Chromic chloride, preparation

Tetraquo-diammino-chromic Chloride, [Cr(NH8)2(H2Q)4]Cl3, is prepared in the same manner as the bromide by decomposing the basic salt. It forms light red monoclinic prisms which are very soluble in water, insoluble in alcohol, and on warming the aqueous solution with a little acid is transformed into the dichloro-diaquo-salt. 

A series of tetraquo-dcrivatives containing two molecules of pyridine is also known. These have formula [Cr py2(H20)4]R3, and correspond in general properties to the diammino series. They are prepared from green hydrated chromic chloride, [CrCl2(H20)4]C1.2lI20, by treatment with pyridine, when it is transformed into the dihydroxo-diaquo-dipyri-dino-chloride, [Cr pya(H20)2(0H)2]Cl, and this on treatment with mineral acid changes into the tetraquo-salt.6... 

This scries is represented by the dithionate, [Cr(NH3)5(QH)]S206. 2H20, which is prepared from aquo-pentammino-chromic bromide by treating it with dilute aqueous ammonia till the colour changes from yellow to deep red a concentrated aqueous solution of sodium dithionate is then added, the liquid stirred and left to stand, when carmine-red crystals separate. These are washed with dilute alcohol and dried in air. The dithionate is slightly soluble in water, and is decomposed by dilute hydrochloric acid with formation of aquo-pentammino-chromic chloride, [Cr(NH3)5H20]Cl3. 

A few of these salts are known. For example, the iodide, [Cr(NH3)s(H20)2(0H)]I2.H20, which is prepared from dichloro-aquo-triammino-chromic chloride, [Cr(NHs)3(H20)Cl2]Cl, by treating it -with pyridine and solid potassium iodide. It crystallises in purple leaflets, soluble in water, and is transformed into triaquo-triammino-chromic bromide by hydrobromic acid.2... 

Nitro-pentammino-chromic Chloride, [Cr(NH8)5X02]Cl2, is prepared from ehloro-pentammino-chromic chloride by heating it for a short time with water acidified with nitric acid, allowing the mixture to cool slowly, and filtering off the undissolved ehloro-pentammino-salt. The filtrate, which contains aquo-pentammino-ehromic salt, is decomposed with sodium nitrite and a 12 per cent, solution of hydrochloric acid after a few minutes the precipitate formed is collected, washed with water, dissolved in water, and filtered into a strong solution of ammonium chloride. The precipitated crystals are collected, washed with water and alcohol, and obtained as yellow octahedral prisms.-The salt, which is soluble in water, decomposes on heating with hydrochloric acid, giving chloro-pentammino-salt.3... 

Bromo - aquo - tetrammino - chromic Chloride, [Cr(NH3)4 H2O.Br]Cl2. is prepared from the bromide by dropping a solution of the salt into fuming hydrochloric acid, when a bright red crystalline powder is formed. 

Dihydroxo-diaquo-diammino-chromic Chloride, [Cr(NH3), (H20)2(OH)2]Cl, is formed by the addition of ammonia or pyridine to an aqueous solution of tetraquo-diammino-chloride, or saturating an aqueous acetic acid solution with rubidium chloride. It forms light red violet crystals which are insoluble in water. The iodide is obtained from the bromide on addition of potassium iodide to a dilute acetic acid solution of the salt as a light red violet precipitate. The thiocyanate, [Cr(NH3)2(H20)2(OH)2]SCN, is amorphous, and is prepared from the bromide by dissolving in aqueous acetic acid and adding potassium thiocyanate. 

Dithiocyanato-tetrammino-chromic Thiocyanate, [Cr(NH3)4 (SCN)o]SCN, is prepared by warming an aqueous solution of ehloro-aquo-tetrammino chromic chloride, [Cr(NH3)4H2O.Cl]Cl2, with excess of potassium thiocyanate, when dark orange-red crystals separate. The crystals are soluble in water, and by the addition of concentrated hydrochloric or hydrobromic acid the corresponding chloride or bromide is produced. These also are orange-red in colour. The nitrate is formed from the chloride by precipitation with nitric acid, and the sulphate by rubbing the chloride with sulphuric acid.4... 

