2. Chem Lab Days Experiments
  3. Recycling Waste Aluminum: Synthesis of Potassium Alum

Experimental Procedure

Your fumehood will contain all the equipment and glass ware you need from the experiment. The location of all the chemicals are indicated throughout the procedure. Work in the fumehood at all time!

Partner 1:

Take the 250 mL beaker and go to the top-loading balance to weigh ~1 g of aluminum pieces. The aluminum is located near the balances, use the tweezers to add the amount you need and note the mass you weighed.

Partner 2:

Collect a bottle of 1.5 M \(\ce{KOH(aq)}\) and a bottle of 9 M \(\ce{H2SO4(aq)}\) from the supply fum hood. The bottles contain the required amounts mentioned below. 

Alum Synthesis

  1. Place the 250 mL beaker containing the aluminum pieces on the hotplate. Add all the 1.5 M \(\ce{KOH(aq)}\) solution contained in the bottle (40 mL) and a stir bar (white pill-looking magnet!).

    Balance the associated chemical equation by adding the missing stoichiometric coefficients:

       \(\ce{Al(s)}\:+\:\)  \(\ce{KOH(aq)}\:+\:\)    \(\ce{H2O(l)}\:\rightarrow\:\)    \(\ce{Al(OH)4^{-}(aq)}\:+\:\)    \(\ce{K+(aq)}\:+\:\)    \(\ce{H2(g)}\)

     

    Note: This reaction a redox reaction! To balance it, you must first balance the two equations below (the half-reactions) in a manner that the electrons are cancelled when you combine them to get the redox reaction.

           \(\ce{Al(s)}\:+\:\textbf{4}\:\ce{KOH(aq)}\:\rightarrow\:\ce{Al(OH)4^{-}(aq)}\:+\:\textbf{4}\:\ce{K+(aq)}\:+\:\textbf{3}\:\ce{e^{-}}\)

           \(\textbf{2}\:\ce{H2O(l)}\:+\:\textbf{2}\:\ce{e^{-}}\:+\:\textbf{2}\:\ce{K+(aq)}\:\rightarrow\:\ce{H2(g)}\:+\:\textbf{2}\:\ce{KOH(aq)}\)

How Did I Do?
  1. Start stirring the solution by pressing the button with a circular arrow on the hotplate. Then set the heating to 150 ºC and heat until the aluminum is all dissolved. The solution may be grey but there should be no metal pieces left. This should take 10–20 min, increase the heat to 180 ºC if needed but do not let the solution boil! Turn off the hotplate once the aluminum has dissolved and remove your beaker. 
  2. Vacuum filtration assembly, showing the Buchner funnel on top of a filter flask connected to the vacuumRemove impurities from the can by using suction filtration. First add a filter paper to the funnel and spray a small amount of DI water to wet the paper. Turn the vacuum on by turning the yellow knob on the outside edge of your fume hood counterclockwise until you feel resistance. Don’t turn the vacuum on too much, to avoid tearing the filter paper. When the vacuum is turned on you should see water drip from the Buchner funnel into the vacuum flask. Then filter your reaction by pouring it in the Buchner funnel.
  3. Using no more than 5 mL cold DI water per rinse, rinse the beaker twice with DI water and pour each rinse through the filter. If the filtrate is not clear (still grey) it should be re-filtered. Yellowish is okay!
  4. Transfer the filtrate (liquid) to a CLEAN 250 mL beaker. Rinse the suction flask with minimal DI water (no more than 10 mL) and add this to the contents of the beaker.
  5. Add the contents of the 9 M \(\ce{H2SO4(aq)}\) solution bottle (20 mL) slowly to the solution (pour small portions from the bottle, if you add the acid too fast, the mixture will boil over!) to the reaction solution. Stir with a glass rod. Heat to 200 °C and continue stirring until all solids dissolve. If it takes more than 5-10 min, ask for help. Cool your solution to room temperature by removing from the hotplate.

    Balance the associated chemical equations by adding the missing stoichiometric coefficients: 

    1.  \(\ce{Al(OH)4^{-}(aq)}\:+\:\)  \(\ce{H2SO4(aq)}\:\rightarrow\:\)  \(\ce{Al(OH)3(s)}\:+\:\)  \(\ce{H2O(l)}\:+\:\) \(\ce{SO4^{2-}(aq)}\)

     

    1.  \(\ce{Al(OH)3(s)}\:+\:\)  \(\ce{H2SO4(aq)}\:\rightarrow\:\)  \(\ce{Al^{3+}(aq)}\:+\:\)  \(\ce{SO4^{2-}(aq)}\:+\:\)  \(\ce{H2O(l)}\)
How Did I Do?
  1. Prepare an ice bath by adding ice and tap water to the 400 mL beaker. Further cool your reaction by placing your 250 mL beaker into the ice bath. You should see a white solid form, which is the potassium alum, \(\ce{KAl(SO4)2.12H2O}\). If you do not see solids after 5-10 min of cooling, agitate the flask or mix the solution with your glass rod.

