Difference between revisions of "Silver"
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Revision as of 18:51, 27 February 2016
Silver is a chemical element and transitional metal, with the symbol Ag and atomic number 47. Silver has the highest electrical and thermal conductivity of any metal and is the most reflective metal.
Silver is resistant to many acids, such as halogen acids and sulfuric acid, but will slowly dissolve in nitric acid forming silver nitrate, reaction sped up by heating. It's stable in pure air and water, but tarnishes when it is exposed to air or water containing ozone or hydrogen sulfide, in case of the latter it forms a black layer of silver sulfide which can be cleaned off with dilute hydrogen peroxide or hydrochloric acid. This phenomenon can be used as an indicator of air quality. Silver can be attacked by strong oxidizers, such as potassium dichromate or potassium permanganate, in the presence of potassium bromide.
Silver fulminate (AgCNO) is a powerful touch sensitive explosive, that has little value in chemistry because it's extremely sensitive to shock, heat, static electricity. It is however used in bang snaps.
Silver halides are photosensitive and are known for their ability to record a latent image, that can later be developed chemically, process that made photography possible.
Silver is soft, white metal. It has the highest electrical conductivity of any known metal and compound (except superconductors) and highest thermal conductivity of all known metals. It melts at 961.78 °C and boils at 2162 °C. It is a dense metal (10.49 g/cm3), though slightly less dense than lead.
Silver can be obtained from certain coins and jewelries, as an alloy, so purification might be required. US quarters and dimes made before 1964 were made of a 90% silver and 10% copper alloy. Nickels made between 1942-1945 ("War nickels") contain 35% silver, the rest being copper and manganese. However it's illegal to destroy money to obtain the metal from them.
Silver is also found in many contacts from various electronic devices, such as switches, circuit breakers, relays, as a sintered alloy, usually with nickel, copper and tungsten or tungsten carbide. There is no standard mixture, sometimes other elements are also present such as cadmium, which makes extraction difficult due to its toxicity. The specific gravity varies among the contacts, some can be as low as 6 g/cm3, while other may reach 10 g/cm3. Determining the exact percentage of silver presents challenges.
Tin-silver-copper solders also contain small amounts of silver in their composition.
Various other electronics that contain appreciable amounts of silver:
- Mylar sheets from old keyboards contain silver. You can get around 4 g of silver from around 25-26 mylars.
- Coin and button batteries (found on motherboards and in small electronics) often use silver oxide and zinc to generate power electrochemically. When these batteries are fully spent, the silver oxide inside has been reduced to silver metal, which can be carefully collected and purified. Another option is simply to dissolve the contents of the battery in nitric acid and precipitate the silver by adding copper, comparatively a much more reactive metal, to a solution of silver nitrate or another soluble silver salt.
- Certain monolithic capacitors may contain silver instead of palladium
- Varistor disks tend to be coated with a small amount of silver
Note that extracting silver (and other precious metals) from electronics may or may not be legal depending on your jurisdiction. Consult your lawyer before you start doing this.
Lastly, silver can also be bought as ingots and bars, thought it's price will vary depending on the market.
Silver can be obtained by reducing it's salt with a reducing agent, such as ascorbic acid.
It can also be obtained by chemical purification of silver alloys used to make jewelry and coins. This is a process done in several steps.
- The coin, ring or another object made of a silver alloy is dissolved in nitric acid. The reaction should be done under a fume hood or outdoors, because of nitrogen dioxide.
- The solution at the end of the reaction is of a bluish-green color, because it contains a mixture of copper, silver and other metal nitrates.
- Sodium hydroxide is added into the solution. All dissolved metals precipitate in the form of hydroxides.
- The precipitate is removed from the solution and dried.
- The dried precipitate is heated to the temperature of 280-300 degrees Celsius. This temperature is enough to decompose the silver (I) oxide, but not enough to decompose the oxides of copper, nickel or whatever other metals are there. After heating, the precipitate turns into a mixture of powdered elemental silver and various oxides.
- The calcinated powder is treated with hydrochloric acid, which dissolves all oxides but does not attack elemental silver. It is the only substance left in the powder.
- The powdered pure silver is washed with water to remove the remnants of acid.
If you are sure that copper is the only contaminant, a simpler process can be used. Do not add the hydroxide, evaporate the solution and crystallize the nitrate mixture and heat it to the temperature of 280 degrees. Copper nitrate will decompose, and silver nitrate won't, and it will be available for extraction with water, recrystallization and decomposition under 500 degrees to produce elemental silver.
- Silver nitrate
- Silver fulminate
- Silver aceltylide
- Silver azide
- Electrolytic silver cell
- Silver crystal growing, caused by insertion of a well-cleaned copper object such as wire into silver nitrate solution. The end product is a plant-like growth of silver crystals in a blue solution(copper(II) nitrate).
- Anti-bacterial surfaces
Silver metal has antiseptic properties and does not react with the organism. Silver compounds are harmful, and in large doses, silver compounds or colloids will deposit in various body tissues, leading to argyria, which results in a blue-grayish pigmentation of the skin.
Silver does not require special disposal, though if stored in sulfide polluted air, it will form a dark layer. The layer can be removed with hydrogen peroxide.
Due to its high price, it's best to try to recycle the silver.