838 PRINCIPLES OF CHEMISTRY 



= Ye 2 Cl 6 , or, generally speaking, 2FeO -f C1 2 + H 2 O = Fe 2 O 3 + 2HC1 

 When such a transformation is required it is best to add potassium 

 chlorate and hydrochloric acid to the ferrous solution ; chlorine is 

 formed by their mutual reaction and acts as an oxidising agent. 

 Nitric acid produces a similar effect, although more slowly Ferrous 

 salts may be completely and rapidly oxidised into ferric salts by means 

 of chromic acid or permanganic acid, HMnO 4 , in the presence of acids 

 for example, 10FeSO 4 + 2KMn0 4 + 8H 2 SO 4 = 5Fe 2 (S0 4 ) 3 + 2MnS0 4 

 4- K 2 S0 4 + 8H 2 0. This reaction is easily observed by the change of 

 colour, and its termination is easily seen, because potassium perman- 

 ganate forms solutions of a bright red colour, and when added to a 

 solution of a ferrous salt the above reaction immediately takes place in 

 the presence of acid, and the solution then becomes colourless, because all 

 'the substances formed are only faintly coloured in solution. Directly all 

 the ferrous compound has passed into the ferric state, any excess of 

 permanganate which is added communicates a red colour to the liquid 

 (see Chapter XXI.) 



Thus when ferrous salts are acted on by oxidising agents, they pass 

 into the ferric form, and under the action of reducing agents the 

 reverse reaction occurs. Sulphuretted hydrogen may, for instance, be 

 used for this complete transformation, for under its influence ferric salts 

 are reduced with separation of sulphur for example, Fe 2 Cl 6 + H 2 S 

 = 2FeCl 2 + 2HC1 + S. Sodium thior,ulphate acts in a similar way : 

 Fe 2 Cl 6 + Na 2 S 2 3 + H 2 O = 2FeCl 2 -f Na 2 SO 4 + 2HC1 + S. Me- 

 tallic iron or zinc, 20 in the presence of acids, or sodium amalgam, <fec., 

 acts like hydrogen, and has also a similar reducing action, and this 

 furnishes the best method for reducing ferric salts to ferrous salts 

 for instance, Fe 2 Cl 6 + Zn = 2FeCl 2 + ZnCl 2 . Thus the transitionfrorn 

 fsrrous salts to ferric softs and vice versd is always possible? 1 



30 Copper and cuprous salts also reduce ferric oxide to ferrous oxide, and are them* 

 selves turned into cupric salts. The essence of the reactions is expressed by the following 

 equations . Fe 2 O 3 + CajO+ 2FeO + 2CuO ; Fe 8 O 3 + Cu = 2FeO + CuO. This fact is made 

 use of in analysing copper compounds, the quantity of copper being ascertained by the 

 amount of ferrous salt obtained. An excess of ferric salt is required to complete the 

 reaction. Here we have an example of reverse reaction ; the ferrous oxide or its salt in 

 the presence of alkali transforms the cupric oxide into cuprous oxide and metallic copper, 

 as observed by Lovel, Knopp, and others. 



J1 We will here mention the reactions by means of which it may be ascertained 

 whether the ferrous compound has been entirely converted into a ferric compound or 

 vice versd. There are two substances which are best employed for this purpose: 

 potassium ferricyanide, FeE^C^Ne, and potassium thiocyanate* KCNS. The first salt 

 gives with ferrous salts a blue precipitate of an insoluble salt, having a composition 

 Fe5C 12 N 13 , but with ferric salts it does not form any precipitate, and only gives a brown 

 colour, and therefore when transforming a ferrous salt into a ferric salt, the completion 

 of the transformation may be detected by taking a drop of the liquid on paper or on ft 

 porcelain plate and adding a drop of the ferricyanide solution. If a blue precipitate be 



