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ANALYTICAL CHEMISTRY, VOL 51, NO 7, JUNE 1979
average recovery and reproducibility, and the sensitivity of t h e analysis. T o some extent, the error associated with estimating the quantity of a component which is captured inefficiently, or is poorly recovered, can be overcome by increasing the sensitivity of the analysis. However, this does not imply that such values are good estimates of the true value of the component. Thus, in order to ensure the accuracy of such data, the scientist must control not only the analyses but, more importantly, the processes involved in obtaining the sample itself.
Using the above facts, the mean and variance of
0= A ( R / p q )
(3)
are derived as follows:
APPENDIX Let X and Y be two independent random variables having unknown distributions with means p x , p, and variances exz, u,‘, respectively. From the theory of mathematical expectations ( 5 ) we can write: px =
where a and b are as given in Equations 6 and 7 in the text.
E ( X ) = expected value of X
(i)
ACKNOWLEDGMENT
Var(X) = E(X2)- E2(X)
(ii)
Bob Teece and Satu Somani brought this problem to our attention and provided helpful discussions, as did John Hurley.
ex2=
E ( X * )= E(XY) =
ex2
+ px2
E(X)E(Y) = P,+>
V a r ( X Y ) = E ( X 2 P )- E2(Xu)
(iii)
(v)
E ( X 2 l f l ) = E ( X 2 ) EIfL) ( =
+ px2)(e,* + PJ
(ax2
(vi)
E(CX) = CE(X)
(vii)
V a r ( C X ) = C 2 V a r ( X ) where C is c o n s t a n t
(viii)
E ( l / X ) -- 1/E(X)
(id
E(l/X2)
;=
l/E(X2)
LITERATURE CITED
(iv)
(4
L. R . Ember, Environ. Sci. Techno/., 12, 757-760 (1978). P. Van Rossum and R. G. Webb, J . Chromatogr., 150, 381-392 (1978). D. K. Basu and J. Saxena, Environ. Sci. Technoi., 12, 791-894 (1978). K. Eckschhger, “Errors, Measurement and Results in Chemical Analysis”, Van Nostrand Reinhold Company, London, 1969. (5) R. V. Hogg and A. T. Craig, “Introducton to Mathematical Statistics”, fourth ed., Macmillan, New York, 1978.
(1) (2) (3) (4)
RECEIVED for review September 18, 1978. Accepted March 26, 1979. This problem arose from work funded by the U S . Environmental Protection Agency under Amendment 2 of grant IO0517077 to the Illinois EPA.
Adsorption of Mercury Vapor by Gold and Silver T. T. Mercer Department of Radiation Biology and Biophysics, University of Rochester, School of Medicine and Dentistry, Rochester, New York 14642
The sticking probability is unity for mercury vapor hitting a gold or freshly-formed silver surface. Adsorption isotherms for gold films indicate that it is not significantly affected by film aging, at least over periods as long as 235 h. Adsorption isotherms for silver films deteriorate rapidly with aging, indicating a marked decline in slicking probability. The absorptive capacity of gold films is approximately 0.7 pg Hg per cm2 of superficial film area. The adsorptive capacity for silver films varies significantly with aging time. Silver wool, silvered Chromsorb P, and silver membrane filters adsorb amounts of mercury equivalent to * I 3 - 1 ‘ I 2 monolayers ( ~ 0 . 3 6pg/cm2).
