VISUAL DEMONSTRATION of the EVAPORATION of MERCURY* WESLEY G. LEIGHTON
AND
PHILIP A. LEIGHTON
M
ERCURY vapor is quite transparent to visible Figure 1 shows the relative intensities at various light and also to the near ultraviolet, with the wave-lengths available from an argon-mercury quartz ex$eption of its strikmg absorption at h 2537 lamp as determined by one of us with a quartz monoand 1849 A., which are the wave-lengtbs of mercury chromator and thermopile-galvanometersystem. The resonance. According to R. W. Wood1 a beam of X values given represent the average for three lamps, and 2537 A. is reduced 50% in intensity in passing through a are corrected for the variations in image length and 5-mm. layer of mercury vapor at room temperature change in transmission of the monochromator with ( p = ca. 0.001 mm.). Wood has also described cer- wave-length.a These intensities are in accord with tain experiments which demonstrate this strong ab- similar data given in the literature.* sorption in a spectacular manner. Thus, a quartz It is evident from Figure 1 that a large proportion &ask, evacuated except for mercury vapor at room (8879 of the total radiation from the lamp lies in the temperature, was photographed with mercury reso%.O% nance radiation, with the result that the flask appeared 90 to be perfectly opaque. Again mercury resonance radiation was allowed to fall on a barium platinocyanide screen, which fluoresced visibly. When air was bubbled 80 through mercury at room temperature in an open bottle placed between the source and the screen, the vapor 70 was observed to cast dense shadows on the fluorescent background. i60 u ." The latter experiment, with a convenient sourc; of B mercury resonance radiation, affords an impressive 2 50 w demonstration. The source used by Wood (106. cit.) consisted of a quartz bulb filled with mercury vapor and .-:40 placed in the exit beam from a quartz monochromator set a t X 2537 A. The vapor, excited by absorption 2 .. -, 30 of the core of the narrow band at X 2537, re-emitted practically pure resonance radiation, which was used in 20 the above-described experiments. This means of obtaining the resonance radiation is 10 not sufficiently convenient for the average laboratory, nor does it give a high intensity. Fortunately, a cono m . I . . venient and intense source of mercury resonance radia5790 5461 43584Od7 3663 3132 2537 4078 3650 31281 tion is obtainable in the argon-mercury quartz lamp, 3021 various modifications of which have been on the market 2967 Wave-length, A. for several years. This type of lamp operates on a FIGURE l . - R E ~ A n v z INTENSrTmS On' Tm3 PRINCIPAL small transformer plugged into the 110-volt A.C. lightLINESEMITTEDBY THE MERCURY-ARGON LAMP ing circuit, consumes less energy than an ordinary light The resonance line at 1849 A. is $so emitted, but is abglobe (about l/lOth the energy of an ordinary quartz . sorbed by air. with production of ozone. mercury arc), and is both steady and long lived.2 07
-
* Contribution from the Chemical Laboratcries of Stanford University and Claremont Colleges. 'WOOD,PhiL Mag., 161, 23, 689 (1912); "Physical optics," 1933, pp. 590-1. 1. Chem. Physics, 2, No. 7, 377 (1934), in describing TAYLOR, the properties of the rare-gas-mercury-vapor discharge, states that the lamp was found constant and reproducible over many hours, some diminution resulting after several hundred hours, due to the well-known deterioration of quartz in the presence of excited Hg atoms. This is consistent with our experience in that an argon-mercury quartz lamp used some 1000 hours over a period of more than two years, still compares favorably in intensity with a new lamu. A sliaht discoloration has aopeared, being most pronoun6ed near the electrodes.
region of h 2537 A., and it is equally evident from Plate I that this radiation consists almost entirely of the non-reversed resonance line, without the wings which this line possesses when obtained from an ordinary
-
*The lamps described were furnished by the Ultraviolet -, -- - .in n&formance ind &&acteri& to the Sc 25 developed by the Hanovia Chemical and ~anufacturi& .Co. f& use as a source in producing Raman spectra. COBLENTZ. J. Am. Med. Assoc., 97, 1966 (1931); Hanovia Chem. and Mfg. Co., circular describing the Sc 2537 lamp.
