- Synthesize alum from aluminum foil.
- Use skills learned in Labs 1-7 to perform the synthesis of a compound
- Select the appropriate filtration technique for the desired separation.
- Use gravity filtration to separate the filtrate from undesired reside.
- Calculate percent yield of a synthesis.
Today we will be synthesizing alum [KAl(SO4)2·12H2O, potassium aluminum sulfate dodecahydrate, MW = 474.4 g/mol] from aluminum foil. Alum has a variety of uses, including a pickling agent (keeps pickels crisp), a chemical flocculant in water purification, an adjuvant in vaccines (enhances immune response), a mordent for dying fabrics (sets the color), and a natural deodorant.
Alums are a group of ionic compounds that crystallize from solutions containing sulfate ion, a trivalent cation (Al3+, Cr3+, or Fe3+), and a monovalent cation (K+, Na+, or NH4+). Alum is also a hydrate – an ionic compound in which water molecules are chemically bound to the compound. The trivalent metal ion has six of the water molecules bound tightly to it; the remaining six molecules are more loosely bound to the other cation and the sulfate anion.
The equations given below show the complete sequence of reactions involved in our experiment.
- Reaction of aluminum with KOH (the dissolution step):
2 Al(s) + 2 KOH(aq) + 6 H2O(l) → 2 KAl(OH)
- Initial addition of sulfuric acid (precipitation of Al(OH)3)
2 KAl(OH)4 (aq) + H2SO4 (aq) →2 Al(OH)
- Further addition of sulfuric acid (dissolving of Al(OH)3)
2 Al(OH)3 (s) + 3 H2SO4 (aq) → Al
- Precipitation of alum on cooling
K2SO4 (aq) + Al2(SO4)3 (aq) + 24 H2O(l) →2 KAl(SO4)
This synthesis will use gravity filtration and suction filtration, which have already been introduced this semester. We will also use a desiccator to store our samples until Week 2. A desiccator is a large glass vessel that is filled with drying agent that removes water from the air. The sample is sealed in the desiccators to ensure that the remaining trace amounts of water are removed from our sample, as well as prevent out sample from reacting with any humidity in the lab.
100, 150, 250, 400 mL beakers
125 mL Erlenmeyer flask
Suction filtration apparatus
6 M H2SO4
2 M KOH
- Goggles, gloves, and aprons MUST be worn at all times in the lab.
- 2 M potassium hydroxide (KOH): Skin corrosion/irritation; Causes severe skin burns and eye damage; Serious eye damage/eye irritation o Wash exposed skin thoroughly after handling. Wear protective gloves, protective clothing, and eye protection.
- 6 M sulfuric acid (H2SO4): Skin corrosion/irritation; Causes severe skin burns and eye damage; Serious eye damage/eye irritation; Hazardous to the aquatic environment –
Acute Hazard o Wash exposed skin thoroughly after handling. Wear protective gloves, protective clothing, and eye protection.
- Hydrogen gas is given off when aluminum metal is reacted with potassium hydroxide solutions. It is extremely flammable. NO OPEN FLAMES. Conduct this addition in the hood.
Procedure: Preparation of Alum
- Cut approximately 0.5 grams of aluminum foil into small pieces. Record mass of the sample to three significant figures. Record observations of the metal pieces.
- Place foil pieces in a 100 mL beaker.
- Fill the Styrofoam cup ¼ to 1/3 full of warm water. Place the beaker in the warm water. DO NOT allow the warm water to spill into the beaker – this will decrease your yield.
- IN THE HOOD, slowly and carefully add 25 mL of 2 M KOH to the BEAKER containing the aluminum pieces. Record observations of the reaction mixture. NO OPEN FLAMES. This reaction produces hydrogen gas.
- Use stirring rod to mix the solution and cover with a watch glass.
- DO NOT put the stirring rod down on the counter.
- Place the stirring rod in a beaker while not in use so residual reaction mixture (corrosive) will not collect on the counter.
- Repeat stirring every few minutes until all the aluminum dissolves.
- If the reaction is too violent, remove from water bath.
- If the reaction is slowing down, replace water in the water bath with fresh warm water.
- Gravity filter the reaction mixture. This technique is SLOW. Be patient and follow directions. Prepare gravity filtration apparatus.
- Using a ring and stand, place the funnel above the 100-125 mL Erlenmeyer flask.
- Fold filter paper in half and in half again. Do NOT crease the folds – this weakens the paper. Open into a cone and place it in the funnel.
- Wet the filter paper with small amounts of deionized water so that it sticks to the sides of the funnel. If the filter paper does not stick to the walls of the funnel, the filtration will be very slow.
- Once filtration is complete, dispose of filter paper in the discard beaker.
- Allow the solution to cool.
- With a plastic pipet, slowly add 8 mL of 6M sulfuric acid, H2SO4 to the filtered solution. Record observations of the reaction mixture.
- DO NOT dip the pipet tip in the solution.
- DO NOT use the pipet tip to stir the solution.
- Dropwise add additional 6M H2SO4 until the solution becomes clear and any solid dissolves.
- Dropwise addition should be no more than 8 mL. If too much H2SO4 is added, yield will suffer.
- CAUTION: The reaction is exothermic; use care! Add slowly.
- Prepare an ice-water bath in a 400 mL beaker. Place the Erlenmeyer flask in the icebath.
- Place 3 mL of 95% ethanol into a test tube and place the test tube in a second ice bath.
- Leave reaction mixture undisturbed for 30 minutes. If crystals do not form, see instructor.
- Write your initials in pencil on a piece of filter paper. Weigh a watch glass and initialed filter paper. Record mass.
- Setup suction filtration apparatus and filter the reaction mixture.
- Using a ring and stand, clamp the side arm flask to the ring stand. Attach the hose to the side arm.
- Place the filter paper in the Buchner funnel with initial side down. Wet the filter paper with small amounts of deionized water so that it sticks to the funnel.
- Remember to break suction BEFORE turning off the water by detaching the hose from the side arm.
- Rinse the residue twice with 1-2 mL of cold 95% ethanol.
- Allow air to draw through the residue for 10 minutes.
- Carefully remove the filter paper and residue from the funnel. Scrap the funnel to ensure that residue clinging to the funnel is transferred to the watch glass.
- Dry the product in the lab desiccator.
- Record observations of dry product and the final mass of the product.
Calculate the theoretical yield of alum [KAl(SO4)2·12H2O, potassium aluminum sulfate dodecahydrate, MW = 474.4 g/mol]. Assume the limiting reactant is the aluminum foil.
Calculate the percent yield of alum in this synthesis.