J. Phys. Chem. 1988,92, 6167-6170
6167
Vapor Pressures of HN03/H,0 Solutions at Low Temperatures David Hanson and Konrad Mauersberger* School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455 (Received: February 1 , 1988; In Final Form: April 4, 1988)
Vapor pressures of liquid, supercooled, and frozen H N 0 3 / H 2 0mixtures of various concentrations have been measured over a wide range of temperatures and pressures. The vapor pressures as well as impurities were analyzed by a mass spectrometer system. Measurements made on liquid and supercooled mixtures were fit according to the Clausius-Clapeyron equation. Different concentrations showed the expected variability of HN03 and H 2 0 vapors as a function of temperature. Vapor pressures of partially frozen mixtures followed a freezing envelope leading to quadruple (eutectic) points. Completely frozen bulk mixtures resulted in the coexistence of two solid phases and vapor. Vapor pressures were measured over both the trihydrate and solid solutions of HN03 in ice and over the trihydrate and monohydrate. The H 2 0 and HN03 pressures were found to be uniquely determined for each of these solid mixtures.
Introduction The recently discovered ozone hole over Antarctica has resulted in a need to investigate the vapor pressures of nitric acid/water mixtures at low temperatures.' The formation of polar stratospheric clouds that contain HN03 is a crucial part of the chemical mechanisms that are proposed to explain the loss of ozone between 12 and 25 km in September and October.2 In these mechanisms, H N 0 3 must be condensed out of the air in order to allow the formation of free chlorine, which then enters a catalytic ozone destruction chain. The cloud particles could also absorb HCl or provide surfaces for certain heterogeneous reactions that are important to these mechanisms.l" have speculated on the possible conA number of densation temperature of HN03 with H 2 0 by extrapolating H N 0 3 / H 2 0 vapor pressure data measured near room temperature' to stratospheric temperatures found during polar night. This involves extrapolations of over 5 orders of magnitude in vapor pressure for a supercooled nitric acid/water mixture. Moreover, there are uncertainties in predicting the qualitative behavior of the vapor pressures upon solidification. The formation of two hydrates, a mono- and a trihydrate, in nitric acid water solutions has been known for some time.*v9 McElroy et al. recognized that there are stable pressure and temperature regions over which each hydrate can exist. Toon et ale3and Crutzen et al.' proposed vapor pressure curves for solid and supercooled H N 0 3 / H 2 0 mixtures. These authors all concluded that particles containing H N 0 3 and H 2 0 could form in the Antarctic stratosphere. Very little experimental information on HN03/H20mixtures at low temperatures is available. Pickering* and Kuster and Kremann9 have measured the freezing temperatures for a wide range of concentrations for the H N 0 3 / H 2 0 system. They clearly identified the formation of mono- and trihydrates as well as the presence of solid ice for low nitric acid concentrations and solid H N 0 3 for high concentrations. Clavelin and Mirabel' collected vapor pressure data of HN03/H20mixtures measured near room temperature. They summarized the data in a set of equations
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(1) Crutzen, P. J.; Arnold, F. Nature (London) 1987, 324, 651. (2) McElroy, M. B.; Salawitch, R. J.; Wofsy, S . C. Geophys. Res. Lett. 1986, 13, 1296. ( 3 ) Toon, 0. B.; Hamill, P.; Turco, R.P.; Pinto, J. Geophys. Res. Lett. 1986, 13, 1284. (4) Molina, M. J.; Tso, T.-L.; Molina, L. T.; Wang, F. C.-Y. Science (Washington, D.C.)1987, 238, 1253. ( 5 ) Tolbert, M. A.; Rossi, M. J.; Malhorta, R.; Golden, D. M. Science (Washingron, D.C.)1987, 238, 1258. (61 Wofsv. S.C.; Molina, M. J.; Salawitch, R. J.; Fox, L. E.; McElroy, M. B: J. Geiphys. Res., in press. (7) Clavelin, J. L.; Mirabel, P. J. Chim. Phys. Phys.-Chim. Eiol. 1979,76, 533.
( 8 ) Pickering, S. U. J. Chem. SOC.1893, 63, 436. (9) Kuster, F. W.; Kremann, R.2.Anorg. Chem. 1904, 41, 1.
