I have not yet obtained a nitrate of iron giving only a white precipitate with prussiate of potash, that is, such as contains only oxide of iron at its minimum of oxydation; for when pure green oxide of iron is dissolved by very dilute nitric acid, a small quantity of the acid is generally decomposed, which is likewise the case in the decomposition of nitre by green sulphate of iron. The solutions of nitrate of iron, however, procured in both of these modes, absorb nitrous gas with rapidity, and by sulphurated hydrogene might probably be converted into pale nitrate.
As it is impossible to obtain concentrated solutions of pale nitrate of iron, chiefly containing green oxide, its powers of absorbing nitrous gas cannot be compared with the muriatic and sulphuric solutions, unless they are made of nearly the same specific gravity.
Nitrous gas is disengaged by heat from the impregnated solution of nitrate of iron, at the same time that much red oxide of iron is precipitated. Whether any nitrous gas is decomposed, I have not yet ascertained; for when unimpregnated pale nitrate of iron is heated, a part of the acid, and of the water of the solution, is decomposed by the green oxide of iron;[128] and in consequence ammoniac, and red nitrate of iron formed, whilst red oxide is precipitated.
X. Absorption of Nitrous Gas by other
Metallic Solutions.
a. White prussiate of iron in contact with water absorbs nitrous gas to a great extent, and becomes dark chocolate.[129]
b. Concentrated solution of sulphate of tin, probably at its minimum of oxydation, absorbs one eighth of its bulk of nitrous gas, and becomes brown, without deposition.
c. Solution of sulphate of zinc absorbs about one tenth of its volume of nitrous gas, and becomes green.
d. Solution of muriate of zinc[130] absorbs nearly the same quantity, and becomes orange brown.
e. These are all the metallic substances on which I have experimented. It is more than probable that there exist others possessing similar powers of absorbing nitrous gas.
Whenever the metals capable of decomposing water exist in solutions at their minimum of oxydation, the affinities exerted by them on nitrous gas and water, will be such as to produce combination. The powers of metallic solutions to combine with nitrous gas at common temperatures, as well as to decompose it at higher temperatures, will probably be in the ratio of the affinity of the metallic oxides they contain, for oxygene.