Introduction

In this experiment, you will prepare a common alum, KAl(SO₄)₂@12H₂O (potassium aluminum sulfate dodecahydrate), from an aluminum beverage can. Alums can be described as a class of compounds that contain the sulfate ion, SO₄^2-, a trivalent (3+) cation such as Al³⁺, Cr³⁺, Fe³⁺, and a monovalent (1+) cation such as K⁺, Na⁺, and NH₄⁺. Include these 12 water molecules when calculating the molar mass of alum.  Alums have many purposes, as shown in the table below. The alum that you will prepare forms white crystals similar to fine salt. The "@12H₂O" indicates that 12 molecules of water are present in the crystals for each KAl(SO₄)₂ formula unit.

Aluminum is the most abundant metal in the earth's crust (8.3% by weight) and is the third most abundant element after oxygen (45.5%) and silicon (25.7%). Pure aluminum is a silvery-white metal and has many desirable physical and chemical properties: it is light-weight, non-toxic, corrosion-resistant, non-magnetic, and malleable. Aluminum is commonly combined with other metals such as copper, manganese, silicon, magnesium, and zinc, which produces alloys with high mechanical and tensile strength. You are probably familiar with many uses of aluminum and its alloys.

When metallic aluminum comes into contact with aqueous solutions of strong bases like potassium hydroxide, KOH, it dissolves to from hydrogen gas (FLAMMABLE!) and a salt containing both aluminum and potassium ions.

2 Al (s) + 2 KOH (aq) + 6 H₂O (l) 6 2 K[Al(OH)₄] (aq) + 3 H₂ (g)

This salt, K[Al(OH)₄], is soluble in solutions of strong acids, such as sulfuric acid, H₂SO₄, and reacts to form aluminum hydroxide, Al(OH)₃, and potassium sulfate, K₂SO₄.

2 K[Al(OH)₄] (aq) + H₂SO₄ (aq) 6 2 Al(OH)₃ (s) + K₂SO₄ (aq) + 2 H₂O (l)

Excess sulfuric acid is required to dissolve the Al(OH)₃, and to form aluminum sulfate, Al₂(SO₄)₃.

2 Al(OH)₃ (s) + 3 H₂SO₄ (aq) 6 Al₂(SO₄)₃ (aq) + 6 H₂O (l)

As this solution is cooled, the aluminum and potassium sulfate salts crystallize out of the solution as the alum. The equation below shows only the ions and the water involved in the crystallization process.

K⁺ (aq) + Al³⁺ (aq) + 2 SO₄^2- (aq) + 12 H₂O (l) 6 KAl(SO₄)₂@12H₂O (s)

You will determine both the percent yield and the melting point of your alum crystals. Please see this site for a good introduction on the determination of melting points.

Procedure

A piece of scrap aluminum will be provided. Use steel wool to remove as much paint as possible. The inside of the can is protected with a plastic coating and you should remove this also. Weigh the cleaned strip of aluminum to the nearest 0.001 grams.

Cut the Al sample into small squares and place the squares in a clean 100-mL or 150-mL beaker. Perform the following in the hood!! Carefully add 20 mL of 4 M potassium hydroxide, KOH. Bubbles of hydrogen gas should evolve. Place your beaker on a hot plate in the hood to speed up the reaction. When hydrogen bubbles are no longer formed, the reaction is complete. This should take 10 - 15 minutes. Remove the beaker from the hot plate and allow it to cool at your bench.

The resulting grayish mixture can be filtered to remove unwanted impurities. If the solution is still warm, cool it by placing the beaker in an ice bath. Set up a 250-mL filter flask and Gooch filter crucible (or other filtration apparatus) as demonstrated. Don't forget to clamp to flask to some kind of support. Filter the solution slowly. Rinse the beaker two times with small portions (<5 mL) of distilled water, pouring each rinse through the filter. Transfer the clear colorless filtrate to a clean 250-mL beaker.

Add 10 drops of methyl red indicator solution to the colorless solution. Methyl red is yellow in basic solutions and red in acidic solutions. To the cool solution, slowly and carefully, with stirring, add 15 mL of 6 M sulfuric acid. The solution should turn red and white lumps of Al(OH)₃ should form. Again working in the hood, heat and stir the mixture to dissolve the white lumps. Excess sulfuric acid may be added dropwise, but no more than 30 mL total, in order to totally dissolve the Al(OH)₃, and give a clear red solution.

Cool the clear solution in an ice bath for at least 20 minutes. Crystals of alum should fall to the bottom of the beaker. If no crystals form, scratch the bottom of the beaker with your stirring rod. If that fails, add a "seed" crystal of alum to facilitate crystallization.

While the solution is cooling, reassemble the filtration apparatus. Be sure to weigh the filter paper at this point. Slowly filter the solution containing the crystals. Rinse the beaker once with 5 mL of cool distilled water and once with 5 mL of isopropyl (rubbing) alcohol, pouring each rinse through the filter. Allow the aspirator to pull air through the crystals until they appear dry, at least 5 minutes. Remove the filter from the flask. More crystals may form in the red filtered solution. If you have time, filter these crystals as just described, and add these to the first crystal crop.

Remove the damp crystals and filter paper from the Gooch filter and place them into a weighed beaker.

Clean the Gooch filters, filter flasks, and rubber rings and return them to the hood.

Place the beaker containing your crystals in your locker until next week. After drying, weigh the filter and crystals, and determine the mass of the alum produced. Calculate the percent yield.

Determine the melting point of the alum crystals using the following procedure. Carefully, push the open end of a capillary tube into your crystals, forcing some of the solid into the tube. Turn the tube over and tap it gently to move the crystals to the sealed end of the tube. Repeat this process until you have about 5 mm of solid in the capillary tube. Carefully, insert the tube into the melting point apparatus. The apparatus will be HOT if others have been using it! Note the temperatures at which the alum first appears to melt (i.e. when liquid first becomes visible) and at which it is completely melted (all liquid). Discard the capillary tube.