NOTES ON THE CHLORIDES OF IRON. 
401 
Three-fifths only of the oxygen in the nitric acid perforin this work, the other 
two still remaining with the nitrogen, and ultimately escaping in the gaseous 
form of binoxide of nitrogen from the vessels used in the operation. The whole 
reaction may be thus expressed :— 
Fe 6 + 9HCl-fN0 6 = 3Fe 2 Cl 3 + 3H0+N0 2 . 
The binoxide of nitrogen arising from the portions of nitric acid first decom¬ 
posed, dissolve in that part of the protochloride of iron not yet acted on, and 
orm a compound of dark colour ; but when the solution is evaporated to a par¬ 
ticular degree of concentration, or excess of some kind of acid is added, the 
black compound is decomposed, and the whole of the binoxide of nitrogen 
escapes. I may here state that I have satisfied myself that this is a true expla¬ 
nation of the chemical reactions which occur in the process. Yet the process 
will not yield, or only by chance yield a pure product. In the first place, an 
exact amount of hydrochloric acid must be used. Now, hydrochloric acid is a 
body which, chiefly from its character as a gas, cannot be conveniently em¬ 
ployed in the pure state; it is always met with in commerce in the state of so¬ 
lution in water, and the strength of the solution is determined by the specific 
gravity ; and if the specific gravity is not exactly what it should be, the result¬ 
ing solution of perchloride of iron will be acid or basic to an unknown extent. 
This circumstance, however, may interfere with any process for the preparation 
of perchloride of iron, or indeed with any other operation in which a given bulk 
of aqueous hydrochloric acid is used. In short, the liquid commonly known as 
hydrochloric acid should never be employed in making solutions of exact 
strength, unless its specific gravity be previously ascertained,—a fact that very 
well serves to show how the simplest operations in Pharmacy require an amount 
of chemical knowledge and skill such as is not always possessed by the followers 
of that calling in the present state of the law of this country. Supposing, how¬ 
ever, the acid to be of known strength, and the requisite amount for a given 
quantity of iron to have been correctly calculated, it must be diluted with 
water, and that water subsequently removed by evaporation. It must be 
diluted, or the iron will not wholly dissolve in the cold; the first portions of 
protochloride of iron formed, being only slightly soluble in the hydrochloric 
acid, crystallize on the iron, and thus prevent the acid from coming into contact 
with the metal; and the solution must not be heated, or protochloride of iron 
escapes. The amount of water necessary to keep the protochloride in solution is 
about half the volume of acid used, if that acid be of the sp. gr. 1T6 or I'll. 
But the water having performed its office, must now be removed, for the action 
of the nitric acid on the hydrochloric acid and protosalt is only complete when 
the mixture is so concentrated that it has about the bulk of the hydrochloric 
acid originally employed: the black compound, otherwise so fugitive, is com¬ 
paratively stable in neutral or only slightly acid solutions, and the binoxide of 
nitrogen which causes it only escapes from such solutions when they are concen¬ 
trated to a syrupy condition. To get rid of this water, heat must of course be 
used ; the liquid must be boiled. But, as we have already seen, solution of 
perchloride of iron is decomposed when evaporated at a boiling temperature. 
Here again, therefore, the process fails. The only means of preventing such a 
decomposition is to add more hydrochloric acid, a proceeding which has the 
additional advantage of facilitating the breaking up of the black compound, and 
thus of more speedily causing the total evolution of the binoxide of nitrogen. 
But we cannot know what amount to add. Slight differences in the rate or 
extent of evaporation greatly influence the evolution of hydrochloric acid from 
the hydrated perchloride of iron, and whether much or little acid must be added 
to make up the loss, we have no ready means of ascertaining. Nor can the 
solution of the iron in two-tliirds of the hydrochloric acid and water be evapo¬ 
rated before the other third and the nitric acid are added ; for in that case proto- 