'
mercury arc, and is therefore capable of being absorbed by cool mercury vapor (Plate I, c and d). Also i t is seen that the argon-mercury lamp is useful as a direct source of mercury resonance radiation, with no need of filters or opticaisystem.*
screen. This picture, which except for motion, shows the vapor shadows practically as they appear to the eye, was obtained on Eastman supersensitive panchromatic film, a t an exposure of 1/20 sec., with a glass (Elmar) lens of relative aperture f 3.5. The actual experiment, in which the shadows are observed rising like smoke, naturally produces a more profound sensation than do the photographs, which cannot show the motion. Should one speculate as to what the eye would see if it were adaptable to radiation a t X 2537 A,,the answer is contained in the photographs of Plate 111. These were taken with mercury resonance radiation using a quartz lens, transparent to X 2537 A,, and focused approximately a t this wave-length by means of the image obtained on a fluorescent screen a t the focal plane of the camera. A piece of white Bristol board
a. Ordinary mercury arc. b. Argon-mercury vapor discharge. c. Same as (b), but with n dish of mercury in the spectrograph. Note the complete absorption of the resonance by the mercury vapor. line, X 2537 d . Continuous hydrogen discharge, with mercury vapor in the spectrograph, showing the selective nature of the absomtion. e. Same as (d), but with no mercury vapor in the light path.
A,.
VISUAL AND PHOTOGRAPHIC DEMONSTRATION O F THlZ EVAPORATION OF MERCURY
To demonstrate visually the evaporation of mercury. it is merely necessary to place an open vessel of the metal between an argon-mercury lamp and a screen coated with anthracene, uranyl sulfate, willemite, or other material with which visible fluorescence is excited by ultra-violet light a t X2537 A. Even a t room temperature mercury vapor is swept out of the dish in clouds sufficient to absorb the resonance radiation strongly, and therefore to =st s h a d o ~ on the fluorescent background. It is desirable, but not necessary, to darken the room in order to secure the greatest possible contrast. Abundant clouds appear if a stream of air is passed slowly over the mercury surface. The effect becomes most striking if both the convection currents and also the rate of evaporation of mercury are increased by warming the dish a few degrees, say to 40-60°C. Under the latter conditions the contrast and intensity prove great enough to photograph the phenomenon. Plate IIa is a photograph of an anthracene saeen and beaker of mercury (4(t50°C.) in white light. Plate IIb is the same scene but with the saeen illuminated by three 5-inch, 30-watt argon-mercury quartz lamps4 mounted close to each other and a t 60 cm. from the
-
* T o avoid possible injury to the eyes, the lamp should be partially enclosed or shielded. A pair of glasses should be worn ora nlass date held in front of the eves when it is nerezsnrv lnnk ,tn -- .-directly ai the arc. ~
~~~
a. Beaker of mercury (at 50°C.) in front of an anthracene screen, photographed in white light, with a glass lens, showing the complete absence of shadows as is obviously to be expected, if the rising vapor is truly gaseous and not a fog of condensed droplets.
b. Photograph (with g h s lens) of mercury vapor shadows on the fluorescent anthracene screen, showing how the effect appears to the eye, when the scene of ( a ) is irradiated with mercury resonance radiation from three argon-mercury lamps.
was placed behind the beaker of mercury to obtaia a background of diffused resonance radiation (A 2537 A.) :instead of the visible fluorescence of the anthrac~ne screen used above, when irradiated with the argonmercury source. Plate IIIa, taken with three lamps so close to the saeen that no vapor shadow was formed, shows the mercury vapor itself as a silhouette, rising directly from the beaker of mercury. Re-radiation of the resonance line bv the v a ~ o.r which . mieht nroduce a white cloud on direct exposure to the lamps, is extinguished in the presence of air.1 In ?late IIIb, the lamps were placed farther away and close together ~
.
-
(as in case of IIb) so as to give a shadow on the screen as well as a silhouette of the vapor. In both cases an Eastman 50 plate was used, an exposure of l/lOth sec. at relative aperture f 7.5 sufficing. As before, the mercury was heated to 40-50°C. to intensify the effect. I t is scarcely necessary to remark that the eye sees neither the silhouette nor the shadow when the nonfluorescent white board is used as the background, although both of these may be viewed on a small willemite screen placed at the focal plane of the camera.
tively thick layers. Rhodamine-B dye may be applied in a dilute alcoholic solution, but its red fluorescence is not so desirable for visual demonstrations. However, with red-sensitive film, mediocre photographs similar to IIb were obtained. No doubt still other fluorescent materials could be used. TOXIC QUANTITIES OF MERCURY VAPOR IN TID3 AIR
The visual demonstration of mercury evaporation should serve effectively to warn against careless indifference in handlm~the metal. It is too easv to disregard a toxic vapor which we neither see nor smell. One notes with interest that S t o ~ kin, ~a very emphatic paper on the subject of mercury poisoning, asserts that many chemists and physicists have suffered from chronic mercury poisoning without being aware of the cause of their disorders. He names Faraday and Pascal as examples, basing this statement on symptoms said to be revealed in their biographies. The same author points out the need for proper ventilation, particularly in rooms in which one may be exposed daily to small traces of mercury vapor. Air saturated with mercury vapor contains 0.015 mg. of the metal per liter of air at 20°C., whiie at 30°C. this is approximately doubled. It is improbable that anything approaching saturation would be reached in the atmos~here - - - - of -- I room, but on the other hand according to standard references7a concentration as small as 0.0007 mg. of Hg .per liter, with daily exposure of 3 to 5 hours, is said to produce symptoms of chronic mercury poisoning after 2 or 3 months or longer, the daily absorption being estimated at 0.8 to 1.3 mg.
