Alum is widely used in the dyeing of fabrics, in the manufacture of pickles, in canning some foods, as a coagulant in water purification and waste-water treatment plants, and in the paper industry (Katz, 2000:1). The pulp and paper industry alone consumes 70% of the more than one million tons of alum produced annually in the U.S. (The Synthesis of Potassium Aluminium Sulfate (Alum) from Aluminium Scrap, 2005). Other uses include soaps, greases, fire extinguisher compounds, textiles, leather, synthetic rubber, drugs, cosmetics, cement and plastics (The Synthesis of Potassium Aluminium Sulfate (Alum) from Aluminium Scrap, 2005).
This particular process for converting aluminum into alum would produce very expensive alum. Alum can be made very cheaply at the moment using clay as the raw material. Consequently, the procedure used in this experiment is not used as an industrial method for recycling aluminum (The Synthesis of Potassium Aluminium Sulfate (Alum) from Aluminium Scrap, 2005).
Although aluminum metal sits well above hydrogen in the activity series, it reacts only slowly with dilute acids because a thin coating of aluminum oxide protects the metal surface. Aluminum reacts with alkaline solutions (figure 1) to produce hydrogen because the excess hydroxide ion first attacks the tough Al2O3 layer so the metal can react (Synthesis of Alum from Aluminium, 2001). Aluminum converts to the tetrahydroxoaluminate ion Al (OH)4-. Slow addition of acid to a solution of this ion causes the precipitation of solid Al (OH)3 followed by the dissolving of the precipitate to form the aluminum ion Al3+. The solid Al(OH)3 will also dissolve in excess base due to formation of Al(OH)4-. We call a hydroxide that can react with either acids or bases amphoteric (Synthesis of Alum from Aluminium, 2001).
2 A1(s) + 6 H2O (l) + 2 KOH (aq) → 2 K [Al (OH)4] (aq) + 3 H2(g)
Method
The methods and techniques used in the experiment were done according to the methods in the practical manual. The only adjustment were made at step 6 and 7 where the 4 x 5 ml portions of water were changed to 2 x 5 ml of water in step 6 as well as in step 7.
Results
The results obtained in this experiment are summarised below and can also be found on the attached sheets.
Mass of aluminium weighed: 0.61 g
Mass of watch glass with crystals: 60.68 g
Mass of watch glass: - 53.51 g
Mass of crystals obtained: 7.17 g
nKOH = CV nH2SO4 = CV nAl =
= (2)(35 x 10-3) = (6)(16 x 10-3) =
= 0.07 moles = 0.096 moles = 0.023 moles
Al is limiting reagent
mAlum (theoretical) = n x Mr
= (0.023)(474.35)
= 10.72 g
% yield =
=
= 66.85%
Observations
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The Erlenmeyer flask with the aluminium and KOH solution on the steam bath produced many fumes (H2) and bubbles until all aluminium was dissolved. Solution turned slightly black with fragments of scrap floating around.
- After the filtration the dirt and pieces of scarp remained on the filter paper.
- Filtrate turned yellow with the addition of methyl red indicator.
-
With the addition of the 6 M H2SO4 the solution turned to a thick pink milkshake colour.
- With constant stirring and additional temperature the solution turned to a clear red/pink solution and gave off a lot of heat (exothermic reaction)
- The end product on the watch glass was white crystals.
Discussion
The KOH and H2SO4 solutions are highly corrosive and attack the skin and clothing. If any contact were made it had to be washed off immediately with water. Furthermore H2 gas is extremely explosive in air and steps 2-3 had to be done in the fume hood to prevent potential accidents.
At the end of the experiment alum crystals of 7.17 g were obtained with a percentage yield of 66.85%. In relation to previous related studies the percentage yield obtained were weak. Ideal yields would have been closer to 100%. The reason for this could have been for inaccurate work during the practical itself. The solution was not cooled down probably before it was filtered with the Büchner filter and was filtered at a high temperature. Furthermore not all the crystals could have been transferred quantitatively with the spatula both after the filtration and at the end of the experiment. The addition of too many water could also have played a role in yielding less product.
The purity of the end product are very pure because of the Büchner filtration that kept all the impurities behind on the filter paper. Purity of the product would be affected if the Büchner filter did not function properly or if the filter paper were not placed properly and let some of the impurities through to the filtrate. This method used in this experiment to synthesise alum from scrap aluminium is a very expensive method and therefore it is not used in the industry. In the industry it is made cheaper by using clay as the raw material. At the end more accurate work by performing the procedures more carefully would improve the results and produce a better yield of the end product.
Conclusion
The aim of the experiment was definitely fulfilled that alum was synthesised from scrap aluminium. The recovery was not that good due to inaccurate work but nonetheless a good average of alum crystals was obtained. The results can definitely be improved in the future to produce a far better yield but the experiment was still a success.
References
Katz, D.A. 2000. Alum from Waste Aluminium Cans. Brooks/Cole
Synthesis of Alum from Aluminium. 2001. [Online]. Available at: . [2011, August 30].
The Preparation of Common Alum from Scrap Aluminium. 2002. [Online]. Available at: , August 30].
The Synthesis of Potassium Aluminium Sulfate (Alum) from Aluminium Scrap. 2005. [Online]. Available at: , August 30].
