1. Amphoteric Behavior of Alum
The amphoteric behavior of alum is shown by varying the pH of its solution
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2. Purpose To observe the solubility of alum in a changing pH environment
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3. An acidic solution of “alum” --- aluminum sulfate, Al2(SO4) 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
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4. Concepts pH
Indicators
Precipitates
Amphoteric Behavior
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5. 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
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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
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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
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8. 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 pH pH 10 to 14
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9. Amphoteric Behavior Alum is soluble in acidic and basic environments
Formation of a solid (pH 4-10)
[Al(H2O)6] OH- <-----> Al(OH)3 + 6H2O
(liquid) (solid)
Solid dissolving (pH > 10)
Al(OH)3 + OH > {Al(OH)4]-
(solid) (liquid) 1 2
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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
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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
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