Other methods consist in adding a few drops of hydrochloric acid or a pinch of common salt. Chloride of silver is instantly formed, which falls to the bottom, carrying impurities with it, or a small quantity of sodium carbonate can be added, and the bottle placed in strong sunlight.
When any of these methods fail, however, it can he restored by boiling " the bath. The solution is poured into an enameled iron dish placed over a lighted gas stove. If the bath is not alkaline it must be made so by the addition of ammonia solution. When thoroughly heated it will become black and turbid. When about a third of the solution has evaporated the solution is cooled and filtered. The strength of the silver is then ascertained, and distilled water added to reduce it to the required strength. Another method is to fuse the bath. The method is similar to the last one. The evaporation is continued to dryness, and the bath fused until all frothiness disappears. The heating apparatus is then removed, and the mass scooped in a lump. When sufficiently cool sufficient of a nitric acid solution i in 12 is added to re-dissolve the silver with the aid of heat. The bath is again evaporated to dryness, dissolved in pure distilled water, and fil tered water is then added until correct strength is obtained, all organic matter having been by this means thoroughly carbonized.
It will often happen that the silver solution becomes so choked up with the soluble salts, resulting from the chemical actions which occur in sensitizing, that none of the methods of purifi cation will be advantageous. In this case it is best put away and the silver recovered at a con venient opportunity. Mr. C. L. Mitchell gives the following methods of recovering the silver from the old baths:—If the bath had been made according to the first or third class, all that is necessary is to render the bath acid with nitric acid, and then add a strong solution of washing soda until the white precipitate of carbonate ceases to be formed. This is allowed to settle, the
supernatant liquid poured off, and the precipitate washed repeatedly until the washings are free from color, and all soluble salts have been removed. The precipitate is then drained on a filter and the moist mass filtered, and all is placed in a dish of porcelain or enameled iron, and dilute nitric acid (i in io of water) added, until effervescence ceases, and the white precipitate is nearly entirely dissolved. This solution is then filtered and evaporated, first to dryness, and then heated to calm fusion, and allowed to cool. The resulting semi-crystalline mass may be considered a pure silver nitrate. If the old silver bath, however, is prepared according to the ammonia-nitrate formula, this method will not answer, as in the presence of ammonia salt not all the carbonate of silver can be precipitated. The best plan, therefore, is to acidify the bath as before with nitric acid, and then add salt until all the silver is precipitated as chloride. This should be well washed, collected on a filter, and dried. It may then he converted into pure metallic silver by fusing it in a crucible with powdered charcoal and potassium nitrate, or it may be placed in a vessel contain ing dilute sulphuric acid (I in 20) and metallic zinc added. Metallic silver will be precipitated in a finely divided state, and the zinc dissolved. From this silver the nitrate can be prepared by dissolving it in dilute nitric acid, and then proceeding in the same manner as described for the silver carbonate.
It should be noted that care and cleanliness will greatly assist in preserving the silver bath in its purity as long as possible. Filter and strengthen each time after using.