0022-3654/88/2092-6167$01.50/0
relating composition, pressures, and temperatures. Phase theory'O applied to the HN03/H20system provides information on the behavior of the vapor over a single solid phase as well as on the vapor pressures over two coexisting solid phases, the so-called three-phase equilibria. Three-phase equilibria are common in binary mixtures, and one characteristic is that the vapor composition and pressure are uniquely determined for a given temperature. These three-phase equilibria also serve as phase boundaries for solid compounds that form from binary mixtures. Consequently, determining the vapor pressures over the appropriate set of coexisting solid phases defines the pressure region over which a particular solid compound is stable. In this paper we report vapor pressure measurements of liquid, supercooled, and frozen H N 0 3 / H 2 0 mixtures. The majority of measurements were performed with bulk mixtures; some vapor deposits of nitric acid and water on glass surfaces were also made. Experimental Section The apparatus shown in Figure 1 consists of a removable glass sample tube, gas handling lines for H 2 0 , H N 0 3 and pump-out lines, a chamber where vapor pressures are established, and a mass spectrometer beam system (MSBS)" for gas analysis. The total pressure in the chamber was measured with a high-precision capacitance manometer, and the partial pressures of water, HN03, and impurities were determined with the MSBS. For most measurements reported, the vapor pressure chamber was a 5-L highly polished stainless steel vessel. The MSBS continuously sampled the gas of this chamber through a small orifice at a rate such that the system could be considered under static equilibrium. Details of the operation of the MSBS system have been given in a number of publications."J2 The acid mixture is at the bottom of the glass sample tube where the cold region is 1 cm high and 3 cm in diameter. The tube in the middle is heated at its base and along the sides in order to prevent condensation, which could influence vapor pressures. Generally, 1 W is enough heat to keep that region clear of acid/water deposits. Heating the glass above the sample was found to be very important for frozen mixtures. The sample tube is immersed in methanol, which is cooled by a flow of cold nitrogen gas. The temperature is regulated with resistive heating to about f0.1 K, and it is monitored with a platinum resistance thermometer that has been calibrated at the ice point and generally has an accuracy of f 0 . 3 K. At temperatures above 223 K thermal diffusion and transpiration along the transition to the room temperature measurement chamber are estimated to be less than 5%.I3J4 At lower temperatures, however, thermal transpiration (IO) Kroger, F. A. The Chemistry of Imperfect Crystals; North-Holland: Amsterdam, 1982. (1 1) Mauersberger, K. Rev. Sci. Instrum. 1977, 48, 1169. (12) Mauersberger, K.; Finstad, R. Reo. Sci. Instrum. 1979, 50, 1612. 0 1988 American Chemical Society
Hanson and Mauersberger
6168 The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
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MASS SPECTROMETER B E A M SYSTEM
VAPOR PRESSURE
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measurements of Hz0l3suggest that the H 2 0 pressure will be as much as 20-22% greater in the room temperature chamber than in the still. Both the H 2 0 and H N 0 3 data have been corrected for this effect. No measurements for thermal diffusion of HN03 in H 2 0 have been found, but this effect is estimated to be less than 5% for all temperatures of interest.14 The bulk liquid mixtures are prepared by diluting either 70% (w/w) (X = 0.40) or fuming (90%(w/w)) nitric acid and are analyzed before and after a run by using standard acid-base titration techniques. The accuracy of the titration is about i0.005mole fraction. Typically, 3 cm3 of liquid is used for the bulk measurements. The MSBS is calibrated for H N 0 3 by using the vapor of purified fuming nitric acid and measuring the pressure with the capacitance manometer. The sensitivity of the mass spectrometer is about 20 times less for HN03 measured at the parent peak of 63 than for the same pressure of water measured at mass 18. The electron ionization fragmentation products of HN03are preferably NOz+ (46) and NO+ (30). Since some of the impurities during a vapor pressure run are N O and NO2, the relatively small signal on 63 was primarily used to determine nitric acid partial pressure. Therefore the experimental detection limit for H N 0 3 was near 5 X lo-' Torr. However, mass 46 was occasionally used to extend this limit. Generally, the H 2 0 partial pressure was determined by subtracting the H N 0 3 and impurity pressures from the total pressure. For low-temperature data the MSBS was also used to determine the H 2 0 pressure directly.
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Results Liquid and Supercooled Liquid. Figure 2a,b shows the results of H N 0 3 and H 2 0 vapor pressure measurements for selected concentrations of liquid and supercooled liquid. The dashed lines are the extrapolations of the Clavelin and Mirabel' data set down to the freezing temperatures determined by Kuster and Kremanr~.~ Similar lines were obtained by McElroy et al.: using the freezing points determined by Pickering.8 For the sake of clarity, the measurements are presented with a solid line for each concentration. The estimated systematic error of the liquid or supercooled data is approximately 5%, and the main source is the uncertainity in the impurities of the calibration gas. Three of the concen-
Figure 2. Vapor pressures of HNO, (a) and H20 (b) for liquid and supercooled mixtures. Mole fractions of HNO, are indicated in the figure. The solid lines are fits of experimental data while the dashed lines are extrapolations of the Clavelin-Mirabel data' set.
