Dissolve the precipitate in dilute hydrochloric acid; peroxidise with a few drops of nitric acid and boil, dilute to about 200 c.c., add ammonia (with constant stirring) till the liquid smells of it, and heat to boiling. Wash as much as possible by decantation with hot water. Transfer to the filter, and wash till the filtrate gives no indication of soluble salts coming through. The filtrate must be colourless and clear. The wet precipitate is very bulky, of a dark-brown colour and readily soluble in dilute acids, but insoluble in ammonia and dilute alkalies. When thrown down from a solution containing other metals it is very apt to carry portions of these with it, even when they are by themselves very soluble in ammoniacal solutions. It must be dried and ignited, the filter paper being burnt separately and its ash added. When further ignition ceases to cause a loss of weight, the residue is ferric oxide (Fe_{2}O_{3}), which contains 70 per cent. of iron. The weight of iron therefore can be calculated by multiplying the weight of oxide obtained by 0.7. The presence of ammonic chloride causes loss of iron during the ignition, and organic matter causes an apparent loss by reducing the iron to a lower state of oxidation. When the iron in the solution much exceeds 0.2 gram the volumetric determination is generally adopted, as the bulkiness of the precipitate of ferric hydrate makes the gravimetric method very inconvenient. VOLUMETRIC METHODS. As already explained these are based on the measurement of the volume of a reagent required to bring the whole of the iron from the ferrous to the ferric state (oxidation), or from the ferric to the ferrous (reduction). Ferrous compounds are converted into ferric by the action of an oxidising agent in the presence of an acid. Either permanganate or bichromate of potash is generally used for this purpose.[65] 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:--