IKON. 299 



The exsiccated ferrous sulphate, U. S. P., is made by expelling 

 nearly all the water of crystallization by heating to 100 C. (212 F.) ; 

 the granulated (precipitated) ferrous sulphate is made by quickly 

 cooling a hot saturated solution of ferrous sulphate, slightly acidu- 

 lated with sulphuric acid, while stirring, when ferrous sulphate sepa- 

 rates as a crystalline powder, which is filtered, washed with alcohol, 

 and dried. 



Experiment 32. In a flask put 10 c.c. of concentrated sulphuric acid diluted 

 with 40 c.c. of water, add iron wire, card teeth, or nails, in portions, until the 

 acid is exhausted, as seen by the cessation of effervescence. Gently heating 

 facilitates the action at the end. Note the bad odor of the hydrogen, due to 

 impurities, and the dark flakes of carbon in the solution. Finally, filter the 

 hot solution and set it aside to crystallize. If crystals do not form, evaporate 

 further. 



Ferrous sulphate readily forms double salts with alkali sulphates, which are 

 not efflorescent, and in the dry state are less readily oxidized than ferrous sul- 

 phate. When a hot, strong solution of 1 part of ammonium sulphate is added 

 to a similar solution of 2 parts of crystals of ferrous sulphate, on cooling, a 

 salt with the composition, (NHJ 2 SO 4 .FeSO 4 .6H. 2 0, separates (Mohr's salt). 

 This is often used when a stable ferrous salt is wanted. 



Ferric sulphate, Fe 2 (SO 4 ) 3 . The solution of this salt, Liquor ferri 

 tersulphatis, is made by adding sulphuric and nitric acids to a solution 

 of ferrous sulphate and heating : 



6FeS0 4 + 3H 2 S0 4 + 2HNO 3 = 3[Fe 2 (SO 4 ) 3 ] + 2NO + 4H 2 O. 



The action of nitric acid is similar to that described above under 

 ferric chloride. The hydrogen of the sulphuric acid is oxidized, and 

 the radical SO 4 unites with the ferrous sulphate, nitrogen dioxide 

 being liberated. 



Experiment 33. Dissolve several crystals of ferrous sulphate in about 20 c.c. 

 of water, add about 5 c.c. of dilute sulphuric acid. Warm the solution and 

 add concentrated nitric acid, in drops, until the dark color first produced sud- 

 denly turns to reddish-brown. Note the red fumes of oxide of nitrogen 

 escaping. The dark color is due to the union of nitric oxide, NO (see reaction 

 above), with unoxidized ferrous sulphate (see test 2 for nitric acid). Heat the 

 solution to expel oxide of nitrogen and excess of nitric acid. Dilute a few 

 drops and test with ferricyanide, as in Experiment 30. 



Solution of ferric sulphate is used in the preparation of Ferric 

 ammonium sulphate, Ferri et ammonii sulphas, FeNH 4 (SO 4 ) 2 .12H 2 O 

 (iron alum, or ammonio-ferric alum), which is made by mixing a solu- 

 tion of ferric sulphate with ammonium sulphate and crystallizing. 

 The salt has a pale violet color and is readily soluble in water. 



