Salts that Produce Acidic Solutions

Salts With a Hydrolyzable Cation

When dissolved in water, acidic salts will yield solutions with pH less than 7.0. This is due either to the presence of a metal cation that acts as a Lewis acid (which will be discussed in a later concept), or, quite commonly, due to a hydrolyzable proton in the cation or the anion. Salts with acidic protons in the cation are most commonly ammonium salts, or organic compounds that contain a protonated amine group. Examples include:

• ammonium (NH₄⁺)
• methyl ammonium (CH₃NH₃⁺)
• ethyl ammonium (CH₃CH₂NH₃⁺)
• anilinium (C₆H₆NH₂⁺)

An example of an acid salt is one containing any of these cations with a neutral base, such as ammonium chloride (NH₄Cl).

Salts With Hydrolyzable Protons in the Anion

Acid salts can also contain an acidic proton in the anion. Examples of anions with an acidic proton include:

• bisulfate (HSO₄^-)
• dihydrogen citrate (H₂C₆H₅O₇^-)
• bioxalate (HO₂C₂O^-)

Each of these anions contains a proton that will weakly dissociate in water. Therefore, salts containing these anions—such as potassium bisulfate—will yield weakly acidic solutions in water.

Determining Acidity or Alkalinity of a Hydrolyzable Ion

From the previous concept, we know that salts containing the bicarbonate ion (HCO₃^-) are basic, whereas salts containing bisulfate ion (HSO₄^-) are acidic. We determine whether the hydrolyzable ion is acidic or basic by comparing the K_a and K_b values for the ion; if K_a > K_b, the ion will be acidic, whereas if K_b > K_a, the ion will be basic.

Anilinium chloride

Anilinium chloride is an example of an acid salt. The NH³⁺ group contains an acidic proton capable of dissociating in solution; therefore, a solution of anilinium chloride in pure water will have a pH less than 7.