Mercury Vaporization
To the Editor: Since 1950 we have distilled and packaged over 5,000,000 (five million) pounds of liquid mercury. We have practiced stringent safety controls in this operation, one of them being covering exposed mercury surfaces with water. Therefore, the article, "The Mercury-Water System," [52, 117 (1975)l by M. L. Sanders and R. R. Beckett was most interesting to us. I t appears to he saying that i t is not necessary to put water over mercury as the water does not "keep it from vaporizing." Our tests show that although water may not "keep it from vaporizing" it nonetheless reduces the vapor reading. Perhaps the vapor is absorbed in the water? For a number of years we have been using a Beckman Mercury Vapor Meter No. K23. We held this unit about 4 in. above ode of our bottling troughs (33 in. X 8'h in. X 3 in.) containing about 100 lh of mercury and with no water on the surface. The meter immediately showed in excess of the low scale which goes to 0.1 mg/M3. We checked a second trough of ahout the same dimensions, with about the same amount of mercury, and covered with about % in. of water. Both troughs had been in this condition overnight. The reading of the mercury trough covered with water was 0.05-0.06 mg/M3. The room showed a reading of 0.03-0.04 mg/M3. . We then repeated the test by putting 16 lhs. of clean mercury into each of two glass howls 8 in. dia. X 5% in. hieh in a room located away from the mercury bottling operation, and one which showed no vapor readings a t all. We covered the mercury in one howl with 16 oz of water.The results are as follows: No. Time Bowl (after set-up) (water covered Hg) (Exposed Hg) 1 1min. 0.084.09 mg/M3 over 0.1 mg/M3 0.20 mg/M3 1hr. 0.07-0.08 mg/M3 2 0.00 0.10-0.20 mg/M3 3 67 hr. Our conclusion, therefore, is that whatever the reason, theories, or laws involved here, we will still continue t o cover exposed bodies of liquid mercury with water as serving the hest interest of our employees. Bruce J. Lawrence Bethlehem Apparatus Company. Ine. Hellertown, Pennsylvania 18055
To the Editor: The treatment of the mercury-water system by Sanders and Beckett [J. Chem. Educ., 52, 117 (197511 is sound thermodynamics hut is misleading as safety engineering. Far from "vaporizing as if the aqueous layer was not even present," mercury will actually vaporize ahout a million times more slowly from beneath a water layer. The concentration of mercury in the moving air in ventilated work areas will thus he a million-fold less if the mercury is covered with water. The figure of one million is reached by considering two beakers partly filled with mercury. One is filled to the brim with water, the other only with air, and clear air moves over the top of the heakers. Since the diffusion coefficient of mercury in water is times that in air, it follows2
[Crank, J., "Mathematics of Diffusion," Oxford University Press, Oxford, 1956, p. 2311 that the rate of transfer to the clean air from the water-covered mercury is of that from mercury covered by still air. This ratio is still further decreased by the fact that in practice the water usually is still whereas the air usuallv is not: even a faint movement of the air will increase the transfer a t least lo2-fold, making the ratio We should not he discouraged from using a millionfold improvement by considering what would happen if the room ever reached equilihrium. An intelligent hio-organism exhaling carbon dioxide will prevent that from happening. Stephen J. Hawkes Oregon State University Corvallis, Oregon 97331
To The Editor: The comments by S. J. Hawkes and B. J. Lawrence are well intentioned hut continue to circumvent the basic orohlem involved with this particular system; that is, the prohlem of applying good enaineerine controls to hazardous situations.-fi is somewhat &alogo& to attempting to control the explosivity hazard of gasoline hy placing steel wool in the opening of a large reservoir of the liquid. Indeed, the steel wool will cut down the amount of gasoline vapor which escapes the reservoir per unit time (at least until saturation of the wool interstices) but this ignores the fact that nothing is specified about the containers' environment; therefore, we have not controlled the potential for the vapor to cause a prohlem a t a later date. I t appears to me that safety engineering is hased upon potential contingencies. Why not minimize potential at the outset? The length of time for the theoretical equilihrium to he established is not the determining factor for safety here; rather, i t is the mercury concentration in the air which exceeds the established TLV (Threshold Limit Value) for mercury inhalation: 0.05 m g / M b r 560 of the equilihrium value. Since the physiological half-life of ingested mercury is approximately 7 months and inhalation is the most efficient route for physiological uptake, it seems reasonable to limit the intake to a minimum by suhstituting objective control measures rather than relying on "a million fold improvement based upon diffusion rate" plus an estimate of "102 for air movement." From the standpoint of safety engineering this argument ignores effective surface area, container geometry, inter-facial agitation, temperature, etc., factors which must be considered if we desire to compute the total evolution of mercury per unit time-the standard method of evaluating potential exposure in the work environment. I t would indeed have been more accurate to state "vaporizing a t a slower rate when a water layer is present" although this was alluded to later in the article when it was statedthat in a closed and controlled system the limiting parameter to attainment of equilibrium ".. . is the diffusion rate of the mercury through the water." Mr. Hawkes' last statement may he true for relatively small mercury-water systems hut extrapolate a hit and place one "intelligent bioorganism exhaling carbon dioxide" (presumably human) in a closed room with a fan blowing recirculating air over the surface of a 500 ft2 tank of mercury covered with a foot of water. The limiting step in attainment of equilihrium within the room will not he the organisms' input of COz. This is obviously an extreme case hut should point out a common error in assuming health in the work environment is self-controlling. The theory brought forth in the article is sound. If doubt exists as to environmental health conditions in actuVolume 52.Number 12, December 1975 / 819
