Preparation of Alum

DEPARTMENT OF PURE AND APPLIED CHEMISTRY
Visayas State University
Visca, Baybay City, Leyte

Name: Alecsa May S. Celaya BS-Chemistry 3 Date Submitted: December 19, 2014
Lab. Schedule: MW 2:00 – 5:00 pm Rating:
Group No. 11 Experiment No. 5
Preparation of KAl(SO4)2•12H2O (Alum)

OBJECTIVES

This experiment aimed to: prepare KAl(SO4)2•12H2O, commonly referred to as alum, from aluminum metal from a canned soft drink, and determine the mass of the alum collected and its percent yield.

METHODOLOGY An aluminum metal scrap was obtained by scraping the paint off a Coca-Cola can. It was then cut into small pieces and was placed in a pre-weighed 250 mL beaker. With the use of an analytical balance, by taring the mass of the beaker, the mass of the metal was determined to be 1.0042 g. It was then carefully added with 50 mL of 1.5 M KOH solution and then heated using a hot plate. The solution was stirred constantly for 30 minutes with a stirring rod while heating until all the scrap metal was dissolved in the solution. Filtration followed while the solution was still hot and the filtrate collected was acidified by addition of 30 mL of 6 M H2SO4 solution.
The white precipitate formed was redissolved by constant heating and stirring until the solution yielded a volume of 50 mL. The solution was then cooled to room temperature and further in an ice bath for 15 minutes. The formation of octahedral crystals were then observed and were then collected through filtration. Halfway through the filtration process, the alum collected was washed with 15 mL of 50:50 water-alcohol mixture. The alum and filter paper were then transferred in a preweighed watch glass and was allowed to be dried overnight and the mass was determined afterwards. The percent yield was calculated using the Equation 1, where the theoretical yield of alum was derived from the stoichiometric relation in Equation 2:

RESULTS AND DISCUSSION Aluminum beverage cans generally have a thin coating of plastic on the inside that protects the aluminum from the corrosive action of the chemicals in the beverage. The outside usually has a thin coating of paint. These coatings were first removed before any chemical reactions with the metal was carried out. In this experiment, KAl(SO4)2•12H2O (alum) was synthesized by the reaction of aluminum scrap with a strong base (KOH) and a strong acid (H2SO4) .During the first part of the synthesis, a cleaned piece of metal was dissolved in the potassium hydroxide solution according to the following complete, balanced equation:
2 Al(s) + 2 KOH(aq) + 6 H2O(liq) → 2 KAl(OH)4(aq) + 3 H2(g)
Net ionic equation:
2 Al(s) + 2 OH-(aq) + 6 H2O(liq) → 2 Al(OH)4-(aq) + 3 H2(g)
The dissolution of Al(s) in aqueous KOH was an example of reaction. The Al metal was oxidized to aluminum with an oxidation number of +3 and the hydrogen in KOH or in water was reduced from an oxidation number of +1 to zero in hydrogen gas. The Al(OH)4- ion formed was a complex ion called “aluminate”. The alkaline solution of Al(OH)4- was clear and colorless after filtration. The products formed in the reaction were potassium tetrahydroxyaluminate KAl(OH)4 and the evolution of H2 but H2 was evolved as a gas and mixed with the atmosphere. (http://employees.oneonta.edu/kotzjc/LAB/Alum_Expt.pdf)
The next step was the addition of H2SO4. Initially, before the addition of all the acid, the complete reaction was:
2 KAl(OH)4 (aq) + H2SO4 (aq) → 2 Al(OH)3 (s) + K2SO4 (aq) + 2 H2O(l)
The reaction above is an acid-base reaction. The H+ ions from sulfuric acid neutralized the base Al(OH)4- which gave a thick, white, gelatinous precipitate of aluminum hydroxide, Al(OH)3. The precipitate of Al(OH)3 dissolved and gave aluminium ions, Al3+ when more of the sulfuric acid was added,.
2 Al(OH)3(s) + 3 H2SO4(aq) → Al2(SO4)3(aq) + 6 H2O(l) Because alum crystals are soluble in water at room temperature, the solution was cooled in an ice bath. Formation of crystals of hydrated KAl(SO4)2•12 H2O (or alum) were evident after approximately 20 minutes of cooling time.
Al2(SO4)3(aq) + K2SO4(aq) + 24 H2O(l) → 2 KAl(SO4)2•12 H2O(s)
Finally, the crystals of alum were removed from the solution by filtration. Halfway through the filtration process, the crystals were washed with 50:50 alcohol/water mixture to remove any contamination from the crystals. It also helped dry the crystals quickly but not dissolve them because alcohol is more volatile than water.Below is the overall reaction for the synthesis of alum. The stoichiometry depicted that for every 2 mol Al consumed, 2 mol of alum were produced indicating a 1:1 stoichiometric ratio.
2 Al (s) + 2 KOH + 2 H2SO4 + 22 H2O → 2 KAl(SO4)2•12 H2O (s) + 3 H2 (g)

Table 1. Data Collected From The Synthesis Of KAl(SO4)2•12H2O (Alum)
Mass of Aluminum Scrap Metal (g)
1.0042 g
Mass of Filter Paper (g)
1.2056 g
Mass of Alum + Filter Paper (g)
6.7597 g
Mass of Alum (g)
5.5541 g
Theoretical Yield of Alum (g)
17.6568 g
Percent Yield
31.46 %

Table 1 shows the collected data from the synthesis of KAl(SO4)2•12H2O. The theoretical yield which shows how much product will be synthesized in ideal conditions was determined to be 17.6568 grams.

However, the actual yield was only 5.5541 grams. The percent yield was then calculated by the formula below:

The percent yield is 31.46% which is relatively low. This could possibly be due to mechanical and human errors. One major contributor to the error was the loss of the total volume of the solution during the heating process because the solution boiled and some splashed out of the beaker and was thus taken out earlier than expected even though the solution wasn’t clear enough. This was done in order to meet the required volume and to avoid any more losses. Also it is possible that not the entire metal component in the Coco-Cola comprised aluminum.
CONCLUSION
Alum, chemically known as KAl(SO4)2•12H2O, was synthesized from the reaction of aluminum metal from a can of Coca-Cola with KOH and H2SO4. The mass of alum collected by the end of the experiment was 5.5541 g with a percent yield of 31.46%.

REFERENCES http://employees.oneonta.edu/kotzjc/LAB/Alum_Expt.pdf. (Opened: December 16, 2014) http://www.bc.edu/schools/cas/chemistry/undergrad/gen/fall/Alum.pdf. (Opened: December 16, 2014) http://chemlab.truman.edu/CHEM130Labs/Alum.asp. (Opened: December 16, 2014) http://www.mesacc.edu/~paudy84101/CHM151LL/6B%20Alum.pdf. (Opened: December 16, 2014)

References: http://employees.oneonta.edu/kotzjc/LAB/Alum_Expt.pdf. (Opened: December 16, 2014) http://www.bc.edu/schools/cas/chemistry/undergrad/gen/fall/Alum.pdf. (Opened: December 16, 2014) http://chemlab.truman.edu/CHEM130Labs/Alum.asp. (Opened: December 16, 2014) http://www.mesacc.edu/~paudy84101/CHM151LL/6B%20Alum.pdf. (Opened: December 16, 2014)