Physical Properties of Silver
|Silver is a white, lustrous metal, and in very thin layers has a violet colour by transmitted light; in thicker layers the colour is purple. Such layers are produced by depositing a silver mirror on glass by the action of sodium potassium tartrate on an ammoniacal solution of silver nitrate. Reduced silver in the form of a fine powder has a grey, earth-like appearance. The metal crystallizes in octahedra belonging to the cubic system. The density of the unrolled metal is 10.4923; that of the metal after rolling, 10.5034. Its melting-point is given as 958.3° C., 960° C., 960.5° C., 960.9° C., 961° C., 961.5° C., and 962° C.; and its boiling-point as 1955° C., and 2040° C. at 760 mm. With the exception of gold, it is the most malleable and ductile of the metals, and can be hammered into leaves 0.0025 mm. thick. Its conductivity for heat and electricity is higher than that of any other substance. Its specific heat is given as 0.05535, 0.0557, and 0.05608, the corresponding values for the atomic heat being 5.97, 6.00, and 6.04. The mean atomic heat from -75° to -183° C. is 5.31. Its hardness on Mohs's scale is 2.7. |
In the state of vapour, silver has a pale-blue colour, and when molten it is luminescent. Liquid silver absorbs oxygen, the gas being evolved as the metal solidifies. The eruption of the gas through the solid exterior crust gives the mass a characteristic appearance, the phenomenon being termed "spitting." At 760 mm. 1 volume of silver dissolves 0.54 vols, of oxygen at 923° C., 20.28 vols, at 973° C., 19.53 vols, at 1024° C., 18.42 vols, at 1075° C., and 17.56 vols, at 1125° C. Solid silver also dissolves oxygen, but hydrogen and nitrogen are insoluble in either solid or molten silver.
Several investigators have described methods for the preparation of pure silver. In the process adopted by Richards, silver nitrate was purified by repeated crystallization, and converted into silver chloride, which was reduced to metallic silver by the action of invert-sugar in presence of sodium hydroxide. After fusion on a layer of pure charcoal or lime, the silver was purified electrolytically, being dissolved at the anode and deposited in crystalline form at the cathode. The elimination of silver nitrate and gases occluded by the metal was effected by fusion in a boat of pure lime, the last traces of oxygen being removed by the action of hydrogen. After this purification, the metal contained not more than 0.0004 per cent, of occluded hydrogen.
Native silver and the variety obtained by decomposing the sulphide with hydrogen are amorphous. Rolled silver and the violet form produced by reduction of the chloride with sugar are also amorphous, but can be rendered crystalline by the action of heat. Treatment with hydrochloric acid of a silver-zinc alloy containing 10 per cent, of silver leaves the silver in the form of a greyish-white sponge. The chemical activity of amorphous silver is much greater than that of the crystalline variety, an example being afforded by the spongy deposit of the metal formed during the electrolysis of silver nitrate under certain conditions.
The most unstable form of the amorphous variety is colloidal silver, prepared by reduction of an ammoniacal or dilute alcoholic solution of silver nitrate with sodium citrate, sodium potassium tartrate, dextrin, tannin, or formaldehyde, with subsequent addition of ferrous sulphate. If forms a chocolate, pale-lilac, blue, or green mass, and dissolves in water to a deep-red solution. The colour depends partly on the mode of preparation, and partly on the age of the specimen. Reduction of the monoxide by 60 per cent, formaldehyde at 35° C. yields a very stable solution of colloidal silver, varying in colour from pale-lilac to rich ruby-red. The effect is not produced by employing acetaldehyde. In presence of Irish moss, hydrazine hydrate yields colloidal silver varying in colour from dark-reddish brown to brownish yellow in transmitted light. The more dilute solutions thus prepared are very stable, and can be kept for two months. By this means it has been found practicable to prepare solutions containing 17 per cent, of colloidal silver, but such concentrated forms lack stability. The colloidal solution decomposes hydrogen peroxide slowly.
Colloidal silver can also be prepared by forming an electric arc between silver poles immersed in water, the solutions being brown with a low current, and green with a stronger current. The electric conductivity of the solution produced by the second method is higher than that of the brown solution. Addition of an electrolyte also converts the brown solution into the green form. The conductivity of such solutions has been attributed to the presence of silver oxide.
When silver is boiled with water for a prolonged period, a colloidal solution is formed. It contains 0.0162 gram of silver per 100 c.c. Another procedure involves heating the metal to redness or a higher temperature and plunging it into cold water. Many other methods of preparing colloidal silver have been described.
Plates of copper or zinc precipitate colloidal silver from solution. The solid forms are brittle, and amalgamate with mercury. Acids convert them into grey silver, without evolution of gas.
A therapeutic preparation of colloidal silver is known as "collargol." Crede's ointment also contains this form of silver, and is employed in the treatment of certain types of septic infection. An astringent antiseptic is prepared by the action of an alkaline tannin solution on aqueous solutions of silver salts.
In photography colloidal silver plays an important part, an instance being the image produced in the ordinary printing-out process.