CHM 107
Experiment 4


In this experiment an alum will be SYNTHESIZED and ANALYZED for its sulfate content.

Aluminum is the most abundant metal in the earth's crust (7.5% by mass).  The abundance of aluminum, along with its attractive combination of physical and chemical properties, accounts for the fact that it is one of the major industrial raw materials used in industry.

Although the mass of a single aluminum beverage can is small, the large number of containers produced each year means that a large tonnage of the metal is utilized.  Since the metal is not consumed rapidly by corrosion, the amount of scrap aluminum grows rapidly while the available supply of the materials for the manufacture of aluminum decreases.  Ecological problems thus created are typical of those of several different metals.  One obvious solution to the problem is to recycle the used fabricated aluminum into other useful metallic objects.  Chemical recovery methods can also be used to recycle aluminum. 

This experiment illustrates one such chemical RECOVERY process.  You will convert metallic aluminum into an industrially important compound, hydrated potassium aluminum sulfate, or common alum.  "Alum" is a generic term that describes hydrated double salts of certain metals.  Alums can be described by the generalized formula,


    (MM'(SO4)2 H2O 

in which M represents a univalent ion such as K+ and M' represents a trivalent ion such as Al3+.  True alums crystallize in a well-defined octahedron structure and many are beautifully colored, particular those containing d-block elements.  The ancient Egyptians, Greeks, and Romans used alums as a mordant in dyeing cloth.  A mordant contains metal ions that bind dyes to the fabric.   Presently alums are used in everything from film processing to pickle making. 

Beginning with an aluminum beverage can, an alum will be synthesized.  Metallic aluminum dissolves in aqueous solutions of strong bases like KOH to form hydrogen gas and a soluble salt containing a complex ion. 


          2 Al + 2 KOH + 6 H2O ---> 2 KAl(OH)4 + 3 H2 

The addition of sulfuric acid converts the soluble salt into an insoluble hydroxide and potassium sulfate, but a slight excess of acid will dissolve the hydroxide to give a clear solution. 

2 KAl(OH)4 + H2SO4 --> 2 Al(OH)3 + K2SO4 + 2 H2O 

As the solution is cooled, the double salt, hydrated potassium aluminum sulfate "ALUM", crystallizes out of the solution.  

     K+ + Al3+ + 2SO42- + 12H2O ---> KAl(SO4)2 12H2O 

The alum is then filtered, dried, and weighed.  The actual yield of alum can be compared to its theoretical yield. 

In the next part of the experiment the sulfate content of the alum will be determined by forming and weighing dry barium sulfate.   

Since the alum is soluble in water, the sulfate content can be determined by precipitation, drying, and weighing an insoluble metal sulfate.  This technique is called gravimetric analysis.  A good insoluble metal sulfate for this analysis is barium sulfate.  A weighed sample of alum is dissolved in water, and then a soluble barium compound such as barium chloride is added to precipitate insoluble barium sulfate. 

Ba2+ + SO42- ---> BaSO4 

This insoluble salt can be readily isolated by filtration and dried without decomposing by heating in a flame.  While other ions such as the carbonate can interfere by forming additional insoluble barium salts, appropriate procedures can be used to remove these ions before the precipitation with barium occurs.  Carbonate ions are removed by decomposing with acid.




Many beverage cans are not 100% aluminum, but are alloys of aluminum that dissolve slowly.  Use a 1 or 2 inch square piece of aluminum beverage can.   Sandpaper it to remove as much paint and lacquer as possible.  The piece of aluminum should weigh about 1.2 grams, record.  Transfer the aluminum to a clean 250 ml beaker.  Add 35 ml of 2 M KOH to the beaker.  CAUTION: KOH CAN DISSOLVE CLOTHING AND SKIN AS WELL AS ALUMINUM.  Place the beaker on a hot plate and heat gently.  The reaction is complete when bubbles of gas cease to form.   Remove the beaker from the hot plate and allow the solution to cool to room temperature.  Gravity filter the solution into a 250 ml beaker.  Use a wash bottle to rinse all of the dissolved aluminum solution out of the beaker.  Avoid using more than 20 ml of water to wash the beaker. 