Mercury as a vapor occurs in the atmosphere a t concentrations in the approximate range of 1-25 ng/m3 ( I , 2); in particulate form, its concentration is about 0.2-11 ng/m3 ( 3 ) . As a n air pollutant, the form in which mercury occurs has a very marked effect on its dynamic behavior. T h e diffusivity of the vapor is greater, and its sedimentation velocity less, by orders of magnitude, than those of particulate mercury. T o predict fall-out patterns about a source of mercury (4)or to estimate lung deposition of inhaled mercury, it is necessary to know, among other things, the relative amounts of mercury in vaporous and particulate forms. A significant fraction of the particulate mercury may be the 0003-2700/79/035 1-1026$0 1.OO/O
product of vapor adsorption at the surface of airborne particles ( 2 ,4-6). T h e relative amount of vapor in adsorbed form will depend on the concentration and size distribution of the particles, which determine the rate a t which mercury, in atomic or molecular form, strikes a particle; and on the composition of the particles, which determines the strength of the bond that holds the adsorbed mercury to the particle surface. In preparing to study this phenomenon, it appeared useful to have a “calibration” aerosol with predictable attachment and adsorption properties. Silver and gold, which are widely used to determine atmospheric levels of mercury vapor ( I , 7-9) because they readily adsorb a n d retain it a t normal temperatures but release it quantitatively above 200 “C ( 7 ) , were obvious choices. However, the information necessary to make the required predictions concerning their capacities as adsorbents of mercury vapor was not available. Mercury collection systems are usually calibrated by passing air containing a known concentration of the vapor through the collector and a mercury analyzer in series until the latter indicates the presence of mercury in the collector effluent (7, 8). The capacity of the collector is then expressed as the mass of mercury retained per unit mass of adsorbent before “breakthrough” occurs. Since adsorption is a surface phenomenon, this is an ambiguous definition of capacity, which is likely to be seriously misleading when the adsorbent has 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
LJ
3-WAY VALVE
I
I
-
",',"u"d;";
1
CCNTCOLLED TCMPERATURE VrbTER B A - 3
MERCURY VAPOP IN AIF
CONTROLLED
m
G E N E C A T O R B Y P A S S LINE
MERCURY
*
1027
VACUUM
A
CONCENTRATION
DIGITAL
bOL-VETEC
1
SILICA GEL HCPCALITE
C L E A N DRY AIR
HOOD EXHAUST
Flgure 1. Mercury generator system
f -
a large surface-to-mass ratio. Moreover, it is evident from the occurrence of breakthrough that t h e attachment probability
Figure 2. Mercury sampling and detection system
m u s t diminish a s mercury accumulates on t h e adsorbent. Because of these uncertainties, the following experiments were carried o u t in an effort to determine (a) t h e probability that a mercury atom is adsorbed upon striking a surface of gold or silver and (b) t h e amount of mercury vapor that can be retained per unit area of such surfaces.
cell and flows radially inward to pass through a center hole in the bottom plate. It then passes through the valving arrangement, where it is split into two streams, one flowing a t a fixed rate through the detector test cell, while the other carries the excess, at a rate controlled by another rotameter (not shown in the figure), to the exhaust system. The mercury detector is an LDC UV monitor with two identical 30-cm absorption cells. Critical orifices limit the flow rate through each cell to approximately 1 L/min. The reference cell has never been exposed to significant concentrations of mercury; only room air, cleaned by passage through an M-11 gas mask canister, enters it. The control unit of the detector system provides seven ranges of sensitivity for which the dc voltage signal from the detector is directly proportional to the concentration of mercury in the test cell. The signal from the detector, which is displayed on a digital voltmeter (United Systems Corp., VOM 261C), is integrated and both the original signal and its integral are recorded continuously (Linear Systems Corp., Integrating Recorder). When a granular or filamentous adsorbent was under study, a weighed amount of it was packed into a glass tube which was arranged in parallel with a similar control tube. An adjustable constriction in series with the latter was used to match pressure drops across the test and control units. Valving and detector systems were as shown in Figure 2. Adsorption by silver membrane filters was measured in an arrangement that was similar in principle but in which filter holders in parallel replaced the test and control glass tubes. Materials. Adsorptive capacities were determined for silver wool, silver membrane filters, and silvered Chromosorb P, in addition to the glass cover slips coated with films of gold or silver. The silver wool (Fisher S-163 Microwool) was unbraided to give the mercury vapor ready access to all surfaces. The individual filaments were 0.005 cm in diameter, as measured by micrometer. Several filaments, totalling 190 cm in length, weighed 0.0457 g, which corresponds to an average diameter of 0.0054 cm. The specific surface of the wool ( = 4/density x diameter) was 0.0705 m2/g. The silver membrane filters (Flowtronics, 0.45 pm) had a specific surface of 0.16 m2/g. Chromosorb P (Johns-Manville Co., 30160 mesh, lot 112, NAW) was silvered using the method discussed by Trujillo and Campbell (8). Its specific surface before silvering was 2.6 m2/g. As silvered Chromosorb P (AgCrP), its specific surface was 3.0 m2/g. Examination of the AgCrP in the scanning electron microscope (SEM) revealed that the silver did not form a uniform coat over the surface of the Chromosorb, but was deposited on it in particulate form. Assuming that the presence of the silver did not significantly reduce the specific surface of the Chromosorb, the specific surface of the attached silver was 4.4 m2/g. All direct measurements of surface area were based on the BET nitrogen adsorption method ( 1 1 ) . They were carried out in an electronic microbalance (Sartorius). The surface area of a gold or silver film was assumed to equal the crosssectional area of the substrate on which it was deposited. Calibration Procedures. The mercury vapor generator was operated a t a fixed temperature and flow rate with all of the output during a timed interval being directed through two midget impingers in series, each of which contained 10 mL of permanganate solution (0.05 N KMnO, and 0.5 N H,S04). At the
