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Tallying a Mercury Spill

Dateline: 06/14/99

By Alan Bruzel

Mercury is a toxic metal, and although its vapor pressure is low at room temperature, a mercury spill can produce enough gaseous mercury to pose a significant health risk.

Evaporation is a surface phenomenon. Evaporation occurs when a few atoms on the surface of a drop of mercury attain enough kinetic energy to escape the attractive forces of their liquid surroundings. These newly admitted members of the gaseous state of matter float away, ready to be inhaled into nearby lungs.

Just how much of a drop of spilled mercury will vaporize and enter the body depends upon tissue absorption and ambient temperature; to name only two of the factors involved. Studies have shown that the lungs absorb and deliver to the bloodstream eighty percent of mercury vapor inhaled, with only seven percent of this absorbed dose being eventually exhaled. The remaining mercury that doesn't exit the body by way of the urine or feces (or in growing hair) comprises a body burden that threatens the neurological system.

Even at room temperature (20 ^oC, 68 ^oF), enough mercury can evaporate from a spill to exceed the 0.1 milligram per cubic meter (mg/m³) workplace exposure limit set by the US Occupational Safety and Health Administration (OSHA).

Our imaginary mercury spill scenario will use the table below, which also compares the relatively non-volatile mercury to some common laboratory chemicals, to calculate how much mercury vapor (at equilibrium) results from a spill. (Note the high vapor pressure of the highly toxic organic mercury compound dimethylmercury.)

Substitute into the Ideal Gas Law, PV = nRT, the required values. Because the Universal Gas Constant, R, used here is 0.08206 L atm / mol K, conversions to the appropriate units are necessary. Kilopascals (kPa) must be converted to atmospheres, and Celsius temperature (^oC) must be converted to kelvin (K).

1.) Pressure, P, equals the vapor pressure of mercury, 0.00016 kPa. Because one atmosphere (atm) is 101.325 kPa, P is 1.6 X 10^-6 atm.

2.) Let's choose one liter for volume, V.

3.) Temperature, T, is 20 ^oC or 293.16 K.

4.) Solving for moles, n, yields 6.6 X 10^-8 moles.

5.) There are 200.59 grams of mercury in one mole of mercury. Therefore, 6.6 X 10^-8 moles constitute 1.3 X 10^-5 grams. In other words, 13 micrograms of mercury vapor will saturate one liter of air at 20 ^oC.

6.) Because one liter is 0.001 cubic meter, our mercury spill produced 13,000 micrograms per cubic meter, or 13 mg/m³ – far in excess of the 0.1 mg/m³ limit imposed by OSHA.

Fortunately, most mercury spills are small and do not reach a state of equilibrium with the air. Nevertheless, good safety precautions encourage the addition of sulfur to the mercury spill (to yield the less volatile sulfide), aspiration of the material, and proper disposal.

Vapor pressures at 20 ^oC (dimethylmercury at 23.7 ^oC)

Recommended Web resources for additional information:

Equilibrium Vapor Pressure of Mercury from Dental Amalgam In Vitro
Abstract from the May 1996 issue of Dental Materials.

Evaluation of Risks Associated with Mercury Vapor from Dental Amalgam
One of the appendices from the US Public Health Service’s January 1993, Dental Amalgam report.

Hazard Information Bulletin for: Dimethylmercury
Case report of the death of a chemistry professor. From OSHA.

Measuring Vapor Pressure of Mercury and Mercury Amalgams
From How Things Work, Louis A. Bloomfield, University of Virginia.

Mercury Analysis Products
Instrumentation for trace level analysis of mercury. From Tekran Inc.

Mercury Study Report to Congress
Executive summary with background of environmental problems posed by mercury and its compounds. From the US Environmental Protection Agency.

More on Working with Dimethylmercury
Alerting users of the high volatility of dimethylmercury. From Chemical & Engineering News.

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