Synthesis of Alum, KAl(SO₄)₂^.12H₂O

• Objectives
• Background
• Procedure

Objectives

• Experimental techniques:
  • gravity filtration: ordinary filtration using filter paper in a funnel to separate solids from a liquid allowed to flow freely (under gravity) through the funnel
  • vacuum filtration: filtration using filter paper in a specially designed funnel (Büchner funnel) to separate solids from a liquid pulled through the funnel under vacuum
  • recrystallization: isolation and purification of a solid by causing it to precipitate slowly from solution
• Theoretical concepts:
  • theoretical yield: quantity of product expected based on quantity of limiting reagent used
  • per cent yield: quantity of product actually recovered expressed as a percentage of the theoretical yield

Background

Calculating yields:

		mol Al	1 mol alum	474.32 g alum
Theoretical yield =	grams Al used x	-------------- x	-------------- x	------------------- = g alum
		26.98 g Al	1 mol Al	mol alum
	actual yield, g
% yield =	--------------------	x 100%
	theoretical yield, g

Procedure

The synthesis will be carried out in the following stages:

• Dissolving aluminum foil
• Gravity filtration
• Reactions that complete alum formation
• Crystallization
• Vacuum filtration

• Dissolving aluminum foil: 2 Al(s) + 2 OH^1-(aq) + 6 H₂O ---> 2 [Al(OH)₄]^1-(aq) + 3 H₂(g)
  • Weigh 0.45-0.50 g Al foil, using the tare function on the balance and not letting the foil come into contact with your hands or the balance pan. Record the mass to +/- 0.01 g.
  • Place the foil in a reaction beaker labeled with your initials.
  • Under the fume hood, pour 13.0 mL of 3.0 M KOH into the reaction beaker.
  • Use only the glass part of your stirring rod to wet the Al. Use your rubber policeman to push back into the solution any Al that adheres to the side of the beaker.
  • On your data sheet, describe the reaction mixture after all the Al dissolves.
• Gravity filtration of the reaction mixture
  • Place a funnel in your funnel holder, and place a labeled beaker beneath the funnel. Fold a 7-cm piece of filter paper as described in the lab handout. Moisten the filter paper with 2-3 mL of distilled water to make it adhere to the funnel, and discard this water.
  • Transfer the reaction mixture to the filter paper in the funnel. Rinse the reaction beaker twice with 2 mL distilled water, and add each rinse to the filter paper. Collect and save this liquid filtrate.
  • Rinse the residue on the filter paper with 2-3 mL of distilled water, joining this rinse to the filtrate.
  • When all the liquid has drained from the filter paper, discard the paper and its contents in the appropriate waste container.
• Completing the formation of alum:
  
  • Slowly and carefully stir 22.5 mL of 4 M H₂SO₄(aq) into the filtrate in approximately 5-mL portions. Observe and record changes in the filtrate.
  • Using a hot plate, heat the acidified filtrate until all solid dissolves.
  • Reaction steps involving the sulfuric acid:
    • Al(OH)₄^1-(aq) + H₂SO₄(aq) ---> Al(OH)₃(s) + HSO₄^1- + H₂O
    • Al(OH)₃(s) + 3 H₂SO₄(aq) ---> Al³⁺(aq) + 3 HSO₄^1- + 6 H₂O
    The overall reaction to form alum is:
    Al³⁺(aq) + K¹⁺(aq) + 2 SO₄^2-(aq) + 12 H₂O ---> KAl(SO₄)₂^.12H₂O(s)
• Crystallization
  • Using tongs, remove the filtrate beaker to a wire gauze on the lab bench. Allow it to begin to cool while you prepare an ice bath, an ice-water mixture in a 400-mL beaker. Place the filtrate beaker into the ice bath, making sure no water from the ice bath gets into the filtrate beaker.
  • Place 3 mL of 95% ethanol into a test tube, and put the test tube in a second ice bath, secured by a test tube clamp.
  • Let the filtrate beaker sit undisturbed in the ice bath until crystals of alum are observed. If no crystals have formed after 30 minutes, consult your instructor.
• Vacuum filtration
  • While the solution is cooling, set up a filtration assembly. Note: you will not use the second, taped filter flask shown as a trap in the lab handout.
  • Determine the mass of a 7-cm circle filter paper. Then place the paper flat in the Buchner funnel so it covers all holes.
  • Moisten the paper. Then connect the filter flask to a water aspirator with rubber tubing. Test the assembly by turning on the aspirator.
  • When crystals stop forming in the cooling solution, have your instructor initial your data sheet, giving approval to begin filtering the alum.
  • Allow water to run through the aspirator to establish a vacuum in the filter flask. Only then begin to transfer the supernatant liquid from the beaker to the Buchner funnel. Use a rubber policeman to transfer all the crystals into the Buchner funnel.
  • Rinse the crystals with 1-2 mL of cooled ethanol. Stop filtration when liquid is no longer dripping from the funnel into the flask. Do not use the water aspirators to dry the crystals. (Excessive use of the aspirators causes flooding in the science building.)
  • Disconnect the hose from the aspirator and then turn off the water.
  • Place the crystals and filter paper in a weighed beaker, and store them in your locker until the next lab period.
  • You will not turn in your lab reports until the next lab period, when you will weigh your dry crystals and complete the data sheet. You will be well advised, though, to complete every part of the lab report that can be completed. Don't forget to bring the lab handouts back to the next lab period, because the lab report is due in the next lab period.