Introduction

An alum is a hydrated double sulfate salt with the general formula

M⁺M^'3+(SO₄)₂^.12H ₂O

Here M⁺ is a univalent cation, commonly Na⁺, K⁺, NH₄⁺ or Ag⁺; and M'³⁺ is a trivalent cation, commonly Al³⁺, Fe³⁺, Cr³⁺, Ti³⁺, or Co³⁺. This experiment describes the preparation of two alums---potassium alum and chrome alum.

Preparation of Potassium Alum

Potassium aluminum sulfate dodecahydrate (potassium alum), KAl(SO₄)₂^.12H₂O, is prepared from an aluminum can and potassium hydroxide. Aluminum metal rapidly reacts with hot aqueous KOH producing a soluble potassium aluminate salt solution:

2 Al(s) + 2 K⁺ (aq) + 2 OH^- (aq) + 6 H₂O(l) -> 2 K⁺ (aq) + 2 Al(OH)₄^- (aq) + 3H₂ (g)

When treated with sulfuric acid, the amphoteric aluminate ion, Al(OH)₄^-, precipitates as aluminum hydroxide, but dissolves with heat.

2 K⁺ (aq) + 2 Al(OH)₄^- (aq) + 2 H⁺ (aq) + SO₄²- (aq) ->

2 Al(OH)₃ (s) + 2 K⁺ (aq) + SO₄^2- (aq) + 2 H₂O (l)

2 Al(OH)₃(s) + 6 H⁺ (aq) + 3 SO₄^2- (aq) 2 Al³⁺ (aq) + 3 SO₄^2- (aq) + 6 H₂O(l)

The potassium aluminum sulfate dodecahydrate alum forms octahedral-shaped crystals when the nearly saturated solution is cooled.

K⁺ (aq) + Al³⁺ (aq) + 2 SO₄^2- (aq) + 12 H₂O (l) -> KAl(SO₄)₂^.12H₂O (s)

Preparation of Chrome Alum

Potassium chromium(III) sulfate dodecahydrate (chrome alum), KCr(SO₄)₂^.12H₂O, is prepared by reducing the dichromate ion from potassium dichromate, K₂Cr₂O₇, to Cr(III) in a sulfuric acid solution using ethanol, C₂H₅OH, as the reducing agent.

8 H⁺ (aq) + Cr₂O₇^2- (aq) + 3 C₂H₅OH (aq) -> 3 CH₃CHO(aq) + 2 Cr³⁺ (aq) + 7 H₂O (l)

The sulfate ion from sulfuric acid and the potassium ion from the potassium dichromate combine with the chromium(III) ion to form violet to dark green octahedral-shaped crystals when the concentrated solution is cooled.

K⁺ (aq) + Cr³⁺ (aq) + 2 SO₄^2- (aq) + 12 H₂O (l) -> KCr(SO₄)₂^.12 H₂O (s)

Procedure

A. Potassium alum (Supply your own aluminum can.)

1. Cut about 1g (±0.01 g) of aluminum metal from your can. Use steel wool to remove material coating the aluminum and cut the aluminum into very small pieces. Place these in a 150-mL beaker. Add 25 mL of 4 M KOH to the aluminum pieces cautiously.
2. Warm the beaker gently to initiate the reaction. Gas is evolved in this reaction; make sure this reaction occurs in a well-ventilated area. What gas is generated? When no further reaction of aluminum is evident, gravity filter the warm solution to remove any insoluble impurities.
3. Allow the clear solution to cool in a 150-mL beaker. Add a few drops of methyl red indicator. While stirring, slowly add 6 M H₂SO₄. Stop adding when solution turns from yellow to pink (roughly 25 mL, but no more than 30 mL). Insoluble aluminum hydroxide should begin to form. Do not redissolve the A l(OH)₃ with excess H₂SO₄.
4. Gently heat the reaction mixture with stirring until the aluminum hydroxide dissolves.
5. Remove the solution from heat; filter only if solids are present. Cool the solution in an ice bath. Alum crystals should form within 20 minutes. If crystals do not form, gently heat to reduce the volume by one-half. For larger crystals and a higher yield, allow the crystallization to occur overnight without cooling in an ice bath.
6. To test the purity of alum, proceed to part C.

B. Chrome alum

1. Add 100 mL of 2.0 M H₂SO₄ to a 250-mL beaker. Measure (±0.01 g) about 15 g of K₂Cr₂O₇, add it to the beaker and stir to dissolve.
2. Cautiously add 10 mL of 95 % ethanol dropwise to the above solution, with constant stirring, in a fume hood. Do not allow the reaction mixture to exceed 50°. If necessary use an ice bath to maintain temperature. Keep flames away.
3. Allow the reaction mixture to cool overnight for crystallization to occur.
4. If crystals do not form, gently heat the mixture on a hot plate (but do not exceed 50°) to reduce the volume to about one-third of the original volume. Cover the beaker with a watch glass and cool for crystallization to occur. Use an ice bath to promote crystallization.
5. To test the purity of alum, proceed to part C.

C. Melting point of the Synthesized Alum

1. Vacuum filter the crystals from the solution. Wash the crystals on the filter paper with two 5-mL portions of a 50% (by volume) ethanol-water solution.
2. Determine the mass of the dry crystals.
3. Determine the melting point of the crystalline alum.

Results

1. Calculate the % yield of each alum.
2. Compare your melting points to literature values.
3. How is the melting point affected by the presence of impurities?
4. In part A.3., why does excess H₂SO₄ dissolve Al(OH)₃?
5. Do some research to determine some common uses of alums. Indicate the types of alums used and the purpose of each.