Introduction to Silver
By Anup Pokhrel
Silver is a member of Group IB of the periodic table, which includes copper, silver, and gold. In its chemical reactions, silver resembles copper in some respects and gold in others. The principal oxidation state of silver is +1, and its stable ion in natural aqueous environments is Ag+. The naturally occurring isotopes of the element are Ag107 and Ag109. Silver occurs in the native state, as a constituent of various natural alloys and in a great variety of minerals combined with sulphur, antimony, arsenic, tellurium, and selenium. The element also forms a number of halides and a basic sulphate in nature. The principal silver minerals are native silver, argentite, acanthite, argentian tetrahedrite and tennantite, proustite, pyrargyrite, chlorargyrite, and argentojarosite.
The information about silver is given below:
|Group||11||Melting point||961.78°C, 1763.2°F, 1234.93 K|
|Period||5||Boiling point||2162°C, 3924°F, 2435 K|
|Block||d||Density (g cm−3)||10.5|
|Atomic number||47||Relative atomic mass||107.868|
|State at 20°C||Solid||Key isotopes||107Ag|
|Electron configuration||[Kr] 4d105s1|
Ores of silver
Horn silver → AgCl
Argentine or silver glance → Ag2S
Ruby Silver → Ag2SbS3
Extraction of silver
It is extracted from argentine (Ag2S).
- Concentration: at first, the ore is crushed into fine pieces and then powdered into ball mills. The concentrated ore is then set for froth flotation method.
- Cyanide formation: the concentrated ore is treated with (0.4 –7)% aqueous sodium cyanide in the presence of air. The reaction is reversible and to make it irreversible Na2S is consumed with the help of air blown.
- Precipitation: the solution obtained above is filtered to remove any insoluble impurities and silver is precipitated by using scrap zinc to the solution.
The obtained silver is filtered, washed, dried and heated strongly with borax powder to KNO3 to get a compact bright mass of silver.
- Refining or purification: purification is done by electrolysis method in which impure silver is taken as the anode and pure silver as the cathode in acidified solution of AgNO3. When current is passed through it, anode dissolves out and the equivalent amount of pure silver is deposited at cathode leaving impurities behind.
Ag – e– → Ag+
Ag+ + e– → Ag
- It is white metal.
- It is the good conductor of heat and electricity.
- It is malleable and ductile.
- Action with air: pure silver does not react with air but when traces of H2S are present, it turns black.
- Action with non–metal:
- The principal uses of silver center on its high electrical and thermal conductivity, high corrosion resistance, intrinsic metallic luster, and the photosensitivity of its halides.
- About 40 percent of the annual world consumption of silver goes into the minting of coins.
- Use of the metal for silverware, ornaments, and jewelry consumes about 30 percent.
- The photographic industry uses another 15 percent in the form of halides, particularly the bromide.
- The remaining 15 percent is used for a variety of purposes. These include electrical conductors and contact points, printed electrical circuits, silver brazing alloys, certain bearing alloys, as a backing for mirrors, and dental alloys.
- The metal and its compounds are also used as an industrial catalyst in various pharmaceutical preparations and as a germicide.
- The uses of silver are increasing annually, particularly in the electronics and photographic industry and in the manufacture of specialized types of seizure resistant bearings or jet and rocket engines and diesel locomotives.
Silver nitrate (AgNO3) or lunar caustic
- When silver is treated with diluted HNO3, aqueous silver nitrate is obtained which on heating up to crystallization point, crystals of lunar caustic can be obtained.
- It is the colorless crystalline solid.
- It is soluble in water.
- It forms stain on the skin due to decomposition of silver and also used for making tattoos.
- The action of heat: it gives silver nitrate on heating and on further heating it forms metallic silver.
- Action with ammonium hydroxide (NH4OH): ammonium hydroxide with silver nitrate forms brown ppt. of silver oxide which dissolves in excess ammonia to form complex diamine silver nitrate.
- Test for halides and sulphides: It help to detect halide and sulphide with the help of color of the ppt.
- Displacement reaction: silver nitrate shows displacement reaction.
- Used as the laboratory reagent.
- Used for silvering mirror.
- Used for making silver halide, which is used in photography.
Silver chloride (AgCl) or Horn silver
- When the aqueous solution of soluble chloride is added to aqueous solution of silver nitrate, silver chloride is obtained.
- It is white amorphous solid.
- It is insoluble in water.
- Action with ammonia: it reacts with ammonia to form diamine silver chloride complex.
- Reaction with potassium cyanide (KCN): silver chloride reacts with potassium cyanide to form colorless complex potassium Argento cyanide.
- Reaction with sodium thiophosphate (Na2S2O3): sodium nitrate reacts with sodium thio-sulphate to form sodium Argento thio-sulphate.
- Action with light: it decomposes on exposure to light.
- It is used to make photographic film and paper.