic solutions if they contain a conjugate acid or a conjugate base of a weak acid or base.

Ammonium bromide produces ammonium and bromide ions in water. Bromide ions are pH neutral, whereas ammonium ions act as a weak acid as they can donate protons.

When sodium acetate is dissolved in water, the sodium ions do not react with water; however, the acetate ion can accept a proton—forming a basic solution.

Salts that contain pH neutral cations and anions form neutral solutions. For example, sodium chloride dissociates into sodium ions and chloride ions when dissolved in water. As both of these ions can neither accept nor donate protons, they form a neutral solution.

Small and highly charged metal ions, like iron(III) and aluminium(III), can also act as weak acids when they become hydrated.

When aluminum(III) is hydrated, it acts as a weak acid and transfers protons from its waters of hydration to the free water molecules, leading to the production of hydronium ions.

The smaller the metal ion and the higher the charges present on it, the greater is its tendency to act as an acid. For example, the K_a for Fe(III) is 6.3 × 10⁻³, whereas the K_a for Ni(II) is 2.5 × 10⁻¹¹.