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Oct 25 2010

Pique the Geek 20101024: Essential Elements: Mercury

Most people think of mercury as nothing but a toxic nuisance.  Actually, that is far from the truth.  While mercury is toxic in many situations, modern life as we know it would be essentially impossible without that element.  It is also a material known from antiquity, and has drawn the interest of learned folks since then.

Mercury is unique in that it is the only metal to be a liquid at room temperature.  Actually, it is liquid from around minus 39 degrees Celsius to around 360, so it has a almost a 400 hundred degree liquid phase.  That is also sort of odd, since many metals have much longer liquid ranges.

Let us investigate this unique material, and see how it impacts our lives.  You might be surprised.

Before we get started, I want to report a very rare find.  Many of you know that I am a numismatist, and Tuesday I found a very rare US cent, one dating from 1867.  The colloquial term is “Indian Head”, but the creator of it, James B. Longacre, called it “Liberty with a Feathered Headdress”.  This is the oldest coin that I have ever found in circulating cents.  Doing the maths, it is at least 143 years old, and still pretty clear.  The Red Book tells me that is worth around $50 or more, but is much more valuable to me just for finding it.

I first reported this Friday evening at my Popular Culture series, but Pique the Geek gets a different audience, at least in some part.  This was the most exciting coin find that I have had in quite some time.  By the way, Popular Culture next Friday will be devoted to the seminal British band, King Crimson.

There were only about nine million of them minted that year (all of them from the mint in Philadelphia), so to find one in circulation is extraordinary.  I estimate that cents have an attrition rate of over 5% annually, so finding this one is a once in a lifetime thing.  I strongly suspect that it came either from a stolen coin collection, or from an estate that, after the owner died, got cashed in using the CoinStar machines.  Anyway, I digress.

Mercury is one of the last elements that has stable isotopes in the Periodic Table, with an atomic number, aka Z, of 80.  Only thallium, with Z of 81, and lead, with a Z of 82 are the heavier nuclei that have stable isotopes.  It was long thought that bismuth, with a Z of 83 was stable, but we now know that is is radioactive, but the most stable isotope has a half life much longer than the lifetime of the universe.  That actually is not surprising, because odd number Z nuclei often are less stable than even numbered Z nuclei, after the second row in the Table.

Mercury has fascinated folks for millenia.  It looks like silver, is heavy (the density is 13.6, more than lead) and is a LIQUID!  It is also easy to smelt from its main ore, cinnabar, which is the sulfide.  Spain and Italy have huge deposits, and all that you have to do to make metallic mercury from cinnabar is to heat it in a clay retort with a little cooling on the top.  Obviously, modern techniques are better, but for centuries mercury was essentially just burnt off of the ore.

Mercury melts at -38.8 degrees C, and boils at 357 degrees.  This is a range of around 396 degrees in its liquid state.  Here is one of the reasons that Wikipedia has to be taken with a grain of salt.  It indicated in the Wikipedia that

…mercury has one of the broadest ranges of its liquid state of any metal.

This simply is not correct.  I selected three metallic elements at random:  iron, cadmium, and silver.  Their liquid ranges are 1465 degrees, 444, and 1040 degrees, respectively.  I used an unimpeachable source for these figures, The Merck Index, so unless I just randomly selected very bad examples I call Wikipedia out for that statement.

Now, that is not to say that everything in Wikipedia is wrong, but it is hardly a primary source for scientific information, although it is usually pretty good.  But back to mercury itself.

When I was a kid, before mercury poisoning killed off all of the dinosaurs, mercury was very commonly encountered.  All fever thermometers were mercury filled, many consumer household and cooking ones were, and alkaline cells had a lot of it in them.  Mercurochrome was that antiseptic of choice for most minor injuries in kids (because it did not sting), and there were even kids home chemistry books telling how you could “plate” a one cent piece with mercury from Mercurochrome to make it “silver”.  I even had a puzzle that was a blue plastic container with a maze molded in it bonded to a transparent top.  The object was to get the globules of mercury into one big one, then guide it through the maze to the center.

Contrary to popular thought, Etch-A-Sketch did not have mercury in it,  The silvery material was powdered aluminum, much less toxic than mercury.  However, aluminum powder does have some hazards associated with it, but not toxicity.

Almost all of the old “silent” light switches were mercury filled, and calomel (mercury(I) chloride) was still used in medicine and was available over the counter.  These days consumer uses of mercury are much more limited.

I have not seen a mercury fever thermometer in a store in ages, most all of them being based on solid state thermistors that change resistance with temperature and can be read with a simple microprocessor and the results sent to a digital display.  No shaking them down!  Mercury was phased out of silent switches decades ago, although it is still common in heating and air conditioning thermostats, even though those are going towards thermistors and digital control as well.

