Ferric compounds are reduced to ferrous by the action of:—
(1) Stannous chloride;
(2) Sulphuretted hydrogen;
(3) Sodium sulphite; or
(4) Zinc.[66]
The processes, then, may be divided into two kinds, one based on oxidation and the other on reduction. In each case the titration must be preceded by an exact preparation of the solution to be assayed in order that the iron may be in the right state of oxidation.
PERMANGANATE AND BICHROMATE METHODS.
These consist of three operations:—
(1) Solution of the ore;
(2) Reduction of the iron to the ferrous state; and
(3) Titration.
Solution.—The only point to be noticed concerning the first operation (in addition to those already mentioned) is that nitric acid must be absent. If nitric acid has been used, evaporate to dryness, of course without previous dilution; add hydrochloric or sulphuric acid, and boil for five or ten minutes. Dilute with water to about 100 c.c., and warm until solution is complete.
The reduction is performed by either of the following methods:—
1. With Stannous Chloride.—Fill a burette with a solution of stannous chloride,[67] and cautiously run the liquid into the hot assay solution (in which the iron is present as chloride) until the colour is discharged. A large excess of the stannous chloride must be avoided. Then add 5 c.c. of a 2-1/2 per cent. solution of mercuric chloride, this will cause a white precipitate (or a grey one if too large an excess of the stannous chloride has been added). Boil till the solution clears, cool, dilute, and titrate.
2. With Sulphuretted Hydrogen.—Cool the solution and pass through it a current of washed sulphuretted hydrogen till the liquid smells strongly of the gas after withdrawal and shaking. A white precipitate of sulphur will be formed, this will not interfere with the subsequent titration provided it is precipitated in the cold. If, however, the precipitate is coloured (showing the presence of the second group metals), or if the precipitation has been carried out in a hot solution, it should be filtered off. Boil the solution until the sulphuretted hydrogen is driven off; this may be tested by holding a strip of filter paper dipped in lead acetate solution in the steam issuing from the flask. The presence of sulphuretted hydrogen should be looked for rather than its absence. It is well to continue the boiling for a few minutes after the gas has been driven off. Cool and titrate.