498 SMITH'S INTERMEDIATE CHEMISTRY 



hydrolysis of the latter enters into double decomposition with the 

 Prussian blue and produces more rust: 



Fe 4 (Fe(CN) 6 ) 3 + 12NaOH -> 4Fe(OH) 3 J + 3Na 4 Fe(CN) 6 . 



Potassium Ferricyanide K 3 I [Fe(CN) 6 ] 111 . The difference 

 between this and the preceding salt can be seen by writing the 

 formulae thus: 4KCN,Fe(CN) 2 and 3KCN,Fe(CN) 3 . In the 

 ferricyanide the iron is trivalent and the negative ion Fe(CN) 6 = 

 is also trivalent as a whole. It is a soluble salt, of red color, 

 made by oxidizing the ferrocyanide. With ferric salts it gives 

 only a brown solution, but with ferrous salts it gives, a deep blue 

 precipitate of ferrous ferricyanide TurnbulFs blue: 



3FeCl 2 + 2K 3 Fe(CN) 6 -> Fe 3 (Fe(CN) 6 ) 2 j + 6KC1. 



We can distinguish ferrous-ion F6++ from ferric-ion Fe +++ by 

 this reaction. An equally sharp distinction is obtained by adding 

 potassium thiocyanate, for, although the ferrous and ferric thio- 

 cyanates are both soluble, the latter is blood red in color (see p. 

 235). 



Blue-Prints. Some ferric salts are reduced to ferrous salts 

 when exposed to light. Thus fertic oxalate Fe 2 (C 2 04) 3 will keep 

 in the dark, but in light gives ferrous oxalate FeC 2 O 4 : 



Fe 2 (C 2 O 4 ) 3 - 2FeC 2 4 + 2C0 2 1 . 



When paper is dipped in ferric oxalate solution and dried, and a 

 W fern (or ink drawing on transparent paper) is placed over the 

 prepared sheet, sunlight will reduce the iron to the ferrous condi- 

 \ tion excepting where the fern or ink lines protect it from the light. 

 When the sheet is now dipped in potassium ferricyanide solution 

 (developer), the ferric oxalate gives only the brown substance 

 which can be washed out. But the parts exposed to the light turn 

 deep blue from the precipitation of ferrous ferricyanide in the 

 paper. The pattern is white on a blue ground. In regular blue- 

 print paper ammonium-ferric citrate takes the place of the oxa- 



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