When the liquid begins to acquire a strong and persistent odor of sulphuretted hydrogen, the precipitation of sulphuret is completed. The black mass must therefore be collected upon a filter, and washed by pouring water over it, until the liquid which runs through gives little or no precipitation with a drop of nitrate of silver.
The silver may also be separated in the form of sulphuret from old hypo baths, by adding oil of vitriol in quantity sufficient to decompose the hyposulphite of soda; and burning off the free sulphur from the brown deposit.
Conversion of Sulphuret of Silver into Metallic Silver.—The black sulphuret of silver may be reduced to the state of metal by roasting and subsequent fusion with carbonate of soda; but it is more convenient, in operating on a small scale, to proceed in the following manner:—first convert the sulphuret into nitrate of silver, by boiling with nitric acid diluted with two parts of water; when all evolution of red fumes has ceased, the liquid may be diluted, allowed to cool, and filtered from the insoluble portion, which consists principally of sulphur, but also contains a mixture of chloride and sulphuret of silver, unless the nitric acid employed was free from chlorine; this precipitate may be heated in order to volatilize the sulphur, and then digested with hyposulphite of soda, or added to the hypo bath.
The solution of nitrate of silver obtained by dissolving sulphuret of silver is always strongly acid with nitric acid, and also contains sulphate of silver. It may be crystallized by evaporation; but unless the quantity of material operated on is large, it will be better to precipitate the silver in the form of chloride, by adding common salt, as already recommended.
On the Use of Test Papers.
ON THE USE OF TEST PAPERS.
The nature of the coloring matter which is employed in the preparation of litmus-paper has already been described at [page 98].
In testing for the alkalies and basic oxides generally, the blue litmus-paper which has been reddened by an acid may be used, or, in place of it, the turmeric paper. Turmeric is a yellow vegetable substance which possesses the property of becoming brown when treated with an alkali; it is however decidedly less sensitive than the reddened litmus, and is scarcely affected by the weaker bases, such as oxide of silver.
In using test papers observe the following precautions:—they should be kept in a dark place, and protected from the action of the air, or they soon become purple from carbonic acid, always present in the atmosphere in small quantity. By immersion in water containing about one drop of liquor potassæ in four ounces, the blue color is restored.