Mercury is no longer used in alkaline cells, but it took a long time to find a depolarizer that worked at well.  A depolarizer works to keep the cell putting out high current levels despite it being under load.  In the old days, cells polarized so badly that they would only put out useful amounts of current for a few seconds, than had to be disconnected and allowed to depolarize naturally before they could be used again.  By the way, that is where the term “flashlight” originated, because the early cells were so poor that you could only use it a few seconds at a time before you had to turn it off and allow them to depolarize, hence the “flash” part.

The one big consumer use of mercury these days is in fluorescent lighting.  Mercury has always been used in fluorescent lighting, but with the advent of the compact fluorescent lamp, more of it is being used.  Nothing has been found to be a suitable substitute for mercury in fluorescent lighting.  It might be useful to take a couple of minutes to explain how fluorescent lighting works.

This lighting method depends on mercury vapor carrying electricity through the lamp, from one electrode to the other.  To do this, the mercury has to be ionized (stripped of one or more electrons) because nonionized gases are electrical insulators.  The ionized vapor carries the current that excites unionized atoms to emit a very strong ultraviolet emission band at 254 nanometers.  Unfortunately, humans can not see this wavelength.  Thus, the inside of the lamp envelope is coated with a phosphor that converts the UV light to visible light, and then this light passes through the envelope.  The advantage is that compact fluorescent bulbs are on the order of around 10% efficient at converting electricity to light, where incandescents are around 2.5% efficient or less.  This is why CFLs do not get nearly as hot as incandescent ones.  CFLs also last longer then incandescents, typically 10,000 hours or more versus only about 1,000 hours.  However, starting CFLs shortens their life, so you really should leave them on if you are coming back into the room in 15 minutes or so.

There are some controversies surrounding CFLs versus incandescents, so perhaps I shall write an installment covering just that.  My power company must think that they are a good idea, because they sent me eight 18 watt (equivalent to 75 watt incandescent ones) with a 10,000 lifetime each free for the asking.

Mercury is still used in some consumer products that you might not think of right off of the top of your head.  The material thimerosol is used as an antiseptic in some materials, such as mascara of all things.  If you remember back about 20 years of so, there was a real problem with women getting eye infections because of contaminated mascara.  The thimerosol prevents bacteria from growing in the material.  Thimerosol has been blamed for the increase in autism in children because it has been used to preserve vaccines, and children get more vaccines than adults.  If you look at the statistics, this notion does not stand.  For one thing, the amount of thimerosol has been either completely eliminated or reduced to very small amounts, but autism rates continue to rise.  A more likely reason is the change in diagnostic guidelines back around 2000 or so and a greater awareness of it.  I maintain that the autism rate has not changed significantly, and that we are just much better at finding it now.

Mercury was important to the alchemists who thought that is had mystical properties.  One school of thought was that everything was composed of some combination of mercury, sulfur, and salt!  It was used heavily in medicine (and still is to a small extent in mercurial diuretics), some practitioners thinking that it had remarkable restorative powers.  Ivan the Terrible had a big mercury habit, and it is suspected that the neurological degeneration that he experienced was a direct result of that.  It was only during the mid 1920s or so that the dangers of mercury began to be recognized to a large extent.

At about that same time, engineers got the bright idea to use mercury instead of water for spinning turbines in power plants!  I am serious!  The logic was sound in that in a heat engine, the theoretical efficiency is related to the difference betwixt the temperature of the working fluid at the start of the cycle and at the end of it.  Since mercury vapor can be brought to a much higher temperature than steam at a given pressure, it was the ideal working fluid.  General Electric actually built a couple of them, but due to hazards associated with mercury the project was abandoned.  Mercury has some other advantages as well, since it does not grow bacterial films and because of its extremely high density can be handled by gravity rather than pumps in some situations.

Mercury has many other uses, most of them now confined to industrial ones, although there is a very common use:  dental fillings.  Mercury amalgams (alloys with other metals) are still widely used in dentistry, although modern substitutes are being used more.  However, none of them seem to have the combination of properties that dental amalgam has.  Multiple studies have shown that the amount of mercury absorbed from dental work, even extensive work, is not harmful to almost everyone, but it seems that a few individuals are particularly sensitive to it.  This is more of an allergy than a real toxic reaction.  The fad of having all of your dental work drilled out and replaced with substitutes was really a bad idea for most people.

