IRON, COBALT, AND NICKEL 343 



under a bell jar over sulphuric acid. This salt, Fe 2 (N0 3 ) 6 ,9H 2 0, then 

 orystallises in well-formed and perfectly colourless crystals, 25 which 

 deliquesce in air, melt at 35, and are soluble in and decomposed by 

 water, The decomposition may be seen from the fact that the solution 

 is brown and does not yield the whole of the salt again, but gives 

 partly basic salt. The normal salt (only stable in the presence o an 

 excess of HN0 3 ) is completely decomposed with great facility by heat- 

 ing with water, even at 130, and this is made use of for removing iron 

 (and also certain other oxides of the form R 2 3 ) from many other 

 bases (of the form RO) whose nitrates are far more stable. The ferric 

 salts, FeX 3 , in passing into ferrous salts, act as oxidising agents, as is 

 seen from the fact that they not only liberate S from SH 2 , but also 

 iodine from KI like many oxidising agents. 25 bis 



cipitate of FeP0 4 , containing water. If a solution of ferric chloride (yellowish-red 

 colour) be mixed with a solution of sodium acetate in excess, the liquid assumes an 

 intense brown colour which demonstrates the formation of a certain quantity of ferric 

 acetate ; then the disodium phosphate directly forms a white gelatinous precipitate of ferrio 

 phosphate. By this means the whole of the iron may be precipitated, and the liquid which 

 was brown then becomes colourless. If this normal salt be dissolved in orthophosphorio 

 acid, the crystalline acid salt Fel^POJ^ is formed. If there be an excess of ferric oxide 

 in the solution, the precipitate will consist of the basic salt. If ferric phosphate be 

 dissolved in hydrochloric acid, and ammonia be added, a salt is precipitated on heating 

 which, after continued washing in water and heating (to remove the water), has the 

 composition Fe4P;jOu that is, 2Fe. i O 3) P 3 O 5 . In an aqueous condition this salt may be 

 considered as ferric hydroxide, Ee 2 (OH) 6 , in which (OH) 3 is replaced by the equivalent 

 group PO 4 . Whenever ammonia is added to a solution containing an excess of ferric 

 salt and a certain amount of phosphoric acid, a precipitate is formed containing the 

 whole of the phosphoric acid in the mass of the ferric oxide". 



Ferric oxide is characterised as a feeble base, and also by the fact of its forming double 

 salts for instance, potassium iron alum, which has a composition Fe J (S0 4 ) 3 ,K2SO 4 , 

 24H 3 O or FeK(SO 4 )2,12H3O. It is obtained in the form of almost colourless or light 

 rose-coloured large octahedra of the regular system by simply mixing solutions of 

 potassium sulphate and the ferric sulphate obtained by dissolving ferric oxide in sul- 

 phuric acid. 



. 25 It would seem that all normal ferric salts are colourless, and that the brown colou* 

 which is peculiar to the solutions is really due to basic ferric salts. "A remarkable 

 example of the apparent change of colour of salts is represented by the ferrous and ferric 

 oxalates. The former in a dry state has a yellow colour, although as a rule the ferrous 

 salts are green, and the latter is colourless or pale green. When the normal ferric salt is 

 dissolved in water it is, like many salts, probably decomposed by the water into acid 

 and basic salts, and the latter communicates a brown colour to the solution. Iron alum 

 is almost colourless, is easily decomposed by water, and is the best proof of our asser- 

 tion. The study of the phenomena peculiar to ferric nitrate might, in my opinion, give 

 ft very useful addition to our knowledge of the aqueous solutions of salts in general. 



23 bis The reaction FeX 5 + KI = FeX 3 + KX + I proceeds comparatively slowly in solu- 

 tions, is not complete (depends upon the mass), and is reversible. In this connection we 

 may cite the following data from Seubert and Rohrer's (1894) comprehensive researches. 

 The investigations were conducted with solutions containing T \j gram equivalent 

 weights of Fe a (S04)3 (i.e.' containing 20 grams of salt per litre), and a corresponding 

 solution of KI ; the amount of iodine liberated being determined (after the addition of 

 starch) by a solution (also T \j normal) of Na^S-jO^ (see Chapter XX., Note 42). The pro- 



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