EXPERIMENTAL The experimental arrangement comprised a mercury vapor generator capable of producing an atmosphere of mercury a t a constant, controlled concentration, test and control samplers in parallel, and a mercury analysis system that provided a continuously recorded measurement of the mercury concentration in air after it passed through a sampler. Mercury Generators. Figure 1 is a diagram of the mercury generator. It is based on one described by Nelson ( I O ) , but differs in that the vapor source is maintained a t below-ambient temperatures. This change makes it possible t o reduce the total amount of mercury vaporized while maintaining a concentration adequate for experimental purposes. A metered flow of air passes over the external mercury source, which is always a t room temperature, and enters the controlled-temperature source where the excess vapor condenses out. If the flow rate is not too high, the vapor content of air within the source is reduced to the saturation level at the temperature of the water bath, which can be controlled to k0.5 "C. A thermistor, fastened to the mercury reservoir near its outlet and connected to a telethermometer, provides a measure of the saturation temperature of the vapor. Although a given concentration of mercury can be obtained a t several combinations of diluting air flow, generator air flow, and water bath temperature, it has been found convenient to fix the two latter conditions, adjusting only the diluting air flow when changes in the output concentration are desired. Compressed air a t 80 psi enters the laboratory after passing through a molecular sieve drying unit (Gilbarco). After the pressure is reduced to 10 psi, the air passes through beds of silica gel and hopcalite and through a membrane filter (not shown in the diagram) before entering the rotameters. The total air flow is always in excess of the amount required experimentally; the overflow is discharged t o an exhaust hood after passing through beds of silica gel and hopcalite to reduce its mercury content. Sampler-Detector System. Figure 2 is a diagram of one of the sampling systems with the associated detector and recording system. The sampling unit contains two diffusion cells enclosed within a brass chamber. Each cell consists of two circular magnetic disks, parallel and co-axial, that are kept a fixed distance apart by three small metal spacers set a t equal intervals about their periphery. For the experiments under consideration, glass cover slips of the same diameter as the disks were inserted between the spacers and the disks. Thin films of gold or silver were sputtered onto the deposition surface of each cover slip used in the test cell; those used in the control cell were uncoated. (Aluminum disks were tried first, because holes can be drilled in aluminum more easily than in glass. However, films on aluminum were crazed and irregular, whereas a film on glass had a mirror finish.) In operation, air enters symmetrically about the periphery of the
1028
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7 , JUNE 1979
where p = 4??D(Ro2- R12)/3Fh,D is the diffusivity of mercury vapor, Ro and RI are, respectively, the outer and inner radii of the diffusion cell, F is the volumetric flow rate through the cell, and 2h is the separation between the cell plates. The first four coefficients and eigenvalues, with which an adequate estimate of P can be made, are
Table I. Calibration Data for Mercury Generator tempeJa- flow rate, no. of exptl concn. ture, C cm3/min runs satn concn. 11 200 4 1.01 i 0.06 11 400 3 1.22 i 0.02 5 200 3 1.04 f 0.05 5 3 400 1.35 i 0.17 6 200 2 0.99 i 0.01 9 200 1 1.00 15 200 1 0.97
i
end of the timed interval, the solution in each impinger was brought up to 25 mL by the addition of 1 N H2S04and three 5-mL aliquots were taken for separate analyses. Hydroxylamine, 0.1 N, was added dropwise to each aliquot until clear. The mercury content was then determined using the dithizone method described by Sandell (12). Transmittance measurements were made a t 500 nm using a Beckman DU Spectrometer, which was calibrated against a standard mercury solution (0.1354 g HgClz in 100 mL of 1 N H,SO,) at appropriate dilutions. On two occasions, a mercury standard was prepared by placing a globule of mercury in a 100-mL syringe, shaking it vigorously, and then allowing it to equilibrate at room temperature. Fifty milliliters of the saturated vapor were then injected slowly into the sampling line upstream of the LDC monitor. The integrated signal, converted to total mercury according to the initial calibration, was compared to the amount contained in 50 mL of air saturated a t the measured temperature of the syringe. For flow rates less than 2 L/min, rotameter settings and critical orifices were calibrated against a mercury-sealed piston volumeter (Brooks Instrument Co.). A dry-gas meter, calibrated against a water-sealed spirometer, was used when flow rates were greater than 2 L/min. A volume of a t least 10 L, equivalent to 10 revolutions of the dial indicator, was