Sunday, December 19, 2010

Mercury Hangs Around



Photo Courtesy of
The National Atmospheric Release Advisory Center (NARAC)
 at Livermore Laboratory

*****


It doesn't take much in the way of common sense to know that dense gases hang low to the ground.  We've all seen gas dispersion models like the one above, etc., and we're all grateful to be able to use them.

However, there is something I'd like you to think about as it relates to gas releases and particularly to very dense gases.  What I'd like you to think about is the rate and pattern of mixing that occurs for such gases.  This is important when evaluating environments with flammable, oxidizing gases and/or toxic gases.  It is particularly important when we're planning for hazardous response strategies when we are dealing with mercury.

In general, two gases with molecular weight values close to each other will tend to mix more quickly than two gases where the molecular weight values are far apart.  For example, in a compressed gas cylinder (all other things being equal) I have found over the years that nitrogen and carbon monoxide will mix more quickly together than carbon dioxide and nitrogen.  There are, of course some limits on this generality- including the concentrations involved and the area of mixing interface.

The molecular weight of air is 28.996.  The molecular weight of mercury is 200.59.  Kind of a large differential, so even as it enters the air it's going to mix slowly.

Next, there's the vapor pressure.  Gases with liquid/vapor phases where the liquid has a high vapor phase such as carbon dioxide (830 PSIG), will enter gas phase quickly when released.  Mercery, on the other hand, has a vapor pressure of .0017 torr (not much of a vapor pressure at all, really) at 25 degrees C.  In other words, it will enter the vapor phase very slowly.  Other things being equal, heat will increase the vapor pressure.

The quicker the gas enters the vapor phase, the more quickly it will disperse unless there are outside factors working against it such as an inversion, the release occurring in a valley or in a confined space.  A hot day, of course, will accelerate this process.

Mercury, at normal temperature and pressure, will release slowly, continually over a long period of time, so it needs to be thought of differently than most of the dense gases.  There are similarities, of course, but the gasification of mercury, I think, is a little bit different animal than most.

Refrigerant gases have high molecular weights like mercury does, but they have higher vapor pressures.

So although we're used to thinking of dense gases (liquified gases shipped under their own vapor pressure) as somewhat similar, we need to give some serious thought whenwe're calculating the gasification of mercury because of its unusual combination of extraordinarily low vapor pressure and high molecular weight. 

I truly think it needs its own dispersion chart.

Wednesday, December 1, 2010

We Used to Play with this Stuff


We Just Do it a Little Differently Now

It looks a little like overkill, doesn't it? Some of us used to play with it as kids. But Mercury (Hg) is not a good toy.  Here's what JT Baker Chemical company says about it in their MSDS:

"DANGER! CORROSIVE. CAUSES BURNS TO SKIN, EYES, AND RESPIRATORY TRACT. MAY BE FATAL IF SWALLOWED OR INHALED. HARMFUL IF ABSORBED THROUGH SKIN. AFFECTS THE KIDNEYS AND CENTRAL NERVOUS SYSTEM. MAY CAUSE ALLERGIC SKIN REACTION."


What many don't understand is that Mercury is most dangerous in its gaseous state because concentrations of it can then be inhaled.  Or as JT Baker's MSDS says:

"Mercury vapor is highly toxic via this route. Causes severe respiratory tract damage. Symptoms include sore throat, coughing, pain, tightness in chest, breathing difficulties, shortness of breath, headache, muscle weakness, anorexia, gastrointestinal disturbance, ringing in the ear, liver changes, fever, bronchitis and pneumonitis. Can be absorbed through inhalation with symptoms similar to ingestion."

But it is still a underappreciated danger.

As far back as 1926, Alfred Stock of the Kaiser-Wilhem Chemistry Institute wrote:

"The insidious horror of mercury is not nearly sufficiently well known and is being taken note of too little in those places where one is particularly threatened by it, in chemical and physical laboratories."

And this is still true today.

Many emergency responders understand the dangers of contact with liquid mercury, but do not yet understand that 1) mercury readily vaporizes, and that 2) it is a terrible danger in the gas state.  Many teams do not have readily available vaporized mercury detectors. 

Three companies that I know of  make fine mercury vapor detectors: Arizona Instruments, Lumex of Ohio, and Ion Sciences.

An article from MedicalNewsToday back in 2004 reported:

"All mercury spills, regardless of quantity, should be treated seriously. Metallic mercury slowly evaporates when exposed to the air. The air in a room can reach contamination levels just from the mercury in a broken thermometer - just a few drops.

When liquid mercury is spilled, it forms droplets that can accumulate in the tiniest of spaces and then emit vapors. Health problems caused by mercury depend on how much has entered your body, how it entered your body, how long you have been exposed to it, and how your body responds to it."



Mercury may be the densest liquid known, but it still has a vapor pressure of .2729 mmHg, which is still sufficient for it to vaporize into the air to the level that it can harm you.  Like all liquids with a vapor head, the quantity of gaesous fumes that enter the air is temperature dependent.  So the hotter the day, the faster the gasification occurs.

More on this next week.