    Balance the associated chemical equation by adding the missing stoichiometric coefficients:

     

     \(\ce{Al^{3+}(aq)}\:+\:\)  \(\ce{K+(aq)}\:+\:\)  \(\ce{SO4^{2-}(aq)}\:+\:\)  \(\ce{H2O(l)}\:\rightarrow\:\) \(\ce{KAl(SO4)2.12H2O(s)}\) 

How Did I Do?
  1. Collect the crude alum by suction filtration. Use no more than 5 mL cold DI water to help you transfer all the solid. Let the vacuum remove the water by suction for ~ 2 minutes.
  2. While your product is being filtered, weigh a large petri dish on the balance and note its mass in the Results section.
  3. Transfer your alum to the petri dish. Weigh the petri dish and your product and note the mass in the Results section.
  4. Dispose of the filtrate (liquid) from you alum isolation in the aqueous waste located in the waste fumehood.

 

The following video presents the procedure performed in the steps above:

Expand/Collapse Transcript

Begin the reaction by taking your 250 mL beaker from your fume hood and bringing it to the top holding balances. Weigh approximately 1 gram of aluminum by picking pieces with the tweezers. It is okay if the mass is not exactly 1 gram, simply note down how much aluminum you weighed.

Back at the fume hood, add the whole content of the bottle of 1.5 molar potassium hydroxide solution to the beaker containing the aluminum. 40 mL of the solutions were pre-measured for you. Add the stir bar to the beaker and set the heat to 150° C and begin the stirring. Use the glass rod to push down on the pieces of aluminum, they will react faster when fully submerged in the solution and the gas produced by the reaction tends to make them float.

As the aluminum reacts, the solution will take a dark color and become cloudy. The reaction is complete when all the aluminum is gone. You can look for shiny pieces of metal in the solution, if you do not see any you can proceed to the filtration. Take your beaker off the hot plate and remove the stir bar with the magnetic retriever. Rinse the stir bar with a small amount of deionized water.

Prepare for vacuum filtration by adding a filter paper to the Buchner funnel. On the outside of the fume hood, you will find a yellow dial that controls the vacuum. Turn this dial counterclockwise until you feel resistance. Wet the filter paper with some deionized water to ensure it is sticking. You should see water dropping down into the vacuum flask rapidly if the vacuum is strong enough.

Transfer your reaction mixture to the Buchner funnel. Rinse with a little bit of water, but not too much! We need to avoid excess water to ensure the potassium alum solid will form later on.

Once the filtration is complete, turn the vacuum off by turning the yellow dial clockwise. Discard the filter paper containing the residue from the pop cans in the solid waste. You will work with the liquid filtrate, it should not be cloudy but the yellow or orangish color is fine. Pour the liquid filtrate into the 250 mL beaker. Make sure you have cleaned the beaker beforehand.

Add the content of the bottle of 9 molar sulfuric acid to the beaker. The bottle contains 20 mL of sulfuric acid solution. Do so slowly in little portions as the reaction can heat up a lot. You will see white precipitate form. Once all the sulfuric acid has been added, place the beaker on the hot plate and turn on the heat to 200° C. Do not add the stir bar, simply mix the solution with the glass rod. Once the precipitate is fully dissolved and the solution is clear, cool the beaker to room temperature.

Use the large Beaker as a scoop to take some ice and add some top water to make the ice bath. Place the beaker into the ice bath to cool its content and maximize the precipitation of the potassium alum. You may need to agitate the solution to see the white precipitate. That is the potassium alum!

Perform vacuum filtration as done earlier, but this time you will be using the solid. Keep the vacuum on until the solid feels like sand when touching it in the Buchner funnel.

While waiting for the filtration, weigh the empty petri dish. Zero the balance first then place the closed petri dish and note its mass on your data sheet. Back at the fume hood, take the Buchner funnel off and transfer the solid from the Buchner funnel to the petri dish.

