The tube is kept red-hot until the stain is very distinct; then the source of heat is removed, and the gas allowed to bubble through the argentic nitrate solution, which it decomposes, as before detailed ([p. 526]). This process is continued until, on placing the delivery tube in a sample of clear nitrate of silver solution, there is no darkening of colour. In certain cases this may take a long time, but the apparatus, once set to work, requires little superintendence. At the conclusion, the whole of the arsenic is separated,—part is in the silver solution as arsenious acid, part in the tube as a ring of metallic arsenic. The portion of the tube containing the metallic arsenic should be cut off with a file and weighed, the arsenic then removed and re-weighed; the loss is the metal approximately. Or, the weight of the film may be estimated by having a set of similar deposits of known weight or quantities, in tubes exactly corresponding to those used in the analysis, and comparing or matching them.
The arsenious acid in the nitrate of silver may be dealt with in several ways. The equation given ([p. 526]) shows clearly that pure arsine, passed into nitrate of silver solution, decomposes it in such a manner that, if either the silver deposited or the free acid is estimated, the quantity of arsenic can from such data be deduced. In operating on organic liquids, ammonia and other products may be given off, rendering either of the indirect processes inadvisable. A very convenient method, applicable in many cases, is to throw out the silver by hydrochloric acid, alkalise the filtrate by bicarbonate of soda, and titrate with iodine solution. The latter is made by dissolving exactly 12·7 grms. of pure dry iodine by the aid of 18 grms. of potassic iodide in one litre of water, observing that the solution must take place in the cold, without the application of heat. The principle of the titration is, that arsenious acid, in the presence of water and free alkali, is converted into arsenic acid—
As2O3 + 4I + 2Na2O = As2O5 + 4NaI.
The end of the reaction is known by adding a little starch-paste to the solution; as soon as a blue colour appears, the process is finished.
Another convenient way by which (in very dilute solutions of arsenious acid) the arsenic may be determined, is a colorimetric method, which depends on the fact that sulphuretted hydrogen, when arsenious acid is present in small quantity, produces no precipitate at first, but a yellow colour, proportionate to the amount of arsenic present. The silver solution containing arsenious acid is freed from silver by hydrochloric acid; a measured quantity of saturated SH2 water is added to a fractional and, if necessary, diluted portion, in a Nessler cylinder or colorimetric apparatus, and the colour produced exactly imitated, by the aid of a dilute solution of arsenious acid, added from a burette to a similar quantity of SH2 water in another cylinder, the fluid being acidified with HCl.
§ 744. Destruction of the Organic Matter by Nitric Acid, and Subsequent Reduction of the Arsenic Acid to Arsine (Arseniuretted Hydrogen), and final Estimation as Metallic Arsenic.—This process, which is essentially a combination of several, has been much improved in its details by R. H. Chittenden and H. H. Donaldson.[784] 100 grms. of the suspected matters, cut up into small pieces, are heated in a porcelain dish of suitable size, stirred by means of a glass rod with 23 c.c. of pure concentrated nitric acid, and heated up to from 150° to 160°. When the matters assume a yellow or orange colour, the bath is removed from the source of heat, and 3 c.c. of pure concentrated sulphuric acid added, and the mixture stirred, when the mass becomes brown, swells up, and evolves dense nitrous and other fumes. The vessel is again heated to 180°, and while hot 8 c.c. of pure concentrated nitric acid are added, drop by drop, with continual stirring. After this addition, it is heated to 200° for fifteen minutes, and the result on cooling is a hard carbonaceous residue wholly free from nitric acid. The arsenic is in this way oxidised into arsenic acid, which is easily soluble in water. The contents of the dish are, therefore, perfectly extracted by boiling water, the aqueous extract filtered, and evaporated to dryness. The next process is to obtain the arsenic in a metallic state:—
[784] American Chem. Journ., vol. ii., No. 4; Chem. News, Jan. 1881, p. 21.