Synthesis of Alum from Aluminum, Chemistry tutorial


Alums are the ionic compounds which crystallize from solutions having sulphate ion, a trivalent cation like Al3+, Cr3+ or Fe3+ and a monovalent cation like K+, Na+, or NH4+. Six of the water molecules bind tightly to the trivalent metal ion; the remaining 6 molecules bind more loosely to the monovalent cation and the sulphate anion. 

In this experiment we will prepare an alum-KAl(SO4)2·12H2O [potassium aluminum sulphate dodecahydrate]-from an aluminum beverage can. This compound is broadly employed in dyeing fabrics, making pickles, making paper and purifying the water.

Aluminum is the richest metal in the earth's crust (8.3% by weight) and is the third richest element after oxygen (45.5%) and silicon (25.7%). Pure aluminum is a silvery-white metal and consists of numerous desirable physical and chemical properties: it is light-weight, non-toxic, corrosion-resistant, nonmagnetic and malleable. Aluminum is generally joined with other metals like copper, silicon, manganese, magnesium and zinc, which generates alloys having high mechanical and tensile strength. You are probably well-known with numerous uses of aluminum and its alloys.


Aluminum beverage cans usually encompass a thin coating of plastic on the inside which protects the aluminum from the corrosive action of chemicals in the beverage. The outside generally consists of a thin coating of paint. These coatings should be eliminated before any chemical reactions with the metal can be taken out. The coatings might be efficiently scraped off by a metal pan cleaner. A cleaned piece of metal is then dissolved in the potassium hydroxide solution according to the following complete, balanced equation:

The full and complete ionic equations are: 

2Al (s) + 2KOH (aq) + 6H2O (liq) → 2KAl (OH)4 (aq) + 3H2 (g)

2Al (s) + 2OH (aq) + 6H2O (liq) → 2Al(OH)4- (aq) + 3H2 (g)

The Al(OH)4- ion is a complex ion termed as aluminate. After filtration to eradicate residual plastic and paint decomposition products, the alkaline solution of Al(OH)4- is clear and colorless. The H2 is evolved as a gas and mixes by the atmosphere. Chemical species in the solution are potassium ions (K+) and aluminate ions [Al(OH)4-] ions (plus any unreacted KOH).

Sulphuric acid is now added and two sequential reactions take place. Primarily, before the addition of all the acid, insoluble aluminum hydroxide is made,

2KAl(OH)4 (aq) + H2SO4 (aq) → 2Al(OH)3 (s) + 2H2O (liq) + K2SO4 (aq)

Al(OH)4- (aq) + H+ (aq) → Al(OH)3 (s) + H2O (liq)

Al(OH)3 to provide a thick, white, gelatinous precipitate of the aluminum hydroxide. As more sulphuric acid is added, the precipitate of Al(OH)3  dissolves to form soluble Al3+ ions.

The full and complete ionic equations are:

2Al(OH)3 (s) + 3H2SO4 (aq) → Al2(SO4)3 (aq) + 6H2O (liq)

Al(OH)3 + 3H+ (aq) → Al3+ (aq) + 3H2O (liq)

to provide aluminum ions, Al3+, in solution. The solution at this point includes Al3+ ions, K+ ions (from potassium hydroxide), and SO42- ions (from sulphuric acid). On cooling, the crystals of hydrated potassium aluminum sulphate, KAl(SO4)2. 12H2O (or alum) are extremely slowly deposited. In the experiment the crystallization method is speeded up by giving a small 'seed crystal' of alum for the newly forming crystals to grow on. Cooling is required as alum crystals are soluble in the water at room temperature.

The full and complete ionic equations are:

Al2(SO4)3 (aq) + K2SO4 (aq) + 24H2O (liq) → 2 KAl(SO4)2. 12 H2O

K+(aq) + Al3+ (aq) + 22SO42- (aq) + 12H2O (liq) → KAl(SO4)2.12H2O(s)

Lastly, the crystals of alum are eliminated from the solution via vacuum filtration and washed by an alcohol or water mixture. This wash liquid eliminates any contamination from the crystals however doesn't dissolve them. It as well helps to dry the crystals quickly, as alcohol is more volatile than water.

