The residue is calcined with a little carbonate of soda and cuttings of pure Swedish filtering paper, the chlorides present being reduced to the metallic state by this treatment. The residue is next taken up with water acidulated with nitric acid, and the solution filtered. An insoluble residue, that may remain, is washed with hot water until the wash-water ceases to precipitate solution of nitrate of silver, and dried. It is then dissolved in boiling nitric acid, the solution diluted with water, and filtered.[K]
Sulphuric acid is added to the filtrate: if no precipitate forms, the absence of lead, in the residue A, is indicated. If, on the contrary, a precipitate is produced, it is collected upon a filter and washed. In order to make sure that the precipitate consists of sulphate of lead, it is treated with a solution of tartrate of ammonia: it should dissolve, forming a solution in which sulphuretted hydrogen produces a black precipitate.
The fluid which has failed to be precipitated by the addition of sulphuric acid, or the filtrate separated from the precipitate formed, can contain only silver. Upon adding hydrochloric acid, this metal is thrown down as a caseous white precipitate, which is soluble in ammonia, but insoluble in boiling nitric acid, and blackens upon protracted exposure to light. The formation of a precipitate possessing these properties, leaves no doubt as to the presence of silver.
Remark.—In the operations described above, as well as in those following, the difficulty in separating minute precipitates from the filter is often experienced. When the precipitate is to be dissolved in reagents that do not affect the paper, such as ammonia, tartrate of ammonia, and dilute acids, it can be brought in solution directly on the filter. In cases, however, where reagents which attack the paper are employed, the precipitate should be separated. This is accomplished by mixing a small quantity of pure silica, obtained by the decomposition of fluoride of silicium by water, with the solution, before filtering. The precipitate becomes intimately mixed with the silica, and can then be readily removed from the paper. The presence of silica does not interfere, it being insoluble in the reagents commonly made use of.
TREATMENT OF FILTRATE B.
A current of sulphuretted hydrogen is conducted for twelve hours through the solution, which is kept at a temperature of 70°. by means of a water-bath. The flask containing the liquid is then closed with a piece of paper, and allowed to remain in a moderately warm place until the odor of the gas is no longer perceptible. The solution is next filtered with the precaution mentioned in the preceding remark, and the precipitate (a) thoroughly washed. The water used in this operation is united to the filtrate, and the fluid (b) examined as directed further on.
TREATMENT OF PRECIPITATE a.
In order to free the precipitate from the organic substances possibly present, at the same time avoiding a loss of any metal, it is dried, moistened with nitric acid, and the mass heated on a water-bath. Some Swedish filtering paper is next added, the mixture well impregnated with sulphuric acid, and then maintained for several hours at a temperature of about 170°. until a small portion (afterwards returned) gives a colorless solution when treated with water. The residue is now heated with a mixture of one part of hydrochloric acid and eight parts of water, the liquid filtered, the matter remaining undissolved washed with dilute hydrochloric acid, and the washings united with the filtrate.
The residue I. and the solution II. are separately examined as directed below.