The chemical formula for silver nitrate (known to chemists as argentic nitrate) is AgNO3. It is one of the most important salts of silver. It is readily made by dissolving the metal in moderate dilute nitric acid, and concentrating the solution, when it separates out in anhydrous tables belonging to the triclinic system. It dissolves in its own weight of cold water, forming a neutral solution, which is partly reduced by the action of hydrogen with the production of metallic silver to silver nitrite. It is soluble in alcohol and ether. It melts at about 224° F. It rapidly attacks and destroys organic matter, and acts as a violent corrosive poison. It stains the skin, hair, etc, black. The salt blackens when exposed to light, more especially if organic matter of any kind be present. (To prevent this decomposition, the bottles containing the crystals should be covered with paper, as previously directed.)
Pure silver nitrate may be prepared from the metal alloyed with copper - such, for instance, as common silver ornaments not made of pure silver. The alloy is dissolved in nitric acid, the solution is evaporated to dryness, and the mixed nitrates are cautiously heated to fusion. A small portion of the melted mass is removed from time to time for examination. It is dissolved in water, filtered, and ammonia is added to it in excess. While any copper salt remains undecomposed, the liquid will remain blue, but when that no longer happens, the nitrate may be suffered to cool, dissolved in water, and filtered from the insoluble black oxide of copper.
Silver nitrate is sometimes adulterated with potash nitrate, and occasionally contains traces of copper and lead. When precipitated by a slight excess of hydrochloric acid, the filtered solution ought to leave no fixed residue, when evaporated on platinum foil, as the whole of the silver would be thrown down, and any impurity would remain in solution. Copper is detected by adding ammonia in excess to the solution, when it will give the liquid a blue tinge.
AgNO3 (atomic weight equal 170; e. g. atomic weight of Ag is 108; atomic weight of N is 14; atomic weight of 0, is 48). Silver nitrate can be made from scraps of silver articles of jewelry, for although silver ornaments are made of silver alloyed with another metal - usually copper - yet the silver nitrate can be obtained pure by the processes described below. Since, however, the heat which is necessary to decompose the copper nitrate often produces silver nitrite, it is advisable that silver nearly free from copper should be chosen in preference.
Ordinary commercial silver nitrate is obtained as a by-product in the operations of parting gold and silver, which are carried on in the refineries, and also in many assay processes. The necessary careful attention for the production of a pure nitrate is not always bestowed on it. Too frequently the crystals are sent out simply dried off from the nitric acid; and in other cases, owing to the addition of charcoal to the vessel containing the silver solution, not unfrequently particles of charcoal, oxidised by nitric acid into a body which shows great affinity for the silver, are also present. Consequently, commercial silver nitrate should be purified by recrystallisation. Perhaps the best article to use would be a silver coin.
Put a piece of silver into a thin glass beaker or retort, pour on it some pure nitric acid, and support the vessel over a spirit or gas flame. If a beaker is used, an iron wire triangle high enough to reach over the top of the spirit lamp can be used, on which place a piece of iron wire gauze, and then stand the beaker containing the silver and acid on top. (A spirit lamp can be bought for 1s., the triangle for 4d., and the wire gauze for 3d., and can be obtained from or through any dispensing chemist.) If a retort is used, a ring stand should be employed to hold it. Continue heating the acid until the coin is dissolved, and crystals appear. Take these crystals out of the vessel, put them into a saucer, and pour on them a little dilute nitric acid, and then redissolve them in water, and crystallise by evaporating this aqueous liquid. A porcelain evaporating dish is the best article to employ for this purpose, or it may be done in a saucer placed on a shovel held over a fire. If the nitrate is not wanted in crystals, it can be obtained thus: After having dissolved the coin in the nitric acid, boil this impure acid solution until a dry mass is obtained, without becoming crystalline, for which purpose the beaker containing this acid and dissolved silver can be stood in a saucepan containing water, and the water allowed to boil, care being taken that it does not boil over into the beaker.
Put this dry mass into a crucible (bought for a few pence) and fuse it pretty strongly. This fusing can be performed in this manner: Make a good, clear coke fire, and stand the crucible in an iron spoon or ladle in the fierce part of the fire, until the fusion is complete. Take out a portion of the fused mass from time to time, dissolve it in water, then filter the solution thus obtained, to free it from copper oxide, and to the filtered solution add a little ammonia. If a blue colour is produced, the fused mass contains copper nitrate still incompletely decomposed. In such case coctinue the fusion until a portion taken out and treated as just mentioned ceases to give a blue coloration to ammonia. When this occurs, dissolve the fused mass in water, when it is ready for use, or can be recrystallised as already-described.
Another process for obtaining silver nitrate from an alloy of copper and silver is to dissolve the alloy in nitric acid and suspend a piece of clean metallic copper until the silver is wholly precipitated. Take out the copper, and wash the silver in plenty of water. This washing is performed by adding water, stirring the precipitated silver salt, and pouring the solution into a filter; then keep adding water to the mass in the filter. Collect the precipitate off the filter paper, and add to it a little solution of silver nitrate to remove any adhering particles of copper. Finally add some nitric acid to the precipitate, and crystallise by evaporation. A third process is to precipitate the silver from the coin by immersion of a piece of copper in the acid solution (as just directed in last experiment); wash the precipitated silver in nitric acid (being careful not to add an excess of acid; it excess has been added, evaporate carefully to dryness to expel it); then to neutralise and remove traces of the copper salt, add silver oxide to the boiling solution (of precipitated silver and nitric acid), and filter it. When the filtrate ceases to give a blue coloration to ammonia, the nitrate may be looked upon as pure.
This solution, if of the right strength, might be used at once for photographic purposes.
The purity of silver nitrate may be easily ascertained by dissolving a portion in distilled water, and precipitating the solution entirely with pure hydrochloric acid. The liquid filtered from the precipitate should leave no residue on evaporation to dryness. Taylor, an authority in photographic chemistry, says: "Nitrate of potash and nitrate of copper would have little effect, except in reducing the strengths of the bath. The peculiar photographic action of bad nitrate of silver is probably to be referred to a different source, viz. to the presence of oxidised organic matter. In the assay process" (of obtaining this silver salt) "fragments of charcoal are introduced to prevent the acid from bumping in the vessel as it dissolves the silver. We have good reason for believing that during this process the nitric acid oxidises the charcoal into a substance which has an affinity for the silver salt." And Taylor has found that silver nitrate so produced is altogether unfit for collodion photography. "The fact is certain, that silver nitrate prepared by dissolving silver in nitric acid and evaporating to dryness without any crystallisation cannot be depended on for photography." In this case recrystallisation of such salt should be resorted to for obtaining the nitrate in a fit and pure state.
A saturated solution of the purified crystals slowly restores the blue colour of reddened litmus paper if the nitric acid be expelled by heating to 240° F. previous to the second recrystallisation. This proceeding, however, is not actually necessary, inasmuch as a trace of adhering nitric acid can always be removed by soda carbonate when making the bath. The action of light upon silver nitrate is very peculiar. If pure, it may be kept unchanged in the crystallised form, or in solution in distilled water; but if the solution contain vegetable or animal (organic) matter, the nitrate is decomposed and blackens. The stains upon the skin produced by handling silver nitrate are produced in this way, and are seen most evidently when the part has been exposed to light. The varieties of organic matter which especially facilitate the blackening of silver nitrate are such as tend to absorb oxygen. - (H.C. S.)