(13) Yasumoto, I. J . Phys. Chem. 1980,84, 589. (14) Jost, W . Diffusion in Solids, Liquids and Gases; Academic: New York, 1952. For a reasonable value of the thermal diffusion constant of 0.10, the change in the concentration of the minor constituent along a ternmrature transition of 190-300 K is 4%.
trations, H N 0 3 mole fraction X = 0.15, 0.20, and 0.25, were measured three times and proved reproducible. Not represented are data measured at mole fractions of X = 0.226, 0.273, and 0.175. These data followed a line as expected. Measurements were limited toward higher temperaturesand pressures, since the
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Vapor Pressures of HN03/H20Solutions beam system becomes nonlinear at high pressures. At low temperatures, near 223 K, the supercooled liquid would freeze. Freezing Envelope. Researchers at the turn of the century8q9 found they could isolate nearly pure mono- or trihydrate by partially crystallizing a solution. This leads to a freezing envelope in vapor pressures as the liquid concentration changes upon cooling when the mono- or trihydrate freezes out as McElroy et al.2 presented. This envelope is a three-phase line with solid, liquid, and vapor present. The freezing envelope was measured in our experiment, and results are presented as a solid curve in Figures 2 and 3 with the experimental data points shown in Figure 3. For comparison, the freezing envelope obtained by the intersection of the Clavelin and Mirabel’ vapor pressures with the freezing points measured by Kuster and Kremanng is presented as the dashed curve in Figure 2. Clearly shown are two eutectic points. Since one is near a mole fraction of X = 0.122, a 0.180 mixture, for example, should, when slowly cooled, crystallize out the trihydrate until 230 K, when the liquid left reaches X = 0.122 and upon further cooling freezes. Similarly, in mixtures below X = 0.12, solid solutions of H N 0 3 in ice (which will be called “ice”) will crystallize until the eutectic point is r e a ~ h e d .One ~ important deviation of our liquid data from the Clavelin-Mirabel data set is the H 2 0 pressure for mixtures around 0.12. Our data show that the H 2 0 pressure is very close to that of ice whereas the extensions of the Clavelin-Mirabel data are about 30% higher. These results are to be expected because the free energy of the water of a liquid HN03/H20solution ought to approach the free energy of ice before “ice” freezes out. McElroy et aL2 also recognized this fact, as their quadruple point Q1lies on the ice line, not on the straight extrapolation of the Clavelin and Mirabe17 X = 0.12 H 2 0 pressure line. Solid. Figure 3a,b shows vapor pressures observed over various solid mixtures along with the freezing envelope. The solid was studied both in frozen bulk mixtures and as a result of vapor deposit. The data show one vapor pressure line only for HNO, (Figure 3a) and H 2 0 (Figure 3b) starting at the eutectic point. These are the curves for three coexisting phases: trihydrate, “ice”, and vapor for the eutectic at X = 0.12 and monohydrate, trihydrate, and vapor for the eutectic at X = 0.40. Frozen mixtures of mole fraction 0.25-0.47 produced vapor pressures of H N 0 3 and H 2 0 represented by the circles. Vapor deposits were also made in this range, and vapor pressures fell on the same lines. Corrections for thermal transpiration were made by using data of a molecule that is approximately the average of H 2 0 and HNO3.I3 The consistency of the vapor pressures for all frozen mixtures and vapor deposits from X = 0.25 to X = 0.47 demonstrate that the equilibrium between the three phases trihydrate, monohydrate, and vapor was effectively realized. One mixture of mole fraction X = 0.53 was studied, and the vapor pressure of the frozen substrate was on that portion of the freezing envelope that ends at the 0.72 eutectic mixture at 207 K. The H N 0 3 pressure at 207 K was 0.018 Torr. Out of this eutectic should emerge an H N 0 3 pressure close to the vapor pressure of solid HN03, as the coexistence of almost pure solid HNO, and the monohydrate is established. Given the vapor pressure of liquid HN0315and the latent heat of fusion for HN0316 of 2503 cal/mol, the vapor pressure of H N 0 3 at 207 K should be 0.020 Torr. Since high HN03 pressures tend to degrade the apparatus, the high HNO, concentrations have not been studied in more detail. They are also of little importance to atmospheric applications. Below 0.25 mole fraction, the pressure of the bulk mixtures was dominated by water; the H 2 0 pressure was close to that of ice for temperatures around 230 K, as shown in Figure 3b; however, at 204 K the best line through the data lies about 10% over the ice17 line. This deviation could be partly due to the inaccuracy
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6169
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Figure 3. Vapor pressures of HN03 (a) and H 2 0 (b) for solid and partially frozen mixtures: (0)measurements of bulk mixtures of mole fractions between 0.25 and 0.47; (X) of mole fractions between 0.05 and 0.22. The coexistence curves are labeled. The ice line is shown for
reference. (15)Kalish, M.A.Zh. Fiz. Khim. 1973,47(1),275. Liquid nitric acid vapor pressures from 231 to 357 K are given by log P = 7.3156 - 1255.4/T - 117169/p. (16)Forsythe, W.R.;Giauque, W. F. J . Am. Chem. SOC.1942,64,48. (171 Jancso. G.:PuDezin. J.: Van Hook, W. A. J. Phvs. Chem. 1970,74. 2984.
