Experiment Number 5

 

Synthesis of Alum, Potassium Aluminum Sulfate

 

 

 

Introduction

 

Alums are a class of compounds that are usually composed of a monovalent metals, a trivalent metal and sulfate.  The most common alum is probably potassium aluminum sulfate which is also called potash alum.  When these compounds are synthesized and crystallized, the crystal structure often includes a significant amount of water in proportion to the alum.  Thus, the alum that we will synthesize in this lab exercise is KAlSO4·12H2O.

 

Alums are used in water purification, leather tanning, mordant dyeing, as astringents, and in baking powder.  Alum occurs in nature as the mineral kalunite. Sodium aluminum sulfate, or soda alum, NaAl(SO4)2·12H2O, is also used in baking powder. Ammonium aluminum sulfate, or ammonia alum, NH4Al(SO4)2·12H2O, is used in tanning, in dyeing and fireproofing textiles, in vegetable glues and porcelain cements, and in water purification. Chromium potassium sulfate, or chrome alum, KCr(SO4)2·12H2O, is used as a mordant in dyeing, in tanning, and in photographic fixing baths to harden gelatin films and plates.

 

 

 

 

Materials

 

Aluminum foil

Sulfuric Acid, 3M

Potassium Hydroxide. 3M

Ethanol/Water (50/50)

Beaker, 250 mL

25 mL Grad. Cylinder

Funnel

250 mL Flask

Filter Paper, fluted

Wash bottle (distilled H2O)

Stirring rod

Hot plate

Balance

Parafilm®

 

 

 

Safety

 

Goggles must be worn at all times during this experiment.  Sulfuric acid is is extremely corrosive and potassium hydroxide is extremely caustic.  Use caution when using these chemicals.  Should anything get on your hands or clothing wash off immediately.

 

 

 

 

Procedure

 

1.         Obtain about 1 g of aluminum foil and weigh to the nearest hundredth of a gram. Tear the foil into small pieces and place in a clean 250 mL beaker.

2.         Add 25mL of 3M KOH solution to the aluminum foil in the 250 mL beaker using a 25 mL pipette.  Allow reaction to proceed until all the aluminum is dissolved.

3.         Flute a piece of filter paper and place in a funnel.  Place the funnel into 500 mL flask.  Filter the dissolved aluminum while hot to remove the carbon.  Wash the  beaker and the filter paper with a small amount of distilled water.  Discard the filter paper and rinse the funnel.  If necessary, this is a place to stop and store solution over night.

4.         Obtain 35 mL of 3M H2SO4 in a graduated cylinder.  Acidify the dissolved aluminum/KOH by adding slowly while stirring the 35 mL of 3M H2SO4.  Use a glass rod to stir the solution.  You will notice that a white precipitate forms and then dissolves in the acid.  You will also notice that the solution becomes very hot as a result of mixing a strong acid with a strong base

5.         If precipitate remains, filter the solution again as described above.

6.         Boil the solution to a volume of about 40 to 50 mL.  Allow the solution to cool slowly. This is a good stopping place.

7.         Once the solution has cooled to room temperature, cover with Parafilm®  or plastic wrap.  You may wish to experiment with placing some of the flasks in the refrigerator.  Crystals should form overnight.

8.         Collect the crystals on a piece of pre-weighed filter paper and wash briefly with the 80% ethanol.  Determine the percent yield relative to the expected yield.

 

 

 

 

 

 

Questions

 

1.                  If a 1:1 ratio of aluminum to alum exists, what mass of aluminum would be required to produce one pound of alum.?

 

2.         What factors would make the yield less than 100%?  greater that 100% (Be specific-human error is not an acceptable answer.)

 

3.         Why is the final filtration a wash with 80% ethanol instead of just water?

 

4.         Calculate the molar mass of potassium chromium duodecohydrate.

 

5.         Describe the shape of your crystals in terms of geometric shapes?