310 METALS AND THEIR COMBINATIONS. 



Chromic hydroxide, Cr(OH) 3 . A solution of potassium dichro- 

 rnate may be deoxidized by the action of hydrogen sulphide, sul- 

 phurous acid, alcohol, or any other deoxidizing agent, in the presence 

 of sulphuric or hydrochloric acid : 



K 2 Cr 2 7 + 4H 2 S0 4 + 3H 2 S = K 2 SO 4 -f 7H 2 O + 3S + Cr 2 (SO 4 ) 3 . 



As shown by this formula, the sulphates of potassium and chro- 

 mium are formed and remain in solution, while sulphur is precipi- 

 tated, the hydrogen of the hydrogen sulphide having been oxidized 

 and converted into water. 



By adding ammonium hydroxide to the solution thus obtained, 

 chromic hydroxide is precipitated as a bluish-green gelatinous sub- 

 stance : 



Cr a (S(Vs + 6NH 4 OH = 3(NH 4 ) 2 SO 4 -f 2Cr(OH) 3 . 



By dissolving this hydroxide in the different acids, the various 

 salts, such as chloride, CrCl 3 , sulphate, etc., are obtained. Chromic 

 sulphate, similar to aluminum sulphate, combines with potassium or 

 ammonium sulphate and water, forming chrome alum, KCr(SO 4 ) 2 . 

 12H 2 O; it is a purple salt, and is isomorphous with other alums. 



Perchromic acid, H 2 Cr 2 O 8 . This acid is of interest because it is analogous 

 to persulphuric acid, H 2 S 2 O 8 , and is formed in the test for hydrogen dioxide. 

 The ethereal solution is obtained when an acidified saturated aqueous solution 

 of potassium dichromate is shaken with ether and just sufficient hydrogen 

 dioxide solution to give an intense blue color. Excess of hydrogen dioxide 

 must be avoided. The ethereal solution is much more permanent than an 

 aqueous solution of the acid. When it is cooled to 20 C. (4 F.) and 

 treated with metallic potassium, a purplish-black precipitate of potassium per- 

 chromate, K 2 Cr,,O 8 , is formed. This is stable only at low temperature, decom- 

 posing at ordinary temperature into oxygen and potassium chromate. Several 

 other salts have been prepared ; they are all very unstable. 



The chemical conduct of chromium, according to the degree of oxidation or 

 the valence of the metal, is like that of manganese. Chromous salts, corres- 

 ponding to the oxide CrO, are known, but, like ferrous salts, they are very 

 readily oxidized and pass to the stable chromic salts, corresponding to the oxide 

 Cr 2 O 3 . The chromates and the acid, derived from the oxide CrO 3 , although 

 stable when alone in solution, readily give up oxygen in acid solutions to 

 reducing agents, just like permanganates, and the chromium gives salts of the 

 lower oxide, Cr 2 O 8 , which are green : 



K 2 Cr 2 7 + H 2 S0 4 = K 2 S0 4 + H 2 Cr 2 7 , 



H 2 Cr 2 O 7 ^ H 2 O + 2CrO 3 ; 2Cr0 3 = Cr 2 O 3 + 3O, 

 Cr 2 3 + 3H 2 S0 4 = 2 (S0 4 ) 3 + 3H 2 O. 