Now add 10 drops of methyl red indicator to the clear solution.  Methyl red is yellow in basic solution and red in acidic solution.  Obtain 25 ml of 6 M sulfuric acid in a beaker.  Add the 6 M sulfuric acid a few milliliters at a time with stirring until the solution turns red.  Avoid adding excess sulfuric acid.  Heat the solution gently on a hot plate and stir vigorously until all the Al(OH)3 has dissolved.   The hot solution should be red and contain no suspended solids.  If it is not red, carefully add a few drops of 6 M sulfuric acid until it is red.  Cool the clear, red solution in an ice bath for 20 minutes with occasional stirring.  Well-defined crystals of alum should form.  After complete crystallization collect the alum crystals by vacuum filtration.  Allow the aspirator to draw air through the crystals for several minutes to help air dry the crystals.  Next transfer the crystals to a clean dry paper towel and allow to dry at least 24 hours.   After the crystals are dry, weigh and record. 

At this point in the experiment you have converted the aluminum in the beverage can into alum, a white crystalline compound.  Next you will determine by gravimetric analysis the sulfate content of an alum sample. 

Weigh about 1 gram of alum (SUPPLIED BY INSTRUCTOR) into a clean 400 ml beaker and add 200 ml of distilled water.  The alum should completely dissolve.   After the alum has dissolved add about 5 ml of 6 M HCl solution.  CAUTION:  HCL IS CORROSIVE AND CAN CAUSE BURNS ON YOUR SKIN OR BURN HOLES IN YOUR CLOTHING.  The purpose of adding the HCl is to decompose any carbonate ions that may be present. 

Next prepare 100 ml of a 0.1 M BaCl2 solution.  Write specific instructions/calculations in your checklist on how to accomplish this task.  Do not include these calculations in your Lab Report.  You will need a 100 ml volumetric flask to prepare the solution. 

Add 50 ml of your 0.1 M BaCl2 solution with constant stirring to the 400 ml beaker containing the dissolved alum.  After addition of the barium chloride, carefully heat the beaker on a hot plate until the contents boil.   Boil the mixture gently for 10 minutes, and allow to cool with occasional stirring.  After the mixture has cooled to room temperature and the precipitate has settled, carefully decant the clear solution into a waste container and discard.  Now gravity filter the precipitate and any remaining solution using ashless filter paper.  Use a wash bottle to assist in the transfer of all of the precipitate from the beaker onto the ashless filter paper. 

Heat a clean crucible and its cover as hot as possible for about 5 minutes.  Let the crucible and its cover cool to room temperature and then weigh, record.  Next transfer the wet ashless filter paper containing the barium sulfate precipitate to the crucible, and fold the paper so that it is contained entirely within the crucible.  Place the crucible on a clay triangle with the cover slightly ajar so that some air can enter.  Heat gently at first to char the filter paper.  Do not allow the filter paper to flame.  After the filter paper is charred, heat the crucible to a red heat.  The filter paper will burn off completely and only a white precipitate of barium sulfate will remain in the crucible.  Allow the crucible and its cover to cool to room temperature, weigh and record.  Reheat the crucible with the full flame for a second time, allow to cool and reweigh (HEATING TO A CONSTANT WEIGHT).   

Data Sheet (to help collect and organize data/calculations)




mass of aluminum                                       _______

moles of aluminum                                     _______

moles of alum                                             _______

mass (theoretical yield) of alum                   _______


mass (actual yield) of dry alum crystals        _______


percent yield of alum                                     _______  





mass of alum (use about 1 gram)                      _______

mass of empty crucible and cover                     _______

mass of crucible, cover, barium sulfate (1st heat)  _______

mass of crucible, cover, barium sulfate (2nd heat)  _______


mass of barium sulfate                               _______

moles of barium sulfate                              _______

moles of sulfate                                         _______

mass of sulfate                                           _______


experimental % of sulfate in alum               _______




molar mass of alum (from molecular formula)         _______

% sulfate in alum (from molecular formula)              _______



% error                                              _______