Alum

Saturated solutions of alums kept at a constant temperature yield large, beautiful crystals of various colors, which are produced by partially substituting various transition metal cations for the aluminum in the alum. The addition of the ammonium alum (where = NH4) to a water supply produces a light, loose precipitate, or oc, of aluminum hydroxide that purifies drinking water. The fluffy, loosely associated nature of the Al(OH)3 is often attributed to hydrogen bonding between the precipitate and water molecules. 

Potassium alum Al2(S04)3K2S04-24H20 (double sulphate of potassium and aluminium) 

At room temperature and in solutions at less than 10% (which is practically the solubility limit of potassium alum), these alums attack aluminium only superficially and uniformly. The dissolution rate is below 0.1 mm per year. 

At high temperatures, and especially in boiling solutions, the metal becomes covered by a rather adherent film that provides protection. 

In very concentrated alum solutions and at high temperatures, pitting corrosion can occur. 

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CoS04,7H20. Few cobalt(III) oxy acid salts are known. 002(504)3,ISHjO is formed by electrolytic oxidation and forms alums Co(N03)3 contains co-ordinated nitrate (C0F3 plus NjOs). 

Ironiffl) sulphate, Fe2(S04)3, 12, 10, 9, 7, 6, 3 and 0 H2O. Formed from Fe(IIl) salts plus (NH4)2S04. Decomposes to FcjOj and SO3 on heating. Sulphate complexes and alums are also known. 

Selenaies(VI) resemble sulphates but can be reduced more easily - to SeOs " -f-115 volts). Alums are formed. Pyroselenates, [OsSeOSeOs] " are known. 

Thallium I) sulphate, TI2SO4. Formed Tl plus hot cone. H2SO4 or TIOH plus H2SO4. Moderately soluble in water forms alums and double sulphates. 

Vanadium II) forms green V2(S04)3 (electrolytic or Mg reduction of V2O5 in H2SO4). Alums and other double salts are formed. 

The hexaquo-cation occurs in the blue-violet alums, for example NH4V(S04)2.12H20... 

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 ... 

Chrome alum is also obtained if the acidified dichromate is boiled with ethanol, the ethanal formed distilling off. 

Hydrated chromium(III) sulphate exhibits different colours and different forms from which varying amounts of sulphate ion can be precipitated by barium chloride, due to the formation of sulphato-complexes. Chromium(III) sulphate can form alums. 

However the Mn (aq) ion can be stabilised by using acid solutions or by complex formation it can be prepared by electrolytic oxidation of manganese(II) solutions. The alum CaMn(S04)2.12H2O contains... 

Hydrated cobalt III) sulphate, Co2(S04)3. JSHjO is obtained when cobalt(II) sulphate is oxidised electrolytically in moderately concentrated sulphuric acid solution it is stable when dry but liberates oxygen from water. Some alums, for example KCo(S04)2.12H,0 can be obtained by crystallisation from sulphuric acid solutions. In these and the sulphate, the cation [CofHjO) ] may exist it is both acidic and strongly oxidising. 

Add a known volume ofo oaM.AgNOj solution (in excess) and boil the solution until the silver chloride has coagulated. Filter through a conical 5 cm. funnel, ensuring that the filter-paper does not protrude above the r m of the funnel. Wash the silver chloride and the filter-paper several times with a fine jet of distilled water. To the united filtrate and washings add i ml. of saturated ferric alum solution. The solution should be almost colourless if it is more than faintly coloured, add a few drops of concentrated nitric acid. Then titrate with 0 02M-ammonium thiocyanate solution until the permanent colour of ferric thiocyanate is just perceptible. (Alternatively the chloride may be determined potentiometrically.)... 

When a solution of, say, 1 g. of hydroquinone in 4 ml. of rectified spirit is poured into a solution of 1 g. of quinone in 30 ml. of water, qulnhydrone C,HA.C,H (0H)3, a complex of equimolecular amounts of the two components, is formed as dark green crystals having a gfistening metallic lustre, m.p. 172°. In solution, it is largely dissociated into quinone and hydroquinone. Quinhydrone is more conveniently prepared by the partial oxidation of hydroquinone with a solution of iron alum. 

Dissolve 100 g. of iron alum (ferric ammonium sulphate) in 300 ml. of water at 65°, Pour the solution, with stirring, into a solution of 25 g. of hydroquinone in 100 ml, of water contained in a 600 ml. beaker. The quinhydrone is precipitated in fine needles. Cool the mixture in ice, filter with suction, and wash three or four times with cold water. Dry in the air between filter paper. The yield of quinhydrone, m.p, 172°, is 15 g. It contains a trace of iron, but this has no influence upon the e.m.f, of the quinhydrone electrode provided that the washing of the crude material has been thorough. The quinhydrone should be stored in a tightly-Btoppered bottle. 

L. alumen, alum) The ancient Greeks and Romans used alum as an astringent and as a mordant in dyeing. In 1761 de Morveau proposed the name alumine for the base in alum, and Lavoisier, in 1787, thought this to be the oxide of a still undiscovered metal. 

The compounds of greatest importance are aluminum oxide, the sulfate, and the soluble sulfate with potassium (alum). The oxide, alumina, occurs naturally as ruby, sapphire, corundum, and emery, and is used in glassmaking and refractories. Synthetic ruby and sapphire are used in lasers for producing coherent light. 

All compounds of chromium are colored the most important are the chromates of sodium and potassium and the dichromates and the potassium and ammonium chrome alums. The dichromates are used as oxidizing agents in quantitative analysis, also in tanning leather. 

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