included in each measurement. Flow rate calibration measurements could be reproduced to within 2 % . All flow rates were checked at the start of each experiment. Test Procedures. All measurements of adsorptive capacity were carried out in the same way. After the water bath came to the desired temperature, the generator and dilution air flows were adjusted to the proper levels with the valving arranged to pass the diluted mercury vapor through the control side of the sampling unit before it entered the detector. When the latter indicated that a constant mercury concentration had been reached, the valving was adjusted to pass the air through the test unit, which had been charged with a known amount of adsorbent. If the capacity of the adsorbent was sufficiently large, there would be an interval during which no mercury reached the detector; thereafter, the concentration would increase continuously until it was close to the input Concentration. When the rate of increase became very slow, the generator was by-passed and clean air was allowed to flow through the adsorbent until desorption ceased. From time to time throughout the adsorption process, the valving was reset to pass the contaminated air through the control unit so that the constancy of the input concentration could be verified. Since the input concentration was known, and the recorderintegrator provided a continuous account of the amount of mercury escaping adsorption, it was possible to establish the cumulative amount of mercury adsorbed as a function of time. With this information, a knowledge of the surface area of the adsorbent, and the continuous record of mercury concentration in the air that had passed through the adsorbent, it was possible to determine penetration as a function of the mass of mercury adsorbed per square centimeter of adsorbent. When the test unit was a diffusion cell as shown in Figure 2, it was necessary to make allowance for the fact that a certain fraction of the mercury atoms passed through the unit without striking any surfaces. Flow rate through the test cell was 0.93 0.02 L/min. I t was also necessary to use the diffusion cells to determine if a mercury atom adheres to a gold or silver surface upon coming into contact with it. If it does, then the initial penetration of the diffusion cell should be given by (13):
*
m
P = Ea,e-bcp ,=1
(1)
bi
ai
2.82776 2 32.1473 3 93.4749 4 186.8050 If P is measured as a function of F , a plot of In P vs. F' rapidly approaches a straight line of slope -b14nD(Ro2 - R12)/3hand intercept 0.9104. Given the slope, the diffusion coefficient can be calculated and compared to values that are already available in the literature. It became apparent early in the study that the initial penetration depended to some extent on the freshness of the adsorbing surface. To study this effect, the elapsed time between formation of an adsorbent film and measurement of initial penetration was varied for a number of experiments. 1
0.9104 0.0531 0.0153 0.0068
RESULTS Calibration of M e r c u r y Generator. The calibration data for the mercury vapor generator are summarized in Table I. T h e tabulated values of temperature and flow rate refer t o instrument settings; they differed slightly from the measured values. T h e saturation concentrations used t o calculate the ratios in the fourth column were based on the measured temperatures in the first column with due allowance for the greater density of air a t the reduced temperature. T h e errors on the ratios are standard deviations on the total number of samples analyzed for a given set of conditions. T h e last two rows apply t o the calibrations t h a t involved injection of mercury directly into the sampling line. On the basis of these results, it was concluded that a t a flow rate of 200 cm3/min t h e mercury content of air passing through the controlled-temperature vessel equilibrated at the saturation concentration corresponding t o the temperature a t the vessel. T h e generator was operated a t t h a t flow rate for all of the adsorption experiments reported here. S t i c k i n g Probability. Figure 3 is a copy of recorder tracing of the early penetration of mercury vapor through a gold-film diffusion cell. Penetration is essentially constant for only a brief period, indicating that the sticking probability decreases as the amount of adsorbed mercury increases. Relative penetration (= initial penetration through the test cell/penetration through the control cell) was determined for several flow rates. All of the measurements were made using glass cover slips coated with freshly-formed gold films. A least-squares regression analysis of the experimental data yielded the following relationship between P and F
[
P = 0.905 (f0.018) e x p -(1764;
33)1
(2)
where F is the flow rate in cm3/min. In cgs units, t h e slope of the line is -2.82776[4rD(RO2- R ? ) / 3 h ] = 29.4 f 0.55 s/cm3
(3)
T h e dimensions of t h e diffusion cell are Ro = 1.10 cm, RI = 0.10R0, h = 0.065 cm. T h e calculated value for the diffusivity of mercury in air is D = 0.135 f 0.003 c m 2 / s
(4)
Mullaly and Jacques (14) and Spier (15) reported values of 0.119 cm2/s and 0.14 cm2/s, respectively. I t was assumed, therefore, t h a t the sticking probability is unity for mercury atoms striking a gold surface. Adsorption Capacities. The adsorption capacities of silver
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
1 '
1029
60 50
40 30
Time
Figure 3. Initial penetration through gold film diffusion cell (Time scale: 1 division = 2 min.). (A) Mercury vapor vented to test diffusion cell. (B) Initial penetration HG RETAINED, p G / C M z
Figure 5. Relative penetration as a function of mercury retained per
unit area of adsorbent gold films
silver films init. init. rel. rel. aging peneaging penesymbol time, h tration curve time, h tration 0