The petri dish containing the crude potassium alum is now weighed again. By subtracting the mass of the empty petri dish, you can obtain the mass of crude potassium alum you made in the lab.

In the next video, we will show you how to make a seed crystal by slow crystallization.


Qualitative Tests

Separate the work between partners. The volunteers will show you where each test is performed, bring a portion of the alum you made to perform each test.

Sulfate, \(\ce{SO4^{2-}(aq)}\), test:

Place a small scoop of the alum in a test tube. Add 5 drops of DI water and stir to dissolve the alum, adding more water if needed. Add 1 drop of \(\ce{BaCl2}\) solution (green top). Note observations in the Results Section. If a white precipitate is observed, the solution contains sulfate ions. \(\ce{BaSO4}\) is insoluble in water!

Potassium, \(\ce{K+(aq)}\), test:

The metal wire loops are stored in an \(\ce{HCl}\) solution. Take one and heat it in the flame until it glows red, before dipping it in your alum sample. Place the metal wire loop back in the flame. Note observations in the Results Section. When heated at high temperature, each element has a characteristic light they will emit. If the flame is light purple/lilac, the sample contains potassium.

Aluminum, \(\ce{Al^{3+}(aq)}\), test:

Place a small scoop of the alum in a test tube. Add 5 drops of DI water and stir to dissolve the alum, adding more water if needed. Add 2 drops of aluminon solution (red top). Add 6 M \(\ce{NH3(aq)}\) (blue top) dropwise until pH is basic. Note observations in the Results Section. If a pink precipitate is observed, the solution contains aluminum ions.


Waste Information

Disposing of waste properly is important for the environment and improper waste disposal can be a potential hazard. In the laboratory, we will have a waste guide to indicate where to dispose of waste and show you where each container is. Please look at the waste guide sheet provided at your benches on where things need to go. If you are ever unsure, please ask for help! :-)


Making a Seed Crystal (If time permits!)

When you are ready to do the quality tests, the volunteer will tell you if you have time to grow a crystal in the lab. They will give you a smaller petri dish to work with.

  1. Weigh ~ 4g of alum into a 100 mL beaker. Add 10 mL DI water to the beaker and heat on the hotplate (set at 190 ºC) and stir occasionally with a glass rod.
  2. When all the solid has dissolved, add a little bit more alum. Continue adding alum in small scoops until the alum no longer dissolves. 
  3. Pour the solution into the small petri dish and cover with the lid. If there is a bit of solid at the bottom of the beaker do not transfer the solids. 
  4. Perform the qualitative tests while you leave the petri dish undisturbed. You should see crystals growing after 10 minutes. 
  5. To take home these crystals, pour out the water before you tape the petri dish. One of these crystals can be used as a 'seed crystal' to grow a large alum crystal!

The following video presents the formation of the seed crystals:

Expand/Collapse Transcript

At the top loading balance, weigh around 4 grams of your crude potassium alum into a 100 mL beaker. If ever you spill solid on the balance, make sure to clean up after yourself.

At the fume hood, measure 10 mL of water using the graduated cylinder. Add to the beaker containing the alum and heat on the hot plate at 190° C. Stir with a glass rod. Once all the alum is dissolved, you can add a few more scoops of the solid until you see that the solid is difficult to dissolve.

The goal is to get as close to a saturated solution as possible. Solubility is higher at greater temperature, so when we cool down the solution the potassium alum will form nice crystals.

Once you have saturated the solution, transfer it to a clean petri dish. After a few minutes, crystals will be seen growing. Make sure not to move the petri dish during this time, otherwise you may force the potassium alum out of solution as a powder instead of letting the crystals grow nicely.

If you do not have time to grow crystals in the lab, you can do it at home! Just dissolve your alum powder with hot water from a kettle. Amounts do not matter much if you have patience. If you leave the solution uncovered without moving it the water will evaporate and a large alum crystal can be obtained. 


Classroom Activity

Since potassium alum is safe, you will be able to bring your product home! In class or at home, you can use a slower method to grow a very large alum crystal. There are multiple ways to do this, but this is what we recommend:

  • Select your nicest alum crystal in your product. Tie it with a string, sewing thread, or fishing line works great. If it is all powdery, you can move to the next step.
  • Dissolve the rest of your alum in hot water (a kettle is perfect to heat). Try not to use too much!
  • Suspen your seed crystal on a string in the solution (if you had one). Wait for crystal growth!
  • Try new methods if you want. You can do internet research for the inventive ways people have used! You can even make ornaments that look like icicles with alum.