Overview of the Acid-Base Properties of Salt

Summary of Acidic and Basic Salts

As we have discussed, salts can form acidic or basic solutions if their cations and/or anions are hydrolyzable (able to react in water). Basic salts form from the neutralization of a strong base and a weak acid; for instance, the reaction of sodium hydroxide (a strong base) with acetic acid (a weak acid) will yield water and sodium acetate. Sodium acetate is a basic salt; the acetate ion is capable of deprotonating water, thereby raising the solution's pH.

Acid salts are the converse of basic salts; they are formed in the neutralization reaction between a strong acid and a weak base. The conjugate acid of the weak base makes the salt acidic. For instance, in the reaction of hydrochloric acid (a strong acid) with ammonia (a weak base), water is formed, along with ammonium chloride. The ammonium ion contains a hydrolyzable proton, which makes it an acid salt.

Salts in Which Both Ions Hydrolyze

The following is a more complicated scenario in which a salt contains a cation and an anion, both of which are capable of participating in hydrolysis. A good example of such a salt is ammonium bicarbonate, NH₄HCO₃; like all ammonium salts, it is highly soluble, and its dissociation reaction in water is as follows:

$NH_4CO_3(s)\rightarrow NH_4^+(aq)+HCO_3^-(aq)$

However, as we have already discussed, the ammonium ion acts as a weak acid in solution, while the bicarbonate ion acts as a weak base. The reactions are as follows:

$NH_4^+(aq)+H_2O(l)\rightleftharpoons H_3O^+(aq)+NH_3(aq)\quad\quad K_a=5.6\times10^{-10}$

$HCO_3^-(aq)+H_2O(l)\rightleftharpoons H_2CO_3(aq)+OH^-(aq)\quad\quad K_b=2.4\times 10^{-8}$

Because both ions can hydrolyze, will a solution of ammonium bicarbonate be acidic or basic? We can determine the answer by comparing K_a and K_b values for each ion. In this case, the value of K_b for bicarbonate is greater than the value of K_a for ammonium. Therefore, bicarbonate is a slightly more alkaline than ammonium is acidic, and a solution of ammonium bicarbonate in pure water will be slightly basic (pH > 7.0). In summary, when a salt contains two ions that hydrolyze, compare their K_a and K_b values:

• If K_a > K_b, the solution will be slightly acidic.
• If K_b > K_a, the solution will be slightly basic.