One way to predict the behavior of a salt in water is to follow the approach taken in many text books. A salt may be thought of as the product of a reaction between an acid and a base.

1. When the parent acid and base are strong, a water solution of the salt is neutral.
2. When a salt is formed from the reaction between a weak acid and a strong base, a water solution of that salt will be basic.
3. When a salt is formed from a strong acid and a weak base, a water solution of the salt will be acidic.
4. When a salt is formed from a weak acid and a weak base, a water solution of the salt could be acidic, basic or neutral depending on the nature of the ions.

The reaction of ions with water leading to an alteration of the pH of a water solution is
called ion hydrolysis.

Example #1: NH₄Cl may be thought of as a salt of ammonia and hydrochloric acid. Ammonia is a weak base and hydrochloric acid is a strong acid. Therefore, using the rules above, the water solution of ammonium chloride will be acidic.

• Hence, we see that it is a relatively simple task to predict how a particular salt will affect the pH of a solution if the acid-base parents can be recognized.

• In addition, it is important, not only to be able to predict the acidity of a salt solution, but also to know WHY this behavior occurs. This laboratory exercise builds on the rules above, expects you to understand and be able to explain why the salts behave the way they do, to be able to determine the net reaction that affects the pH of the solution and to be able to expand the list of cations and anions for which prediction of acidic, basic, and neutral behavior can be predicted.

• To explain the hydrolysis behavior of salts requires the consideration of three processes when a salt is dissolved in water:

1. Salts dissociate 100% into ions in water:

Reaction 1 reminds us that the dissolved salt is dissociated into its ions (assumed to be 100% ).

2. Does this cation undergo hydrolysis, i.e., does the cation interact with water molecules? YES

Reaction 2 is a hydrolysis reaction in which ammonium ions react with water to yield hydronium ions, H₃O⁺, thereby, changing the pH of the solution. The equilibrium constant for this reaction indicates that the reaction will occur to a significant extent and the net effect is to raise the [H₃O⁺] and to decrease the pH of the solution.

3. Does this anion undergo hydrolysis? NO

Reaction 3 is reactant driven and the equilibrium constant is exceedingly small. This reaction will not occur to any significant extent and has no net effect on the pH of the solution.(You cannot make a strong acid or base in ion hydrolysis as strong acids and bases will dissociate 100%)

Therefore, the overall result of dissolving NH₄Cl in water is that the [H₃O⁺] will be increased by the reaction of the cation with water causing the solution to be acidic. As the general rule says, salts of weak bases and strong acids will yield solutions which are acidic.

• One very important advantage of the method outlined above is that the it allows one to determine the NET REACTION that affects the pH. The net reaction that caused the solution in the example above to be acidic is the hydrolysis of the cation:

• Since the equilibrium constant for this reaction is known, K = K_{a of} NH₄⁺, it is possible to set up an ICE table to determine the numerical value of pH of a solution of this salt, if the concentration is known.

Example #2: Sodium acetate is a salt of a strong base and a weak acid and the following reactions are considered when this salt is dissolved in water:

1. Salts dissociate 100% into ions in water

Reaction 1 reminds us that the dissolved salt is 100% dissociated into ions.

2. Does the cation undergo hydrolysis? NO

Reaction 2 has an exceedingly small equilibrium constant and this reaction will not occur to any significant extent and, therefore, has no net effect on the pH of the solution.

3. Does the anion undergo hydrolysis? YES

Reaction 3 is an important hydrolysis reaction. The equilibrium constant for this reaction indicates that the reaction will occur to a significant extent and the net effect is to raise the [OH-] and to increase the pH of the solution.

• Therefore, the overall result of dissolving NaC₂H₃O₂ in water is that the [OH-] and the pH will be increased. As the general rule says, salts of weak acids and strong bases will yield solutions which are basic.

• The net reaction that has brought about that change in pH is the one in which the anion reacted:

• Since the equilibrium constant for this reaction is known (K = K_b of CH₃CO₂^-), it is now possible to set up an ICE table to determine the pH of a solution of this salt, if the concentration is known.

Example #3: Salts containing small, highly charged cations are known to alter the pH of aqueous solutions. For example, if Fe(NO₃)₃ is dissolved in solution, the Fe³⁺ ion will hydrate extensively according to the equation. [Hydrate means that the ion will attract water molecules.]

• The small highly charged ion then brings about the following reaction which renders the solution acidic:

• Other examples of ions which react similarly are Al³⁺, other +3 cations and, to some extent, most of the cations except those of the IA family and the larger IIA ions.

General Rules for Ion Hydrolysis

Anions

1. Anions of weak acids will react with water, i.e. will hydrolyze to a significant extent and produce OH- ions thus increasing the pH of the solution, i.e., are basic anions. Example:

2. Anions of strong acids will not hydrolyze to a significant extent and will, therefore, have no effect on the pH, i.e., are neutral anions.

Cations

1. Cations that yield weak bases and produce H₃O⁺ lower the pH of the solution, i.e., are acidic cations. (example: NH₄⁺(aq))

2. Cations that are small and highly charged and extensively hydrated undergo hydrolysis by the loss of H+ from the bonded water and lower the pH of the solution, i.e., are acidic cations. (Example: Al³⁺ and Fe³⁺)

3. Cations of the IA family and the larger ions of the IIA family do not hydrolyze and will, therefore, have no effect on the pH, i.e., are neutral cations.