As the solution of potassium permanganate, which is deep red in colour, is dropped into the colourless iron solution, it is quickly decolorized while the iron solution gradually assumes a yellowish tinge, the first drop of the permanganate solution in excess giving it a pink tint.
In the reduction by stannous chloride the solution of the ore in the flask is heated to boiling, and a strong solution of stannous chloride is added until the solution is completely decolorized; then 60 cc. of a solution of mercuric chloride (so grammes to the litre) are run in and the contents of the flask poured into a dish containing 600 cc. of water and 60 cc. of a solution containing 200 grammes of manganous sulphate, i litre of phosphoric acid (1.3 sp. gr.), 400 cc. of sulphuric acid, and 1600 cc. of water.
The residue is then dissolved in water, decolorized by animal charcoal and saturated at 50 C. with oxalic acid.
The urea oxalate is recrystallized and decolorized and finally decomposed by calcium carbonate (J.
The precipitate is dissolved in boiling water, decolorized by potassium permanganate and decomposed by barium carbonate.