Blue Compounds of Cyanogen and Iron. 161 



We may call it, according to the customary nomenclature, 

 ferridcyanide of iron and potassium. It has been mentioned 

 above, that on treating it with potash, ferrocyanide of potas- 

 sium is formed, and hydrated oxide of iron is set free. It is 

 evident that in this reaction 2 atoms of iron are replaced by 

 3 of potassium, whilst the oxygen of the potash combines 

 with the iron. 



On heating this blue compound with a solution of yellow 

 prussiate it changes it into ferridcyanide • if the blue compound 

 be in excess no trace of yellow prussiate remains behind. 

 3*473 grms. ferridcyanide of iron and potassium were sus- 

 pended in an excess of ferrocyanide, and digested with it for 

 a considerable time. Collected on a filter, it was of a pale 

 blue colour and gave 1*39 grm. sulphate of potash, corre- 

 sponding to 0*624 potassium, of which, before the treatment 

 with ferrocyanide, it only contained 0*411. Hence it is evi- 

 dent that the reaction consists in potassium being taken from 

 the yellow prussiate, and half of the ferridcyanide of iron and 

 potassium changed into ferrocyanide. This reaction may 

 even be applied with advantage to the preparation of larger 

 quantities of pure red prussiate, for it presents neither of the 

 disadvantages attending the decomposition by chlorine, viz. 

 the admixture of chloride of potassium, which is unfavourable 

 to crystallization, and also the well-known green substance, 

 which is formed by the slightest excess of chlorine. The 

 blue salt may of course be used repeatedly for this operation, 

 as it is immediately restored by warming with nitric acid. 



By long- continued boiling with nitric acid, this blue com- 

 pound is changed into a rich dark green, which contains a 

 greater proportion of cyanogen and a small quantity of potas- 

 sium. It is reduced by the action of light, which gradually 

 changes its colour to a blue. For analysis, it was heated with 

 concentrated sulphuric acid until all cyanogen was destroyed ; 

 the iron and potassium were determined as peroxide and sul- 

 phate of potash. 



I. 2*966 grms. gave 1*527 peroxide of iron and 0*147 sul- 

 phate of potash, corresponding to 35*72 per cent, iron and 

 2*23 per cent, potassium. 



II. 3*172 grms. gave 1*637 peroxide of iron and 0*159 sul- 

 phate of potash, corresponding to 35*97 P er cent, iron and 

 2*25 per cent, potassium. 



Burned with chromate of lead it gave the following num- 

 bers : — 



I. 0*421 grm. gave 0*361 carbonic acid and 0*058 water, 

 corresponding to 23*3 per cent, carbon and 13*6 water. 



II. 0*151 grm. gave 0*121 carbonic acid and 0*020 water, 

 corresponding to 23*2 per cent, carbon and 13*3 water. 



