Sodium chrome alum

The term alum, itself, refers to potassium alum, potassium aluminum sulfale, KA1(S04)2 12H O. Oilier common alums are ferric ammonium alum, NH4 Fe(S02)2 12HiO. and sodium chrome alum, NaCr(SOa)212ITO. 

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. 

Sodium thiosulfate is still used in chrome leather tanning as a reducing agent in two-bath processes to reduce dichromate (hexavalent chromium) to chrome alum (trivalent chromium) (see Leather). 

Use a 3% to 5% solution of sodium metabisulfite, a 3% solution of chrome alum, or a mildly acid stop bath, such as Kodak SB-1 Nonhardening Stop Bath. Or use a very gentle-acting stop bath, suggested by Jay Dusard, comprised of a solution of 10.0 grams of sodium bisulfite in 1.0 liter of water. 

Other chemicals that can be used to make stop baths are boric acid, citric acid, and sodium bisulfite. Chemicals that should not be used are chrome alum and potassium salts, such as potassium metabisulfite or potassium sulfite, either of which can cause green stains on some enlarging papers. In addition, the introduction of potassium salts into a fixing bath can convert the bath to potassium thiosulfate, which is nearly inactive as a fixing agent.1... 

Dry test (flame colouration) Potassium compounds, preferably the chloride, colour the non-luminous Bunsen flame violet (lilac). The yellow flame produced by small quantities of sodium obscures the violet colour, but by viewing the flame through two thicknesses of cobalt blue glass, the yellow sodium rays are absorbed and the reddish-violet potassium flame becomes visible. A solution of chrome alum (310 g 1), 3 cm thick, also makes a good filter. 

In a large lead-lined vessel, 300 grams of sublimed, still moist, anthracene (calculated on the basis of 100 per cent product) is stirred with 6 liters of water and 600 grams of sodium bichromate. The mixture is heated to 80°C. over a Fletcher burner, and 1800 grams of 50 per cent sulfuric acid is added over a period of 10 hours. Chromic-acid should always be present, and the mixture should be stirred with a wooden or glass rod. Finally, the mixture is boiled for 2 hours, replacing the water lost by evaporation. The precipitate is then filtered off and washed thoroughly. The mother liquor can be worked up to recover chrome alum or chromic sulfate. 

Experiment 185. — (a) Add a little sodium hydroxide solution to a solution of chrome alum. The precipitate is chromic hydroxide. Describe it. Add an excess of sodium hydroxide solution and shake. Describe the result Boil, and state the result. 

Compare the action of sodium hydroxide on ordinary alum and on chrome alum. 

Oxidation states of chromium - -2, - -3, and -f-6. Oi es of chromium chronate, FeCr204, and crocoite, PbCr04. Chromium metal and its alloys ferrochrome, alloy steels, stainless steel. The aluminothermic process (Goldschrtiidt process). Electrolytic chromium. Chromium trioxide, chromic acid, dichromic acid, potassium chromate, potassium didiromate, sodium chromate, lead chromate. Equilibrium between chromate ion and dichromate ion. Chrome-tanned leather. Chromic oxide (chrome green) chromic ion, chrome alum, chromic chloride, chromic hydroxide, chromite ion. Chromous compounds. Peroxy-chromic acid. 

Chromic oxide jellies may be formed by adding sodium or potassium hydroxide or ammonia to a solution of chromic sulphate or chloride containing sodium acetate or by adding sodium or potassium hydroxide, hut not ammonia, to a solution of chrome alum. The jelly is violet if prepared by the addition of ammonia or of a slight excess of the alkali metal hydroxide if the latter is added in larger quantity the jelly is green. The jellies dissolve in hydrochloric acid, but re-form on neutralising the solution if sufficient sodium acetate is present. 

Cobalt Chromite, CoO.CrgOg, is obtained by the addition of sodium carbonate to a mixed solution of chrome alum and cobalt chloride, and igniting the precipitate. ... 

Chromic Orthophosphates.—Violet chromic phosphate, CrPO. BHgO, is produced as a lavender amorphous precipitate when cold solutions of equal weights of chrome alum and ordinary sodium phosphate are mixed ... 

Derivation (1) Interaction of solutions of chromium chloride and sodium phosphate (2) by mixing chrome alum and disodium hydrogen phosphate. Violet, amorphous powder (not the hexahydrate) is formed that becomes crystalline on contact with water. On boiling, it is converted into green crystalline hydrate. 

But the admirable researches of Gay-Lussac and of Mitscherlich have established the fact, that in many instances, different compounds assume the same form. Thus, the following substances, and many others, take the form of the cube, tetrahedron, or regular octohedron, which are geometrically connected. Chloride of sodium (sea-salt), chloride of potassium, sal ammoniac bromide of potassium iodide of potassium sulphuret of lead fluoride of calcium bisnlphuret of iron arseniuret of cobalt sulphate of alumina and potash (alum) ammonia alum chrome alum, iron alum sesqnioxide of iron, sesquioxide of aluminum, sesquioxide of chromium. In like manner, other crystalline forms are found to be common to many different compounds, although none occurs so frequently as the cube and its congeners. 

T. Klobb obtained mixed crystals of the anhydrous sodium and ammonium chromium sulphates, (NH4,Na)S04.Cr2(S04)3, by melting together chromic sulphate or chrome-alum with ammonium and sodium sulphates. The hexagonal crystals are isomorphous with the anhydrous alums. A. N. Bach found the transition temp, to be between 60° and 70°. 

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