The mixture after being acted upon by boiling water, is to be left to settle, to allow the oxide to be deposited, for it is so fine as to pass through a filter. If the iron contained manganese, this would be found at first in the alkaline solution; but manganese spontaneously separates by exposure to the air. The alkaline liquor must be supersaturated with muriatic acid, and evaporated to dryness. The liquor acidulated, and deprived of its silica by filtration, is to be supersaturated with ammonia; when the alumina will precipitate in the state of a subphosphate. When the liquor is now supersaturated with acetic acid, and then treated with acetate of lead, a precipitate of phosphate of lead almost always falls. There is hardly a bit of iron to be found which does not contain phosphorus. The slightest trace of chrome is detected by the yellow colour of the lead precipitate; if this be white there is none of the colouring metal present.
100 parts of the precipitated phosphate of lead contain, after calcination, 19·4 parts of phosphoric acid. The precipitate should be previously washed with acetic acid, and then with water. These 19·4 parts contain 8·525 parts of phosphorus.
Cast iron sometimes contains calcium and barium, which may be detected by their well-known reagents, oxalate of ammonia, and sulphuric acid. In malleable iron they are seldom or never present.
The charcoal found in the residuum of the nitro-muriatic solution is to be burned away under a muffle. The solution itself contains along with the oxide of iron, protoxide of manganese, and other oxides, as well as the earths, and the phosphoric and arsenic acids. Tartaric acid is to be added to it, till no precipitate be formed by supersaturation with caustic ammonia. The ammoniacal liquor must be treated with hydrosulphuret of ammonia as long as it is clouded, then thrown upon a filter. The precipitate is usually very voluminous, and must be well washed. The liquor which passes through is to be saturated with muriatic acid, to decompose all the sulphurets.
The solution still contains all the earths and the oxide of titanium, besides the phosphoric acid. It is to be evaporated to dryness, whereby the ammonia is expelled, and the carbonaceous residuum must be burned under a muffle. If the iron contains much phosphorus, the ashes are strongly agglutinated. They are to be fused as already described along with carbonate of potash, and the mass is to be treated with boiling water. The residuum may be examined for silica, lime, barytes, and oxide of titanium. Muriatic acid being digested on it, then evaporated to dryness, and the residuum treated with water; will leave the silica. Caustic ammonia, poured into the solution, will separate the alumina, if any be present, and the oxide of titanium; but the former almost never occurs.
Manganese is best sought for by a distinct operation. The iron must be dissolved at the heat of boiling water, in nitro-muriatic acid; and the solution, when very cold, is to be treated with small successive doses of solution of carbonate of ammonia. If the iron has been oxidized to a maximum, and if the liquor has been sufficiently acid, and diluted with water, it will retain the whole of the manganese. This process is as good as that by succinate of ammonia, which requires many precautions.
The liquor is often tinged yellow by carbon, after it has ceased to contain a single trace of iron oxide. As soon as litmus paper begins to be blued by carbonate of ammonia, we should stop adding it; immediately throw the whole upon a filter, and wash continuously with cold water. What passes through is to be neutralized with muriatic acid, and concentrated by evaporation. It may contain besides manganese, some lime, or barytes. It should therefore be precipitated with hydrosulphuret of ammonia, the hydrosulphuret of manganese should be collected, dissolved in strong muriatic acid, filtered, and treated, at a boiling heat, with carbonate of potash. The precipitate, well washed and calcined, contains, in 100 parts, 72·75 parts of metallic manganese.
The copper, arsenic, lead, tin, bismuth, antimony, or silver, are best separated by a stream of sulphuretted hydrogen gas passed through the solution in nitro-muriatic acid, after it is largely diluted with water. The precipitate must be cautiously roasted in a porcelain test, to burn away the large quantity of sulphur which is deposited in consequence of the conversion of the peroxide of iron into the protoxide. If nothing remains upon the test, none of these metals is present. If a residuum be obtained, it must be dissolved in nitro-muriatic acid, and subjected to examination. But, in fact, carbon, sulphur, phosphorus, silicon, and manganese, are the chief contaminators of iron.
Chloride of silver affords the means of determining the proportion of carbon contained in iron, and of ascertaining the state in which that substance exists in the metal. Fused chloride of a pale yellow colour must be employed. The operation is to be performed in close vessels, with the addition of a great deal of water, and a few drops of muriatic acid. The carbonaceous residuum is occasionally slightly acted upon. We may judge of this circumstance by the gases disengaged, as well as by the appearance of the charcoal.
Ductile iron and soft steel, as well as white cast-iron which has been rendered gray by roasting, when decomposed by chloride of silver, afford a blackish-brown unmagnetic charcoal, and a plumbaginous substance perfectly similar to what is extracted from the same kinds of iron, by solution in acids. A portion of this plumbago is also converted into charcoal of a blackish brown colour, by the action of the chloride. Hence this agent does not afford the means of obtaining what has been called the poly-carburet, till it has produced a previous decomposition. But we obtain it, in this manner, purer and in greater quantity than we could by dissolving the metal in the acids. The only subject of regret is, that we possess no good criterion for judging of the progress of this analytical operation.