Amphoteric Behavior of Alum The amphoteric behavior of alum is shown by varying the pH of its solution 11/9/2018

Purpose To observe the solubility of alum in a changing pH environment 11/9/2018

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 11/9/2018

Concepts pH Indicators Precipitates Amphoteric Behavior 11/9/2018

pH acidic neutral basic pH Scale 14 7 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 acidic neutral basic pH Scale 11/9/2018 14 7

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 11/9/2018

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 11/9/2018

Aqueous <----> Solid <----> Aqueous 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 11/9/2018

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 11/9/2018

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 11/9/2018

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 11/9/2018