Amphoteric Behavior of Alum

1. Amphoteric Behavior of Alum The amphoteric behavior of alum is shown by varying the pH of its solution

2. Purpose • To observe the solubility of alum in a changing pH environment

3. An acidic solution of “alum” --- aluminum sulfate, Al2(SO4)3 --- is neutralized and thereby produces a precipitate. The neutralized solution is then made gradually basic and the solid again dissolves! What you will see: A red, acidic solution of alum A blue, neutral solution of aluminum hydroxide precipitate which is cloudy A purple, basic solution of soluble tetrahydroxoaluminate complex Demonstration

4. Concepts • pH • Indicators • Precipitates • Amphoteric Behavior

5. pH • pH measures the concentration of hydronium ions (H3O+) in a solution • The pH scale is based on acids and bases which is based on the dissociation of water • H2O + H2O <-------> H3O+ OH- • Hydronium and hydroxide ions are in balance • A greater balance of H3O+ ions in a solution will make the solution acidic • A greater balance of OH- ions in a solution will make it basic • Fewer H3O+ ions in a solution will make it basic while fewer hydroxide ions will make the solution acidic • The pH scale ranges from 0 to 14 • At pH 7 the the concentrations of hydroxide and hydronium ions equal one another • pH affects the solubility of Aluminum Hydroxide • Aluminum Hydroxide is soluble in acidic and basic, but insoluble in neutral neutral acidic basic pH Scale 0 14 7

6. Indicators • Indicators are weak organic acids whose colors vary with pH • The color and range of an indicator are unique to that specific indicator • The colors that indicators exhibit are intense and unique • The presence of an indicator will not effect the experiment • A universal indicator has many different indicators mixed together and can detect changes along the entire pH scale

7. Precipitates • Solid products of a reaction that occur in solution are termed “precipitates” • Aluminum hydroxide precipitates out of solution as the sodium hydroxide increases the basicity of the solution • Aluminum hydroxide precipitate itself has the interesting property that it dissolves in strong acid and strong base but exists as insoluble precipitate at more intermediate pH values

8. Amphoteric Behavior • The ability of a compound to be soluble in acid and base environments and yet be insoluble solid (precipitate) in neutral • Metal hydroxides can exhibit amphoteric behavior • Aluminum Hydroxide is amphoteric • Aluminum Hydroxide is a solid in the pH range of 4-10 and a liquid in acid and base Aqueous <----> Solid <----> Aqueous pH 0 to 4 pH 4-10 pH 10 to 14

9. Amphoteric Behavior • Alum is soluble in acidic and basic environments • Formation of a solid (pH 4-10) [Al(H2O)6]3+ + 3OH- <-----> Al(OH)3 + 6H2O (liquid) (solid) • Solid dissolving (pH > 10) Al(OH)3 + OH- -----> {Al(OH)4]- (solid) (liquid) 1 2

10. Conclusions • The amphoteric nature of aluminum can be demonstrated by varying the pH of the solution in the presence of an indicator • Amphoteric behavior is the ability of a compound to be soluble in both high and low pH but not in between the two • The solid that forms in a solution and makes the solution cloudy is termed a “precipitate” • Indicators serve to visualize changes in pH by changing color with changing pH values

11. Comments • Other metal hydroxides demonstrate amphoteric behavior including hydroxides of zinc, lead, chromium and tin • The amphoteric behavior of metal hydroxides is beneficial in extracting the metal ions from aqueous solutions • Indicators are accurate to within .5 pH • The rates of many reactions can be altered by adjusting the pH