Potassium Hexacyanochromate III

Stjbmitted by John H. Bigelow Checked by John C. Bailar, jB.f 

Cruser and Miller have described a method for the preparation of potassium hexacyanochromate(III) whereby chromium(VI) oxide is dissolved in hydrochloric acid, reduced with alcohol, evaporated to dryness, taken up with water, and poured into a hot solution of potassium cyanide. The resulting solution is digested several hours, filtered, and allowed to evaporate in air. The following method is a modification of that given by Christensen. It differs from that of Cruser and Miller in that a dichromate is reduced with sulfur dioxide and that chromium(III) acetate instead of chromium(III) chloride is poured into potassium cyanide solution. In following the directions outlined here, special care should be exercised to obtain pure chromium(III) acetate and to avoid decomposition of the relatively unstable potassium hexacyanocbromate(III). 

Twenty-five grams of potassium dichromate is dissolved in approximately 500 ml. of water and reduced by passing sulfm dioxide into the solution, which is then boiled to remove the excess sulfur dioxide. The boUing solution is stirred vigorously while chromium(III) hydroxide is precipitated by slow addition of aqueous ammonia. J A piece 

I Care should be taken to have only a slight excess of ammonia present because of the tendency of chromium to form soluble complex ammine compounds. Vigorous boiling of the solution until only a faint odor of ammonia is perceptible will prevent this. 

For purification, the crude crystals may be recrystallized from water. Two or three recrystallizations are necessary to obtain a pure product. Alternatively, the salt may be purified by dissolving in warm water and adding 3 or 4 volumes of alcohol. The material obtained in this way is not quite as pure or as definitely crystalline as that obtained from water alone. 

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Potassium hexacyanochromate (III) (BHjO) [ 13601-11 -1 ] M 418.5. Crystd from water. 

Potassium hexacyanochromate(III) is available by a literature method.A solution of [Cu(tren)(0H2)][C104]2 is prepared by mixing aqueous solutions of Cu(C104)2 6H20 (2.60 g, 7.00 mmol) in 20 mL of water and tren (1.02g, 6.98 mmol) in 10 mL of water. The dropwise addition of a hexacyanochromate(III) solution, prepared by dissolving 0.38 g of K3[Cr(CN)6] (1.16 mmol) in 15 mL of water, to the copper solution results in the immediate formation of a turquoise precipitate. Once the addition is complete, the precipitate is collected by filtration washed successively with cold water, ethanol, and ether and then air-dried to afford a blue powder of [ Cu(tren)(CN) 6Cr][C104]9 H2O Yield 1.50 g, 55%. 

Potassium hexacyanochromate(III) is a yellow solid, highly soluble in water, that crystallizes as large square platelets. It crystallizes in the orthorhombic system, with the space group Pcan. The unit parameters are a = 8.53, b = 10.60, c = 13.68 A. Because of its disorder behavior, it presents a complex crystallographic problem. The compound shows a j/cn band at 2131 cm. The molar extinction coefficients in aqueous solution of the two observable d d bands at 376 nm ( A2g —> 72) and 309 nm (4 2 T l ) are 93 and 62 L mol ... 

Use the information in Table 16.4 to write the formula for each of the following coordination compounds (a) potassium hexacyanochromate(III) ... 

A solution containing only the neutral and cationic cyanoaquo complexes is obtained when more than a 3 1 ratio of perchloric acid is added to a solution of potassium hexacyanochromate(III). The chilled hydrolysis solution, prepared as described previously, can be first passed through a short anion-exchange column in the perchlorate cycle to remove any possible anionic complexes and then loaded onto the cation exchange column in the sodium cycle. Because the separation of the cationic species takes more time than the separation of the anionic complexes, due to the greater number of species present on the column, it is advantageous to use a water-jacketed column and to carry out the separations near 0°C. 

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