Occurrence and Uses
Silver (Ag) is found throughout the world, but most of it is produced in Mexico, the western United States, Bolivia, Peru, Canada and Australia. Much of it is obtained as a by-product from argentiferous lead, zinc and copper ores in which it occurs as the silver sulphide, argentite (Ag2S). It is also recovered during the treatment of gold ores and is an essential constituent of the gold telluride, calaverite ((AuAg)Te2).
Because pure silver is too soft for coins, ornaments, cutlery, plate and jewellery, silver is hardened by alloying with copper for all these applications. Silver is extremely resistant to acetic acid and, therefore, silver vats are used in the acetic acid, vinegar, cider and brewing industries. Silver is also used in busbars and windings of electrical plants, in silver solders, dental amalgams, high-capacity batteries, engine bearings, sterling ware and in ceramic paints. It is employed in brazing alloys and in the silvering of glass beads.
Silver finds use in the manufacture of formaldehyde, acetaldehyde and higher aldehydes by the catalytic dehydrogenation of the corresponding primary alcohols. In many installations, the catalyst consists of a shallow bed of crystalline silver of extremely high purity. An important use of silver is in the photography industry. It is the unique and instantaneous reaction of the halides of silver on exposure to light that makes the metal virtually indispensable for films, plates and photographic printing paper.
Silver nitrate (AgNO3) is used in photography, the manufacture of mirrors, silver plating, dyeing, colouring of porcelain, and etching ivory. It is an important reagent in analytical chemistry and a chemical intermediate. Silver nitrate is found in sympathetic and indelible inks. It also serves as a static inhibitor for carpets and woven materials and as a water disinfectant. For medical purposes silver nitrate has been used for the prophylaxis of ophthalmia neonatorum. It has been utilized as an antiseptic, an astringent, and in veterinary use for the treatment of wounds and local inflammations.
Silver nitrate is a powerful oxidizing agent and a fire hazard, in addition to being strongly caustic, corrosive and poisonous. In the form of dust or a solid it is dangerous to the eyes, causing burns of the conjunctiva, argyria and blindness.
Silver oxide (Ag2O) is used in the purification of drinking water, for polishing and colouring glass yellow in the glass industry, and as a catalyst. In veterinary medicine, it is used as an ointment or solution for general germicidal and parasiticidal purposes. Silver oxide is a powerful oxidizing material and a fire hazard.
Silver picrate ((O2N)3C6H2OAg·H2O) is used as a vaginal antimicrobial. In veterinary medicine it is used against granular vaginitis for cattle. It is highly explosive and poisonous.
Silver exposure may lead to a benign condition called “argyria”. If the dust of the metal or its salts is absorbed, silver is precipitated in the tissues in the metallic state and cannot be eliminated from the body in this state. Reduction to the metallic state takes place either by the action of light on the exposed parts of the skin and visible mucous membranes, or by means of hydrogen sulphide in other tissues. Silver dusts are irritants and can lead to ulceration of the skin and nasal septum.
Occupations involving the risk of argyria can be divided into two groups:
- workers who handle a compound of silver, either the nitrate, fulminate or cyanide, which, broadly speaking, giving rise to generalized argyria from inhalation and ingestion of the silver salt concerned
- workers who handle metallic silver, small particles of which accidentally penetrate the exposed skin, giving rise to local argyria by a process equivalent to tattooing.
Generalized argyria is unlikely to occur at respirable silver concentrations in air of 0.01 mg/m3 or at oral cumulative doses lower than 3.8 g. Persons affected by generalized argyria are often called “blue men” by their fellow workers. The face, forehead, neck, hands and forearms develop a dark slatey-grey colour, uniform in distribution and varying in depth depending on the degree of exposure. Pale scars up to about 6 mm across may be found on the face, hands and forearms due to the caustic effects of silver nitrate. The fingernails are a deep chocolate-brown colour. The buccal mucosa is slatey-grey or bluish in colour. Very slight pigmentation may be detected in the covered parts of the skin. The toenails may show a slight bluish discolouration. In a condition called argyrosis conjunctivae, the colour of the conjunctivae varies from a slight grey to a deep brown, the lower palpebral portion being particularly affected. The posterior border of the lower lid, the caruncle and the plica semilunaris are deeply pigmented and may be almost black. Examination by means of the slit-lamp reveals a delicate network of faint grey pigmentation in the posterior elastic lamina (Descemet’s membrane) of the cornea, known as argyrosis corneae. In cases of long duration, argyrolentis is also found.
Where persons work with metallic silver, small particles may accidentally penetrate the exposed skin surface, giving rise to small pigmented lesions by a process equivalent to tattooing. This may occur in occupations involving the filing, drilling, hammering, turning, engraving, polishing, forging, soldering and smelting of silver. The left hand of the silversmith is more affected than the right, and the pigmentation occurs at the site of injuries from instruments. Many instruments, such as engraving tools, files, chisels and drills, are sharp and pointed and are liable to produce skin wounds. The piercing saw, an instrument resembling a fret saw, may break and run into the worker’s hand. If the file slips, the worker’s hand may be injured on the silver article; this is especially the case with the prongs of forks. A worker drawing silver wire through a hole in a silver draw-plate may get splinters of silver in his or her fingers. The pigmented points vary from tiny specks to areas 2 mm or more in diameter. They may be linear or rounded and in varying shades of grey or blue. The tattoo marks remain for life and cannot be removed. The use of gloves is usually impractical.
Safety and Health Measures
In addition to the engineering measures necessary to keep the airborne concentrations of silver fumes and dust as low as possible and in any case below the exposure limits, medical precautions for preventing argyria have been recommended. These include, in particular, the periodic medical examination of the eye, because the discolouration of the Descemet’s membrane is an early sign of the disease. Biological monitoring seems to be possible via the faecal excretion of silver. There is no recognized effective treatment of argyria. The condition seems to stabilize when exposure to silver is discontinued. Some clinical improvement has been achieved by use of chelating agents and intradermal injection of sodium thiosulphate or potassium ferrocyanide. Sun exposure should be avoided to prevent further discolouration of the skin.
The main incompatibilities of silver with acetylene, ammonia, hydrogen peroxide, ethyleneimine and a number of organic acids should be kept in mind in order to prevent fire and explosion hazards.
The most unstable silver compounds, such as silver acetylide, silver ammonium compounds, silver azide, silver chlorate, silver fulminate and silver picrate, should be kept in cool, well-ventilated places, protected from shock, vibration and contamination by organic or other readily oxidizable materials and away from light.
When working silver nitrate, personal protection should include the wearing of protective clothing to avoid skin contact as well as chemical safety goggles for the protection of the eyes where spillage may occur. Respirators should be available at workplaces in which engineering control cannot maintain an acceptable environment.