On the Oxidation of Ferrous Salts 245 



inaccuracy attending its use in hydrochloric acid solution, 

 which is as dilute as the sulphuric acid solution is when it is 

 commonly titrated. But the chief want of the analyst is the 

 power to titrate strongly acid hydrochloric solutions of iron 

 with permanganate, and I find that this can be done without 

 difficulty. As long as there is ferrous salt in the solution the 

 permanganate devotes itself exclusively to it; but as soon as 

 it is all transformed into ferric salt the permanganate at once 

 attacks the hydrochloric acid, and the characteristic odour of 

 euchlorine is at once perceived. If the permanganate is added 

 with sufficient care to avoid local supersaturation, the appear- 

 ance of this odour indicates with very considerable sharpness 

 the moment when all the iron has been oxidised. It can, 

 however, also be indicated clearly to the eye. If one or two 

 drops of a dilute solution of pure ferricyanide of potassium be 

 added to the ferrous solution so as to colour it blue without 

 producing a precipitate, then on titrating with permanganate 

 the disappearance of the ferrous salt is indicated by the simul- 

 taneous disappearance of the blue colour. This is illustrated 

 in the following table. In it the concentration of the solutions 



TABLE I. 



M-nmlrcul.- p.T litiv : a- o-5H,SO 4 ami 

 tl< ' lor each rxpniment 10 c.c. of ferrous sulphat. 



-"lution (a), containing 0-0407 gramme iron, were . 

 i with noim.tl Milphmir acid (O'5H 2 SO 4 ) or with 



hydrochloric 'H< 1). dilutol with water so as to ! 