-
3
.
a. Silhouetrc of vapor rising from liquid mercury at 5VC. A screen of wlme Brisml b o w l f u m e d the background. This ~ 3 irradiated s with three areon-mercurv lamna nhccd clor to the screen. The camera was e uipped with puam lens to transmit the radiationat X 25371.. reflectedfrom the white screen. b. Silhouette and shadow of vapor, obtained as in (a), excepting that the lamps were placed at a distance sufficient to cast a shadow of the vapor onto the background. At the same time the vapor itself was silhouetted by the reflected resonance radiation. Neither of the effects in (a) and ( b ) were visible to the eye.
a
For the visual demonstration, the post brilliant screen which we have tried was made by simply rubbing h e l y powdered fluorescent willemite on rough white paper or on finely woven cloth. A brilliantly flnorescing anthracene screen may be similarly prepared. A more permanent willemite screen was made by spreading the dry powder over a moist film of water-glass on Bristol board.= Finely powdered crude anthracene (Kahlbaum's kaufliches Anthracen) was successfully sprayed on Bristol board, using Midland cellulose nitrate "dope" diluted with ethyl acetate. When this coating was dry, the fluorescence was brightened by rubbing the surface with a cloth. The photographs of Plate I1 were obtained with such a screen because of its uniformity, although actnally willemite gives greater contrast because its response to radiation at X 2537 8. is greater. Uranyl sulfate may be applied by any of the above methods but is satisfactory only in compara-
-
HEINEMAN. RosEnr E. S.. University of Arizona, describesthe successful preparation of a willemite screen by dusting the mineral on a wet lacquer surface. (Privately communicated.)
ANALYTICAL APPLICATIONS
The visual demonstration described serves as a very sensitive and instantaneous method of detecting small quantities of mercury, either metallic or combined, simply by observing the shadows due to mercury vapor rising from a heated sample. Mercurous salts are found to give dense vapor shadows at. temperature ranges lower than those required for m&curic salts, probably due to displacement of the theoretical equilibrium,
Bivalent mercury may of course b$ reduced to the mercurous state or to the free metal to enhance the thermal d e c k . One is amazed at the definite though brief shadows obtained by heating objects such as old locker keys, old wire, and even bits of dust from chemical laboratories in which mercury has been used. Similarly, personal articles of jewelry, such as rings and watch chains, are found to reveal whether the wearer has been a frequenter of the laboratory or has otherwise been exposed to mercury. Pure metals such as Fe, Cu, Pb, Zn, Sn, and Cd do not interfere, but any vapor ~. a STOCK, 2. angm. Chem.,39, 461 (1926).
'
HENDERSON m HAGOARD, Noxious gases," Chem. Cat. Monograph, 35. 1927, pp. 177-182; SOLLMAN, "Manual of Pharmacology," Saunders, 1926, p. 1033.
benzene) which absorbs in the region of X (such 2537 A. will cause some confusion. In this latter case the spectrographic method, illustrated in Plate Id, is preferable. Finally, a quantitative estimation of small concentrations of mercury vapor (say, of the order of 1 microgram of mercury per liter of air) is feasible by use of the double photocell method of Hughes and Thomas,% or more simply, by means of afluorescence photometer in which the intensity of fluorescence produced by the resonance radiation after i t has passed through a known - length of the air sample, is compared with a HuonEs~mTHOMAS. Pkys. Rev., 30,466 (1927).
standard intensity, either visually or by photocell. The extreme sensitivity and rapidity of the visual method should make it especially useful in such problems as the detection of mercury in cosmetics, in medicinal preparations, and in excreta, both as a qualitative tool and also as a means of studying the adequacy of chemical operations involving the quantitative estimation of small quantities of mercury. By performing tests in an atmosphere of nitrogen or hydrogen, which transmit the line a t A 1849 A,, the sensitivity should be made even greater. The authors will report elsewhere in further detail on these analytical applications.
