178 METALS AND THEIR COMBINATIONS. 



To explain the constitution of dichromates we have to assume 

 that chromic anhydride, CrO 3 , is capable of forming two acids : 



CrO 3 + H 2 O = H 2 CrO 4 = Chromic acid. 

 2CrO 3 + H 2 O = H 2 Cr 2 7 = Dichromic acid. 



Chromium trioxide, Acidum Chromicum, CrO 3 = 1OO (Chromic 

 acid, Chromic anhydride), is prepared by adding sulphuric acid to a 

 saturated solution of potassium dichromate, when chromium trioxide 

 separates in crystals : 



K 2 Cr 2 7 + H 2 S0 4 = K 2 S0 4 + H 2 O + 2CrO 3 . 



Thus prepared, its forms deep purplish-red, needle-shaped crystals, 

 which are deliquescent, and very soluble in water ; it is powerfully 

 corrosive, and one of the strongest oxidizing agents ; the solution in 

 water has strong acid properties ; it combines with metallic oxides, 

 forming chromates and dichromates. 



Experiment 29. Dissolve a few grammes of potassium dichromate in water 

 and add to 4 volumes of the cold saturated solution 5 volumes of strong sul- 

 phuric acid ; chromium trioxide separates on cooling. Collect the crystals on 

 asbestos, wash them with a little nitric acid, and dry them by passing warm 

 dry air through a tube in which they have been placed for this purpose. 



Chromic oxide, Cr 2 O 3 (Sesquioxide of chromium), is obtained by 

 heating potassium dichromate with sulphur, when potassium sulphate 

 and chromic oxide are formed : 



K 2 Cr 2 O 7 -f S = K 2 SO 4 + Cr 2 O 3 . 



By washing the heated mass with water, the chromic oxide is left 

 as a green powder, which is insoluble in water and in acids ; it is a 

 basic oxide combining with acids to form salts ; it is used as a green 

 color, especially in the manufacture of painted glass and porcelain. 



Chromic hydroxide, Cr 2 (OH) 6 . A solution of potassium dichro- 

 mate may be deoxidized by the action of hydrosulphuric acid, sul- 

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

 of sulphuric or hydrochloric acid : 



K 2 Cr 2 O 7 + 4H 2 SO 4 + 3H 2 S = K 2 SO 4 + 7H 2 O + 38 + Cr 2 (SOJ 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 hydrosulphuric acid having been oxidized 

 and converted into water. 



By adding ammonium hydroxide to the solution thus obtained, 





