On the Reduction of Silver in the Wet Way.
Every chemist is familiar with the reduction of chloride of silver in the form of powder by means of metallic zinc in the presence of a little free acid. It is not easy to bring two such substances as the silver salt and the metal into close contact, and after the work is accomplished the removal of the excess of zinc has its difficulties. Dr. Grager suggests a modification of the old method that ought to be more generally made known. The chloride of silver is dissolved in ammonia and poured into a well-stopped bottle, and into this is introduced an excess of metallic zinc, in not too small fragments, so that any reduced metal adhering to it may be readily washed off.
The decomposition begins immediately, and is rapidly accomplished, especially if the contents of the flask be well shaken up. Three hours will suffice to reduce one-quarter of a pound of chloride of silver. It is easy to ascertain when the reduction is ended, by testing a portion of the ammoniacal solution with hydrochloric acid. As soon as no cloudiness or curdy precipitate is formed, the work may be regarded as completed.
A slight excess of ammonia is said to be favorable. The reduced silver must be washed with water until all odor of ammonia has disappeared. The pieces of zinc are removed by pouring the contents of the flask through a funnel, the opening of which is too narrow for the passage of the zinc fragments, while the reduced silver can be easily washed through. The finely divided silver can be digested in hydrochloric acid to restore it to a pure white color, and it is then ready for solution or fusion, and will be found to be perfectly pure. In dealing with large quantities it would be economical to recover a portion of the ammonia by distillation. In the same way an ammoniacal solution of nitrate of silver can also be reduced by zinc, and the silver obtained pure, even when the original solution of the nitrate contains copper—provided a small quantity of silver be kept in the bath.
It is better where copper is present not to take all of the zinc that may be requisite for the reduction of the silver. It will prove a great convenience to be spared the necessity of converting the silver into the chloride, as it is no easy task to wash out this salt on filters—and it will be found to be applicable to alloys which do not contain more than 25 per cent. of silver.—From Prof. Joy in the Journal of Applied Chemistry.