CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC, 283 



Sulphuric acid the following reaction takes place 2CrO 3 + 3H 2 S0 4 

 =Cr 2 (SO 4 ) 3 + 3 -f 3H 2 O. It will be readily understood from this that 

 a mixture of chromic acid or of its salts with sulphuric acid forms an 

 excellent oxidising agent, which is frequently employed in chemical 

 laboratories and even for technical purposes as a means of oxidation. 

 Thus hydrogen sulphide and sulphurous anhydride are converted into 

 sulphuric acid by this means. Chromic acid is able to act as a powerful 

 oxidising agent because it passes into chromic oxide, and in so doing 

 disengages half of the oxygen contained in it : 2CrO 3 = Cr 2 O 3 -f O 3 . 

 Thus chromic anhydride itself is a powerful oxidising agent, and is 

 therefore employed instead of nitric acid in galvanic batteries (as a 

 depolariser), the hydrogen evolved at the carbon being then oxidised, 

 and the chromic acid converted into a non-volatile product of deoxida- 

 tion, instead of yielding, as nitric acid does, volatile lower oxides of 

 offensive odour. Organic substances are more or less perfectly oxidised 

 by means of chromic anhydride, although this generally requires the aid 

 of heat, and does not proceed in the presence of alkalis, but generally 

 in the presence of acids. In acting on a solution of potassium iodide, 

 chromic acid, like many oxidising agents, liberates iodine ; the reaction 

 proceeds in proportion to the amount of Cr0 3 present, and may serve 

 for determining the amount of CrO 3 , since the amount of iodine liberated 

 can be accurately determined by the iodometric method (Chapter XX., 

 Note 42). If chromic anhydride be ignited in a stream of ammonia, it 

 gives chromic oxide, water, and nitrogen. In all cases when chromic 

 acid acts as an oxidising agent in the presence of acids and under the 

 action of heat, the product of its deoxidation is a chromic salt, CrX 3 , 

 which is characterised by the green colour of its solution, so that the 

 red or yellow solution of a salt of chromic acid is then transformed into 

 a green solution of a chromic salt, derived from chromic oxide, Cr 2 3 , 

 which is closely analogous to A1 2 O 3 , Fe 2 O 3 , and other bases of the com- 

 position E 2 O 3 . This analogy is seen in the insolubility of the anhydrous 

 oxide, in the gelatinous form of the colloidal hydrate, in the formation 

 of alums, 7 of a > volatile chloride of chromium, &c. 7 bis 



7 As a mixture of potassium dichromate and sulphuric acid is usually employed 

 for oxidation, the resultant solution generally" contains a double sulphate of potas- 

 sium and chromium that is, chrome alum, isomorphous with ordinary alum 

 K 2 Cr 2 O 7 + 4H 2 SO 4 + 20H 2 O = 63 + K 2 Cr 2 (SO 4 ) 4 ,24H 2 or 2(KCr(SO 4 ) 2 ,12H 2 0). It is pre- 

 pared by dissolving potassium dichromate in dilute sulphuric acid ; alcohol is then added 

 and the solution slightly heated, or sulphurous anhydride is passed through it. On the 

 addition of alcohol to a cold mixture of potassium dichromate and sulphuric acid, the 

 gradual disengagement of pleasant-smelling volatile products of the oxidation of alcohol, 

 and especially of aldehyde, C 2 H 4 O, is remarked. If the temperature of decomposition 



For Note 7 bis see p. 285. 



