308 METALS AND THEIR COMBINATIONS. 



Chromium trioxide, Chromii trioxidum, CrO 3 = 99.34 (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 2 7 -f H 2 S0 4 = K 2 S0 4 + H 2 + 2CrO 3 . 



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

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

 to animal and vegetable matter, and one of the strongest oxidizing 

 agents ; the solution in water has strong acid properties, but neither 

 chromic nor dichromic acid are known in a pure state as an aqueous 

 solution of chromium trioxide, on concentration breaks up into the 

 oxide and water. 



Experiment 36. 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. 



Chromates and dichromates, When chromium trioxide is dissolved in 

 water, dichromic acid is mainly formed thus : 



200. + H 2 = H 2 Cr 2 7 , 



which gives the ions 2H* and Cr 2 O 7 ". The Cr 2 O 7 x/ ion is yellowish red in 

 color. There is, however, a slight amount of chromic acid formed, thus : 



Ci0 3 + H 2 = H 2 CrO 4 , 



which gives the ions 2H' and CrO 4 ". The ion CrO 4 " is yellow. Chromic acid 

 is known through its salts, the chromates, which give the ion, CrO/'. 



Potassium and sodium chromate in solution show a basic reaction which is 

 not due to any weak acid character of chromic acid, but to the fact that chro- 

 mates have a great tendency to pass to salts of dichromic acid. They are de- 

 composed to some extent by water, thus : 



2K 2 CrO 4 + H 2 == K 2 Cr 2 O 7 + 2KOH. 



If an acid, even a weak one, is added to the solution, the decomposition be- 

 comes practically complete by the removal of the KOH by union with the 

 acid. The color changes from yellow to red, and, upon concentration, the 

 rather moderately soluble dichromate crystallizes out in the case of the potas- 

 sium salt. Potassium dichromate is almost neutral in reaction ; it is, therefore, 

 not an acid chromate. In fact acid chromates are not known in which respect 

 chromic acid diners from sulphuric acid. The acid salt of the composition, 

 KHCr0 4 , which we would expect to be formed by acidifying a solution of the 

 chromate, changes at once into the salt of dichromic acid, thus : 

 2KHCr0 4 = K 2 Cr 2 7 -f H 2 O. 



Although potassium dichromate contains no acid hydrogen, it acts essen- 

 tially like an acid salt toward alkalies. When potassium hydroxide is added 



