Silver

"Molecular" Silver.—Silver can be obtained in a finely divided, very reactive state as a greyish powder either by precipitation of a solution by a reducing agent, such as ferrous sulphate, or by bringing a piece of a more electropositive metal such as iron or zinc into contact with a silver halide below faintly acidified water. This active form of the metal is usually dried at a low tempera ture after thorough washing, and is frequently used in organic synthesis for removing halogen elements from alkyl compounds with duplication of the organic radical.

Colloidal silver may be produced by precipitating a solution of silver nitrate with various organic substances such as tartrates, citrates, tannin, etc. The precipitates vary greatly in colour, and after washing become soluble in pure water. A brown colloidal solution of silver may also be obtained by passing an electric arc between silver terminals under pure water.

Silver in any form acts catalytically upon solutions of hydrogen peroxide ; a steady stream of oxygen flows from the points of contact and finally complete decomposition results.

Chemically Pure Silver.—In exact determinations of the atomic weights (q.v.) of elements (halogens, etc.), it is frequently neces sary to obtain silver in the purest possible state and a great deal of work has been done to this end. For a long time the most satisfactory process was the reduction of a solution of silver nitrate by means of ammonium sulphite, but this process, due to J. S. Stas, has been superseded by that of Richards and Wells (1905), which is carried out as follows: A solution of recrystal lized silver nitrate is precipitated by hydrochloric acid and the resulting chloride thoroughly washed. This chloride is then placed in a silver dish and reduced to metallic silver with pure invert sugar and caustic soda. The reduced silver is fused on a block of lime in the reducing flame of a blowpipe, and thereby brought to 99.999% purity; this is made the anode and cathode of a cell, the electrolyte being a solution of silver nitrate prepared from some of the same purified silver. On electrolysis a crystalline powder of electrolytic silver is formed, and is thoroughly washed, dried and fused in a lime boat contained in a tube through which a current of hydrogen is circulated. The small bars of silver after

scrubbing and washing with dilute nitric acid, ammonia and water are dried in vacuo at about 400°.

Alloys.

Silver alloys readily with most metals, but in many cases the alloys are of little practical value ; thus arsenic, anti mony, bismuth, tin and zinc produce very brittle alloys which are quite unsuitable for further working. Copper on the other hand increases its hardness, toughness and fusibility, and such alloys are therefore almost exclusively used for coinage and jewellery. The proportion of silver in alloys is stated in terms of its "fineness" which means parts of silver in i,000 parts of the alloy. Thus the British coinage till 1920 was of a fineness of 925 or 92.5% silver and 7.5% copper, and was well adapted to its purpose, but the silver currency was then lowered to a fineness of 500, and other metals besides copper, especially nickel, were introduced. This alloy is much less satisfactory than that of the old standard for, apart from its unsatisfactory working properties, it is much more readily discoloured.

Silver is sometimes alloyed with gold and with the platinum metals, with which it produces alloys which can be worked satis factorily. Thus the sovereigns formerly minted in Australia were of 22-carat quality, containing 8.33% silver instead of copper, and thus having a lighter colour; and a dental alloy of 50% silver-platinum was at one time in general use.

Compounds of Silver.

Several oxides of silver have been reported from time to time. A suboxide, has been stated to exist by several observers, but their results have not been gen erally accepted. The best characterized oxide is the compound argentic oxide, which is the base of all the more stable salts of silver. It is produced by precipitating a solution of silver nitrate by an alkaline or alkaline-earth hydroxide, as a blackish powder which can be crystallized from solutions of ammonia, forming violet crystals. This oxide is appreciably soluble in water (I part in 300) giving a solution which turns red litmus blue, and which precipitates most of the heavy metals as hydroxides. This oxide is a strong base and forms stable salts with acids. It begins to decompose into its elements at 25o°.