PREPARATION OF ALUM
FROM ALUMINUM METAL
October 2nd, 2012
The objective of the laboratory is to synthesize alum (KAl(SO4)2.xH2O) from aluminum powder and to determine the proportion of water in the alum crystals. Alum is a product from the reaction between potassium hydroxide and sulfuric acid. The reaction include several steps, as followed:
Aluminum powder reacts with potassium hydroxide to generate Al(OH)4- ions and release hydrogen. 2 Al(s) + 2 KOH(aq) + 6 H2O 2 K[Al(OH)4](aq) + 3 H2 (g)
A gelatinous precipitate of aluminum hydroxide was created when sulfuric acid was added to the aqueous solution of Al(OH)4- ions. 2 K[Al(OH)4](aq) + H2SO4 (aq) 2 Al(OH)3 (s) + K2SO4 (aq) + 2 H2O
Later, excessive addition of the acid causes the precipitate to dissolve in the solution. 2Al(OH)3 (s) + H2SO4 (aq) Al2(SO4)3 (aq) + 6 H2O
Precipitation of alum was resulted from cooling in ice water bath. K2SO4 + Al2(SO4)3 + 2x H2O 2 KAl(SO4)2.xH2O
It is noticeable that alum is a hydrate (a hydrate consists of water molecules in its ionic structure), which leads to its solubility in water. However, a minimum amount of cold water will cause the alum to crystallize. The amount of water incorporated in the alum structure should be clearly defined to derive the full formula of alum, which makes it possible for calculations of theoretical, actual and percent yield of alum.
The experiment was constructed based on the guidelines from Franklin and Marshall Lab Manual1. In a 400 mL-beaker, 0.5 g of aluminum and 2.01g of potassium hydroxide was prepared and mixed together. An amount of 25 mL of distilled water was poured into the beaker in the hood. The mixture was then continuously stirred to help disperse the heat generated from the exothermic reaction. As observed, hydrogen was liberated from the solution, along with aluminum powder gradually darkening and disintegrating into insoluble flakes. It took the solutions 15 minutes to complete when there were no signs of hydrogen released. The solution was then filtered into a new 250 mL beaker. The residue left on the filter paper was carefully washed into the filtrate.
A portion of 10 mL of 9M sulfuric acid was added slowly and attentively to the filtrate, with gentle stirring. The presence of acid will neutralize the solution, generating a gelatinous precipitate known as Al(OH)3. The precipitate was later dissolved when excessive addition of acid was poured into the solution, combined with gentle heating on hot plate. The acidity of the solution was confirmed when tested with litmus paper: the paper turned into red. The solution was filtered for the second time to eliminate any undissolved residues remaining.
The solution was set aside to cool at room temperature. The crystallization process was conducted by placing the solution beaker into an ice water bath for 20 minutes. After crystallization, white, soft crystals were formed. The mixture was filtered through a Buchner funnel.
A wash solution was prepared by combining 5 mL of ethyl alcohol and 5 mL of distilled water. The crystals were washed twice with proper wash solution. Then, the solution was put through suction again to dry out completely. The crystals were spread in a recrystallization disk. Large crystals were broken into small ones with a stapula. The crytals were allowed to air dry in one week.
The weight of the air-dried crystals was then recorded.
Two porcelain crucibles were supported on ceramic triangles and heated to red heat with a Bunsen burner for 10 minutes each. The crucibles were set aside cool, then was placed into the desiccator to cool to room temperature. Their weighs were recorded. An amount of 0.5 g of the crystallized alum was placed into each of the crucibles. The crucibles (with alum inside) were carefully heated on ceramic triangles to red heat. The alum inside the crucibles appeared to melt, transforming into a kind...