Thursday, November 25, 2004
This is an archived issue and therefore not the current one. For the current issue, click here.
Silver When Old
In a nutshell, silver is a great conductor and has been important to society for thousands of years. It is mined around the world and is part of many significant natural molecules. That's enough of an intro, so let's go right into it!
Rule Number One
An important rule involving silver, and I can't stress this enough, is don't eat it! I don't see why you would, but there are a few who need the warning. Silver itself does not cause much of a problem, but it does while paired with other ions. One such condition you can get is called argyria. It is not fatal, but you're stuck with greyish spots on your skin for the rest of your life! This is where silver ions are permanently embedded in the skin, and sun exposure makes them this slate color.
Colloidal silver (extremely tiny silver ion fragments floating in water, in this case) was used for medicinal purposes early last century, claiming it could cure a huge number of diseases and kill bacteria, but it is no longer permitted for this purpose. There are those who still recommend it for curing these diseases, as it is still available as an "all-natural" dietary supplement, and you can find two sides of the issue of colloidal silver's risks and benefits.
From what I've gathered after some light investigation, however, it is wise to avoid taking colloidal silver like this. While really the only risk involved seems to be argyria, which the only symptom of it is permanent blue-grey skin, it is still not a fun thing to have, I imagine. Think of the social effects! Also, studies have shown there is little or no difference between these colloidal silver products and ordinary tap water, as unfiltered tap water contains trace amounts of silver anyway. Furthermore, any arguments I've seen in favor of medicinal colloidal silver are either based on very outdated information and anecdotes or are horrendously fallacious. Don't get me wrong. I'm not saying all dietary supplements are necessarily harmful or even ineffective. I just find it hard to believe that something that is actually very inexpensive (to make, that is, despite what stores may charge for it!) and is supposedly a miracle worker against cancer or AIDS would not already be prevalent throughout the medical world. If it truly lived up to this, despite risks of argyria, I'm sure reputable pharmaceutical companies would be scrambling for it, and would have knocked out the risk of argyria somehow long ago. Besides, even FDA approved drugs are often found to have previously unseen (or at least unmentioned) risks. How much would to trust a drug it has rejected? But I suppose that's just my two cents on that one. I could be wrong. Judge for yourself. But, by all means, always consult a good doctor on this sort of thing first!
In nature, there is no significant amount of silver in the human body.
Argentum and Friends
Silver can't do a lot by itself. It needs to be paired up with other ions for its many natural activities.
Silver sulfide (Ag2S) is the culprit when silver gets tarnished. Hydrogen sulfide molecules in the air (or any airborne sulfides) react with the silver bowl, leaving the residue of silver sulfide you see when it's time to polish the bowl. Eggs contain a lot of sulfur, and they can make this process occur really fast! A form called black silver sulfide is useful for separating silver ions from other positive ions because of being very insoluble in aqueous solutions.
H2S + Ag2(s) ----> Ag2(II)S + H2
So when the silver bowl gets tarnished, it is time to polish it to make it all shiny again. Do you know how the polish does away with the tarnish? Normally, the silver would be unable to take an anion from hydrogen, but this is an apparent exception. What happens when the bowl is tarnished is the silver is oxidized (loses electrons) and, to fill the lack, takes electrons from the airborne sulfides, such as hydrogen sulfide. This produces silver sulfide, which is nice and neutral, but now the silver bowl is ugly. So you need to give silver a different electron donor, but first coax the sulfide away. An environmentally sound method is to line a glass pan with aluminum foil, put in some boiling water, and add a teaspoon of salt (NaCl) and teaspoon of baking soda (NaHCO3). Put in tarnished silver items, but be sure they are touching the aluminum. What's this! The tarnish is disappearing! Of course. The aluminum is drawing the sulfide off the silver, but needed the NaCl and NaHCO3 as electrolytes to help the process along. Hooray! The silver is shiny again! For now.
Silver chloride (AgCl) is a cubic crystalline solid that is almost insoluble in water but is soluble in ammonia, potassium cyanide, and sodium thiosulfate. It occurs naturally as cerargyrite. When exposed to light, it turns from white to a deep greyish blue, a quality that makes it very useful in photography. It is also used for silver plating. This reaction to light exposure is due to decomposition into metallic silver and atomic chloride.