Mercury amalgamates with most metals (iron is a notable exception, being inert to mercury).  Back when metallic mercury was common, people were cautioned to keep it away from jewelery because gold and silver amalgams are very soft, and rings and such would become pitted if exposed to mercury.  Because of this property, mercury is used to recover gold (illegally in many cases).  In the old days, gold panners often would use copper pans on which they would rub mercury.  The copper/mercury amalgam would catch and hold the very fine gold dust that was too fine to pan effectively.  After a day of panning, they would heat their pans over the campfire and evaporate the mercury, leaving a gold button.  Of course, they were exposing themselves to huge amounts of mercury vapor at the same time.  Perhaps this is why so many of them went nuts.

Mercury vapor is hardly ever encountered these days, since so little mercury is around.  The amount of mercury in a CFL rarely is more than 5 milligrams, and often less.  One milliliter of mercury has a mass of 13.6 grams, or 13,600 mg, enough to fill at least 2700 CFLs.  Do not panic if you break a CFL.  There is more danger cutting yourself on the glass than there is from mercury poisoning by a large margin.  Broken old fashioned fever thermometers contained hundreds of times more mercury than a CFL, and all that our mums did was sweep up the debris and put it in the trash.  No one got mercury poisoning from it.

There is one environment (other than industrial occupational exposure situations) that I can think of that could be a problem insofar as mercury vapor inhalation might be a problem:  old high school and college chemistry laboratories, especially ones with wooden floors.  Back in the day, mercury thermometers got broken all of the time, and wood, with its porous structure, can trap tiny droplets of metallic mercury, which very slowly vaporizes.  Even this is not a very serious threat, because it take not only exposure, but repeated exposures to anything except the highest concentrations of mercury vapor to do any harm.  Yes, do do excrete mercury.

The real threat from mercury to consumers is not mercury intentionally added to products, but the mercury in food, especially fish.  Top predators are the ones with the highest mercury concentrations, because they get it from many, smaller fishes that are also contaminated.  Since mercury excretion is rather slow, these large fish accumulate mercury to levels that might prove dangerous to some persons, especially pregnant women and very small children.  As a matter of fact, there are federal advisories about maximum intake of certain fish for target populations.  Basically, FDA recommends that pregnant women, women who plan to become pregnant, and small children not eat shark, tilefish, king mackerel, or swordfish at all.  Albacore tuna (aka solid white tuna) is recommended at no more than six ounces per week.  Shrimp, catfish, chunk (light) tuna, pollock, and salmon are pretty much OK, so 12 ounces per week are recommended.

This does not mention local guidelines issued by states on fish caught in their waters.  The Arkansas Game and Fish Commission publishes in its regulation book the maximum recommended intake for things like black bass in the various waters of the state, and I am sure that most other states do, too.  If you eat local fish, get the book and note where the fish were caught.

Where does the mercury in fish originate?  There are several sources, but by far the largest one is fossil fuel fired power plants.  Natural gas is the least offensive (but still can have a little), oil is worse, and coal is the biggest offender.  Coal does not have a whole lot of mercury in it, but we burn a HUGE amount of coal.  Essentially all of the mercury in the stack effluent rains back down to our rivers, lakes, and the oceans where it enters the food chain.  Microbial processes convert the inorganic mercury (relatively nontoxic) into organic mercury compounds, such as methylmercury and ethylmercury (extremely toxic).  These forms of mercury are fat soluble (inorganic mercury is not) and accumulate in tissues, especially fatty tissues, from where it is not readily excreted.  A process known as biomagnification concentrates it in the top predators.  When humans eat it, it is readily absorbed and retained.  Since the brain has an extremely high fat content, there it goes.  Perhaps thimerosol is less to blame than coal, if there a link betwixt mercury and autism.

This is just a thumbnail review about a very important element, mercury.  It has been used for both good and ill for thousands of years, and the good outweighs the ill.  Much of modern science and technology would not have been possible, or would have been much more difficult to achieve, without it.  The first high vacuua were produced with mercury vapor diffusion pumps, Michelson floated his interferometer on it do disprove the ether theory, and I personally have used many mercury containing instruments during my work as a scientist.  Do no panic because you break a CFL, trust me.

Well, you have done it again!  You have wasted many more einsteins for perfectly good photons reading this mercurial piece.  And even though FauxNewsBabe Julie Banderas remembers that cholera is pronounced “kol-er-uh” rather than “cole-air-a” when she reads me say it, I always learn much more than I could possibly hope to teach writing this series, so please keep those comments, questions, corrections, and other thoughts coming.  Remember, no science or technology issue is off topic in the comments.  (The part about Banderas is absolutely correct.  Whilst I was finishing up this piece, she had a news update about the cholera outbreak in Haiti and used that pronunciation, twice, not ten minutes ago).

Warmest regards,

Doc

Crossposted at Docudharma.com and at Dailykos.com

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