Preparation of Alum

PURPOSE OP EXPERIMENT Prepare KA1(S04)2 12H20, an aluminum alum, from aluminum metal. (Note Experiment 14 describes the gravimetric analysis of the sulfate content in alum.) 

In this experiment you will prepare potassium aluminum sulfate dodecahydrate, KAl(804)2 121120, a double salt, starting with aluminum metal. Aluminum metal reacts rapidly with a hot aqueous solution of potassium hydroxide, KOH, to produce a soluble potassium aluminate salt. 

When this salt is reacted with sulfuric acid, H2SO4, aluminum hydroxide, A1(H2O)3(OH)3, precipitates 

and Purcell, K. F., Chemistry and Chemical Reactivity. Saunders College Publishing, Philadelphia, 1987, sections 4.1-4.3, 21.1a, and 22.2c. 

Cool the solution in an ice bath for 25-30 minutes without any agitation. If the solution is maintained as motionless as possible, well-formed octahedral crystals of alum should grow in the beaker. (If no crystals form, reheat the solution to evaporate an additional 10. mL of water, and recool the solution in the ice bath.) After all of the solid alum has formed, filter the mixture using either gravity filtration or suction filtration, and wash the resulting crystals with 15 mL of a 50 50 (by volume) water-alcohol solution (Laboratory Methods I). Weigh a clean, dry watch glass on a beam balance, and record the mass in TABLE 13.1. 

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The Preparation of Alums.—Since aluminum sulfate is a commercial chemical, the aluminum alums can be prepared by crystallizing the salt along with the double molecular weight of an alkali sulfate. Chromium and ferric sulfates are not well crystallized and have limited uses, so that they are not readily obtained. The preparation of their alums involves the preliminary preparation of the sulfates themselves. 

Preparation of alum ( potassium aluminum sulfate) crystals ... 

The dichromate ion oxidises iron(II) to iron(III), sulphite to sulphate ion, iodide ion to iodine and arsenic(III) to arsenic(V) (arsenate). Reduction of dichromate by sulphite can be used to prepare chrome alum, since, if sulphur dioxide is passed into potassium dichromate acidified with sulphuric acid, potassium and chromium(III) ions formed are in the correct ratio to form the alum, which appears on crystallisation ... 

The slurry is pumped iato another stock chest, where wax ia emulsion form, usually about 0.5—1.0% wax-to-fiber weight, and 1—3% PF resia are added. PF resia is also added on the basis of resia soHds-to-dry fiber. Thea a small amouat of alum is added, which changes the pH (acidity) of the slurry, causiag the resia to precipitate from solutioa and deposit on the fibers. Resia is required ia greater quantity than ia the Masonite process because only light bonding occurs between fibers prepared ia a refiner. The fiber slurry is thea pumped to the headbox of a Fourdrioier mat former, and from this poiat the process is similar to the Masonite process. 

When A. S. Marggraf tried to prepare alum from alumina and vitriolic acid, he found that unless he added fixed alkali he obtained no crystals (19). In 1777 Lavoisier clearly stated that potash is an essential constituent of alum (18, 20). In analyzing a water containing aluminum sulfate, which the younger Cassini had sent him from Italy, Lavoisier added some potash When he evaporated the solution, he obtained crystals of alum and realized that this was a verification of the results of Marggraf and of Macquer. 

Although G. E. Stahl and Caspar Neumann both believed that alum contained lime, J. H. Pott was unable to prepare it from lime and vitriolic acid, but always obtained merely selenite (calcium sulfate) (74). When Stahl leached with water a broken clay tube he had used for distilling spirit of vitriol (sulfuric acid), he obtained crystals of alum (74). Pott, too, prepared alum from clay and sulfuric acid (74). 

As previously stated, Wohler was unable to obtain metallic aluminum by Oersted s method. However, since the latter encouraged him to continue his attempts, he prepared some anhydrous aluminum chloride by Oersted s method, and devised a new plan for isolating the metal. After adding an excess of hot potassium carbonate solution to a boiling hot solution of alum, he washed and dried the precipitated aluminum hy-... 

The electrochemical replacement of one metal by another was noticed by Pliny in antiquity. He mentioned that iron looks like copper if it is smeared with vinegar or alum previously ground in a bronze mortar [275], Metal replacement seems to have also been used in the Roman era for tin-plating bronze, as well as underlying the recipes for the preparation of iron surfaces to receive amalgam gilding that are given in the 9th century [276],... 

Uses Of the Stassfurt salts.—The magnesium compounds in the Stassfurt salts are used for the preparation of magnesium and of its salts. The potash salts are an essential constituent of many fertilizers used in agriculture, etc. 22 and potassium chloride is the starting-point for the manufacture of the many different kinds of potassium salts used in commerce—carbonate, hydroxide, nitrate, chlorate, chromate, alum, ferrocyanide, cyanide, iodide, bromide, etc. Chlorine and bromine are extracted by electrolysis and other processes from the mother liquids obtained in the purification of the potash salts. Boric acid and borax are prepared from boracite. Caesium and rubidium are recovered from the crude carnallite and sylvite. 

In the leaching processes, large volumes of liquid have to be handled, and the mother liquors contain large amounts of salts in soln. much potassium chloride is rendered less useful since these liquors are boiled down and utilized in other ways— principally as fertilizers. The mother liquors also yield rubidium alum. The cost of the operation, small as it is, does not enable potassium sulphate to be prepared of a sufficient degree of purity to enable it to displace the potassium chloride and sulphuric acid process which can be made to yield almost the theoretical amount of sulphate. Consequently, most of the schonite—natural or artificial—is sold as a fertilizer. 

Preparation of Chromium Potassium Alum. Pour 25 ml of water into a 50-ml beaker and dissolve 2.5 g of potassium dichromate in it. Add a concentrated sulphuric acid solution to the mixture (one-and-a-half the stoichiometric amount). First cool the mixture to room temperature, and then put it into water with ice and add ethanol dropwise from a dropping funnel until the solution acquires a violet colour (sulphur dioxide can also be used as the reducing agent). Keep the temperature below 40 °C (why ). After adding the ethanol, let the solution stand to your next lesson. Write the equation of the reaction. 

Yellow Ink.—Yellow ink, according to Dr. Ure, is prepared by dissolving three parts of alum in one hundred parts of water, adding twenty-five parts of bruised Persian or Avignon berries, boiling the mixture for an hour, filtering the liquor, and dissolving in it four parte of gum-arabic. 

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