Experimental Procedure:

CAUTION: Excess care should be employed in handling the potassium hydroxide (KOH) and sulphuric acid (H2SO4). KOH is corrosive and will dissolve clothing and skin! Sulphuric acid will as well burn and dissolve clothing. Wash your hands carefully after using either of these solutions! 

A piece of scrap aluminum will be given. Make use of steel wool to take away as much paint as possible. The inside of can is protected by a plastic coating and you must get rid of this as well. Weigh the cleaned strip of aluminum to the close 0.001 grams.

Cut the Al sample into small squares and put the squares in a clean 100 ml or 150 ml beaker. Perform the given in the hood!! Cautiously add 20 mL of 4 M potassium hydroxide, KOH. The bubbles of hydrogen gas must evolve. Put your beaker on a hot plate in the hood to speed up the reaction. Whenever hydrogen bubbles are no longer formed, the reaction is complete. This must take 10 to 15 minutes. Take away the beaker from the hot plate and let it to cool at your bench.

The resultant grayish mixture must be filtered to get rid of unwanted impurities. If the solution is still warm, cool it by putting the beaker in an ice bath. Set up a 250 ml filter flask and Gooch filter crucible or Buchner funnel as illustrated. Do not forget to clamp the filter flask to some type of support. Filter the solution slowly. Rinse the beaker two times by small parts (< 5 mL) of distilled water, pouring each and every rinse via the filter. Transfer the clear colorless filtrate to a clean 250 ml vessel.

To the cool solution, slowly and cautiously, with stirring, add 15 ml of 6 M sulphuric acid. White lumps of Al(OH)3 must form in the solution . Again working in the hood, heat and stir the mixture to dissolve the white lumps. Surplus sulphuric acid might be added drop wise, however no more than 30 mL total, in order to completely dissolve the Al(OH)3, and provide a clear solution.

Cool the clear solution in an ice bath for as a minimum of 20 minutes. Crystals of alum must fall to the bottom of the beaker. If no crystals form, scratch the bottom of the beaker by your stirring rod. If that fails, add a 'seed' crystal of alum to facilitate the crystallization.

As the solution is cooling, reassemble the filtration apparatus. Be sure to weigh the filter paper at this point. Slowly filter the solution having the crystals. Rinse the beaker once by 5 ml of cool distilled water and once by 5 ml of isopropyl (rubbing) alcohol, pouring each and every rinse via the filter. Allow the aspirator to pull air via the crystals till they appear dry, as a minimum of 5 minutes. Take out the filter from the flask. More crystals might form in the filtered solution (filtrate). If you have time, filter these crystals as just illustrated, and add these to the first crystal crop.

Take out the damp crystals and filter the paper from filter and put them into a weighed beaker. Put the beaker containing your crystals in your locker till next week. After drying, weigh the filter and crystals and find out the mass of the alum produced. 

Calculate the percent yield:

Find out the melting point of the alum crystals by using the given procedure. Cautiously, push the open end of a capillary tube into your crystals, forcing some of the solid to the tube. Turn the tube over and tap it softly to move the crystals to the sealed end of the tube.

Repeat this procedure till you have approximately 5 mm of solid in the capillary tube. Cautiously, insert the tube into the melting point apparatus. The apparatus will be 'hot' if others have been utilizing it! Note the temperatures at which the alum first appears to melt (that is, whenever liquid first becomes visible) and at which it is fully melted (all liquid). Remove the capillary tube in the glass bin.

Get the literature value for the melting point of alum. Compare your experimental melting point to the literature value. Compute the percent error.


Make a brief report explaining your observations throughout the experiment. Report the actual outcome, the theoretical yield and the percent yield.

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