Silver iodide (AgI) is useful in seeding clouds, a meteorological method of coaxing precipitation. Its natural form is iodyrite. Silver iodide is even less water soluble than silver chloride and is more sensitive to light (both qualities are also true of silver bromide). It forms yellow hexagonal and yellow-orange cubic crystals.
Silver bromide (AgBr) occurs naturally as bromyrite. It forms yellow cubic crystals.
Silver nitrate (AgNO3), also called lunar caustic, is important in photography. Dissolving silver in nitric acid and evaporating it can form AgNO3 crystals. A very weak solution is used as antiseptic eyedrops for newborn babies to help prevent them from contracting gonorrhea or chlamydia from their mothers. It is poisonous and corrosive.
Silver fulminate (Ag2C2N2O2) is a powerful explosive that can be formed during the silvering process. It is related to cyanide, as you can tell from its formula.
Silver fluoride (AgF) is the most water soluble silver halide.
Sterling silver is an alloy commonly used for jewelry and silverware. It contains about 92.5% silver and the rest is copper and/or another metal. Other crude silver mixtures are used for coins and other items.
Magnificent Maestro Argento!
Maestro. Conductor. Haha. Get it? Ugh. Anyway, did I mention silver is an excellent conductor? Well, it is. It's even better than copper, which is most commonly used for this purpose. But it wouldn't be a good idea to use it, since it is expensive and rare. Even so, it has the highest electrical and thermal conductivity of all metals! It has the lowest contact resistance and is stable in pure air and water. Silver is a poor reflector of ultraviolet light but the very best of visible light, despite being easily tarnished and thus losing this great feature. Silver is also very ductile (can be shaped into wire) and malleable (can be beaten or rolled out flat), and, when pure, has a white metallic luster. It is even harder than gold, although both gold and palladium are more ductile and malleable.
Melting point: 961.93ºC
Boiling point: 2212ºC
Atomic Weight: 107.87g
Specific Gravity: 10.50
Solid Density: 10490 kg/m3
Molar Volume: 10.27 cm3
Velocity of Sound: 2600 m/s
Mineral Hardness: 2.5
Electrical Sensitivity: 1.6 x 10-8 Ω m
Reflectivity: 97%
Thermal Conductivity: 430 W/(m K)
Coefficient of Linear Thermal Expansion: 18.9 x 106 K-1
Enthalpy of Fusion: 11.3 kJ/mol
Enthalpy of Vaporization: 255 kJ/mol
Enthalpy of Atomization: 285 kJ/mol
Silver When New
Silver occurs in 0.007 parts per billion (ppb) by atoms in the entire universe. One ppb is in the sun. Crustal rocks and meteorites each contain 20 ppb of silver. There are only 0.0057 ppb in sea water and 0.003 ppb in streams.
Humans have mined and extracted silver for at least five thousand years. Today, the big silver producers in the western hemisphere are Mexico, Canada, the United States, and Peru. In ancient times, silver was obtained from mines in present-day Turkey and Aegean Sea islands. Sometimes, the silver is completely by itself but is very often in ores paired with other ions. Argentite (Ag2S) and Horn Silver (AgCl) are common natural silver ores. Other important ores feature silver paired with lead, lead-zinc, copper, gold, and copper-nickel. Silver is yielded when electrolytically refining copper.
And to All a Good Night!
Now that you know all about our 47th element, or at least have gotten a taste for it, you can enjoy your festive bowls and ornaments on another level. Fear the tarnish no more! So put up your tree, bake cookies, watch your favorite Christmas special, and, most of all, have a very Merry Christmas!
Sources
- WebElements - http://www.webelements.com
- Wikipedia - http://www.wikipedia.org
- American Chemical Society - http://www.chemistry.org
- Moore, J.W., Stanitski, C.L., Wood, J.L., Kotz, J.C., Joesten, M.D. 1998. "The Chemical World: Concepts and Applications". Second Edition. Saunders College Publishing.
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