Dithiocyanato-diethylenediamino-chromic Chloride, [Cr en2 (SCN)2]C1.HC1, is prepared from the thiocyanate on treatment with concentrated hydrochloric acid, filtering off undissolved substance, and cooling the filtrate, when a crystalline powder separates. This is recrystallised from water and obtained in ruby-red glistening needles. From a solution of this salt may be precipitated the nitrate, sulphate, bromide, or dichromate by means of the corresponding acid or potassium dichromate. 

Potassium Tetrathiocyanato-dipyridino-chromium, [Cr py2 (SCN)4]K.2H,0, is prepared by heating potassium ehromithiocyanate with anhydrous pyridine. The melt is reerystallised from a little wTater, and on cooling a stable red powder separates. The thiocyanato-groups are all within the complex, and the salt in solution therefore gives no coloration with ferric chloride. It is decomposed by chlorine in presence of cold water with formation of tetraquo-dipyridino-chromic chloride. If crystallised in the dry state from pyridine it forms transparent red crystals of the addition compound, [Cr py2(SCN)JK.4py, which quickly effloresce in air. 

The bromide, [Cr3 en4(0H)2]Cl4.2H20, is stated by Dubsky to contain 2 molecules of water in the molecule, and by Pfeiffer 2-5 molecules. This salt is prepared by treating a dilute aqueous solution of cis-dibis-aquo-diethylenediamino-chromic bromide with pyridine.3 It separates in lustrous bluish-violet crystals, which are soluble in water with neutral reaction. Halogen acids transform it into the m-diacido-salt for example, with hydrochloric acid it forms dichloro-diethylenediamino-chromic chloride, [Cr en2Cl2]Cl.H20. Other salts of the series may be obtained from the bromide. 

References Christensen, Z. anorg. Chem., 4, 229 (1893) to prepare chloro-pentammino-chromic chloride without the use of liquid ammonia, Christensen, J. prakt. Chem., (2) 23, 54 (1881) Jorgensen, J. prakt. Chem., (2) 20, 105 (1879). 

If chromic chloride is not available as a starting material for this procedure, the compound may be prepared in solution by reducing BOg of potassium dichromate with a mixture of 250ml of concentrated hydrochloric add and either 100ml of 95% alcohol, or HOg of oxalic acid 2-hydrate. The reduced solutions are then used directly in the reduction procedure only the zinc need be added. 

According to A. Schrotter, the chromic chloride should be prepared as free as possible from oxide, and from water by heating it in a glass tube on an oil-bath until no more water is evolved. The tube is then cooled, and ammonia passed for a time. The tube is then heated, and ammonia passed until no more ammonium chloride is evolved. The cold mass is removed from the tube, crushed to small fragments, and again treated with ammonia. This operation is repeated several times. 

Formation of the atropisomeric o-kexaphenylenes. When the chromium ate complex (2), prepared from 2,2 -dilithiumdiphcnyl and chromic chloride, is treated with a transition metal halide (CoClj, FcClj), the atropisomers of o-hexaphenylene (3a) and (3b) are formed ... 

Reduction of Chromic Chloride, CrClj.—Wohler obtained metallic chromium by reduction of chromic chloride with potassium, while Fremy obtained the metal in a crystalline condition by passing sodium vapour over chromic chloride in an atmosphere of pure dry hydrogen. Wohler prepared similar crystals—minute octahedra— by fusion of chromic chloride with zinc under a layer of mixed sodium and potassium cHorides, and removal of the zinc from the alloy so formed by means of nitric acid. Metallic calcium reduces chromic chloride at a dull red heat without formation of an alloy. ... 

Both positively and negatively charged colloids have been prepared. The former results when the hydrated oxide is pejotised with chromic chloride, or may be formed by hydrolysis of the chloride or nitrate the latter is prepared by peptising the hydrous oxide with sodium or potassium hydroxide, or by adding sodium hydroxide to chromium nitrate solution in presence of arsenious acid and then dialysing. ... 

Chromium Chlorsulphates may be prepared either by the action of sulphuric acid on chromic chloride, or by the interaction of hydrochloric acid -with chromium sulphate. Isomeric modifications of these salts exist, for it has been shown that in certain cases the — SO4, and in others the —Cl, is ionised. 

---