pH of Salts in Water

Chemicals and Solutions

• ammonium chloride
• sodium chloride solid or KBr (table salt will not work for this demo!)
• sodium acetate solid

Materials

• three tall hydrometer cylinders
• three spatulas
• deionized water
• cresol red indicator solution or phenol red

Procedure

1. Put water into each of the hydrometer cylinders and add several drops of cresol red indicator solution. The water should become orange.
2. To one hydrometer cylinder add sodium acetate. The solution turns red showing that the solution is basic.
3. To another cylinder add sodium chloride. No color change is observed.
4. To the last cylinder add ammonium chloride. The solution turns yellow showing that the solution is acidic.

Discussion

When a salt derived from a strong base and a weak acid dissolves in water, the solution becomes basic. In the dissociation of sodium acetate in water, sodium ions and acetate ions are formed. The sodium ion has no acidic or basic properties. The acetate ion is the conjugate base of acetic acid, and has an affinity for H+ ions. The hydrolysis reaction is given by:

\( \ce{ CH3COO-_{(aq)} + H2O_{(l)} -> CH3COOH_{(aq)} + OH-_{(aq)} } \)

Since this reaction produces OH- ions, the sodium acetate solution is basic.

When a salt derived from a strong acid and a weak base dissolves in water, the solution becomes acidic. In the dissociation of ammonium chloride, chloride ions and ammonium ions are formed. The chloride ion has no affinity for H+ ions. The ammonium ion is the conjugate acid of ammonia and reacts with water in the following manner:

\( \ce{ NH4+ + H2O -> NH3 + H3O+ } \)

Since this reaction liberates hydronium ion, the solution is acidic.

Salts derived from a strong acid and a strong base give neutral solutions. Salts derived from a weak base and a weak acid may be acidic or basic. To determine whether a salt will give an acidic or basic solution, compare the Ka and Kb values. If Ka > Kb it will give an acidic solution.