METHOD OF DETERMINING IRON IN WATERS. 349 



with the rinsings, and oxidized with permanganate to a 

 very slight excess, and then filtered, so as to separate any 

 precipitate, and also to remove the excess of permanganate. 

 The fluid thus obtained is next tested as before by adding 

 the whole or a known part of it to one of the cylinders 

 containing the ferrocyanide. When the water, after being 

 filtered, has still a cloudy appearance, as is the case with 

 sewage and polluted rivers &c, a known quantity of the 

 filtered water must be evaporated to dryness and ignited, 

 the residue dissolved in a small quantity of hydrochloric 

 acid and filtered, washed with water, and the free acid in 

 the filtrate as nearly as possible neutralized with ammonia, 

 and then i cub. cent, of sulphuric acid added, after which 

 it must be oxidized with permanganate, then filtered, if 

 requisite, to remove excess of permanganate, and the iron 

 estimated as before. A green colour may be sometimes 

 obtained, instead of the pure blue ; this is owing to a slight 

 quantity of unreduced permanganate being present, which, 

 however, is of no consequence, as, with a little practice, 

 the green tint may be compared with the blue, and correct 

 results obtained; still the comparison may be rendered 

 easier by adding i to 2 drops of permanganate to the 

 cylinder into which the standard iron is run, and which 

 by this means will also assume a green tint. Experiments 

 were made with reference to this point ; and it was found 

 that the presence of not more than a few drops of unre- 

 duced permanganate has little or no effect on the results 

 obtained, the only consequence being the change of tint, 

 but not of depth of colour. 



Potassium permanganate is employed as the oxidizer, 

 instead of nitric acid, because (i) the oxidation is per- 

 formed much quicker than it would be if nitric acid were 

 used, as in the latter case the liquid would have to be 

 heated ; (2) it can be added to exactly the right point, 

 which could not easily be done with nitric acid ; (3) an 



