Converting Aluminum to Aluminum Potassium Sulfate

Review Questions

Not for Credit



Aluminum is hardly a precious metal, on par with gold, silver, or platinum. In fact, it is so common today that we use it for our cheap aluminum foil and soda cans. In the 19th century however, it was a luxury. Aluminum was made into expensive bracelets alongside gold, and Napoleon III was presented with a set of aluminum forks and spoons and a valuable centerpiece crafted from aluminum, gold, and bronze. This stark contrast in availability stems from the Hall-Heroult process developed in 1886, allowing the economically viable, commercial production of aluminum. Society was thrilled with this practical new metal, and many household products were made from it-from teakettles to playing cards. Less than a hundred years after the design of this process, the United States experienced an abundance of aluminum, at which time people even made rugs out of it!

You will have the opportunity to work with this unique metal today. From aluminum foil, you will make a useful compound commonly called alum (specifically, aluminum potassium sulfate, KAl(SO4)2). There are many practical uses of alum, the product you will be isolating today. It is used industrially in manufacturing dyes and dyeing fabric. Alum is also used in making paper, cement and explosives. Interestingly enough, this chemical, with all of its diverse uses, is also employed in pickling cucumbers!

The conversion of aluminum foil to alum involves several chemical steps. The procedures for each step of today's experiment are more complicated in comparison to the type of lab work you have done so far. It is a good idea to see what is happening qualitatively before you begin.

In the first chemical step of the experiment, you will add a strong base, potassium hydroxide (KOH), to your aluminum foil. You will heat this mixture on a hot plate. The aluminum and hydroxide will combine to form Al(OH)4- and H2(g). The chemical equation for this reaction is shown below:

2Al(s) + 2KOH(aq) + 6H20(l) 2K+(aq) +2Al(OH)4-(aq) + 3H2(g)

The next step in the procedure is the addition of sulfuric acid (H2SO4), to your sample. Adding acid causes two chemical reactions to occur. First, the Al(OH)4-(aq) species is converted into Al(OH)3 (s), which is a white precipitate. The chemical equation for this conversion is:

2Al(OH)4-(aq) + H2SO4(aq) 2Al(OH)3(s) + 2H2O(l) + SO42-(aq)

This white precipitate will disappear as the acid is stirred into the solution, however, as Al3+(aq) and water form from the addition of acid to Al(OH)3 (s). The second chemical change caused by the addition of acid is:

2Al(OH)4-(aq) + 3H2SO4(aq) 2Al3+(aq) + 3SO42-(aq) + 6H2O(l)

Finally, we see the formation of hydrated alum crystals (KAl(SO4)2·12H2O(s)) as the solution is cooled.

K+(aq) + Al3+(aq) +2SO42-(aq) +12H2O(l) KAl(SO4)2·12H2O(s)

Note that the alum formed is Al(SO4)2·12H2O(s) The ·12H2O means that 12 molecules of water are associated with each molecule of the potassium aluminum sulfate. These molecules are the water of hydration, and in many compounds are associated with a color change. You will measure this product after it "air dries" but still has its twelve associated water molecules- the water of hydration. .