of the thermal transpiration correction applied to our data. The thermal transpiration data of Yasumoto13 for H 2 0 were used to correct the H,O and HNO? pressures, since the vapor is almost exclusively HiO. Frozen cdncentrations studied in this range
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The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
TABLE I: Coefficients A and B from a Least-Squares Fit of log P = A - B / T for Both HNOJ and H,O“
HN03
H20 std std A B dev, % A B dev, % liq (0.40) 9.484 2624 2 9.460 2671 3 liq (0.25) 10.437 3141 7 9.128 2446 2 tri-/monohydrate 12.904 3411 11 9.534 2699 14 “ice”/trihydrate 11.352 3725 10.099 2592 7 (upper bound for “ 0 , ) ice (ref 17) 10.431 2668.7 OThe ice line (Jancso et al.”) is included for reference. The standard deviation of the fit represents the scatter in the data.
included the following mole fractions: 0.05,O. 10,O.12,O.15, and 0.22. Earlier measurements of frozen mixtures in this concentration range yielded significantly higher and much more variable H N 0 3 pressures than those presented in Figure 3. This is in contrast to the trihydrate/monohydrate coexistence vapor pressures, which always followed the lines shown. We believe that the equilibrium between “ice” and trihydrate is more difficult to establish, since the composition of the trihydrate is significantly different when formed in an X = 0.13 liquid from that formed in a 0.25 liquid. Conversely, the variation in composition of the trihydrate formed in a 0.38 liquid to that formed in a 0.25 is smalL9 An X = 0.24 liquid, for example, would begin to crystallize the trihydrate a t a composition slightly below 0.25. A 0.13 liquid would crystallize out a trihydrate of a lower composition, assuming 0.20 for argument. Therefore, if a 0.24 liquid is slowly cooled below its freezing temperature, the trihydrate is frozen out and the composition of the liquid changes as the mixture is brought along the freezing envelope until the eutectic point is reached. The trihydrate formed is not homogeneous, and a complete equilibrium between the solids “ice” and a uniform trihydrate is not established. In fact, if the composition of the trihydrate that coexists with “ice” is near 0.20, a completely frozen 0.24 mixture at thermodynamic equilibrium would consist entirely of 0.24 trihydrate crystals and the H N 0 3 and H 2 0 pressures would lie inside the range of pressures for which the trihydrate is stable. Therefore, there is reason to believe that the H N 0 3 pressure over the true equilibrium between “ice” and trihydrate is lower than the data presented in
Hanson and Mauersberger Figure 3. A dashed line has been drawn in the figure through the lowest HN03 pressures, and this is considered to be an upper bound to the H N 0 3 pressure over the coexistence of “ice” and trihydrate. An experiment is currently being developed that will measure the pressures over very small amounts of material deposited on glass so that the equilibrium between the solids might be better realized. Preliminary results indicate that the upper bound shown in Figure 3 is close to the true equilibrium H N 0 3 pressure. Systematic errors for the solid data include pressure meathe temperature uncertainty of h0.3 IC, which surements (fl%), translates into 4% in pressure at lowest temperatures, calibration gas uncertainty (h4%), and thermal transpiration errors at lowest temperatures (f3%). The total root mean square error is about 6-7%. Table I summarizes the straight-line fit of log P vs 1 / T for liquid (including supercooled) mixtures of 0.25 and 0.40 mole fraction and the solid three-phase lines. The results of the study of frozen mixtures presented above show the expected behavior of three-phase equilibria: when two condensed phases are present such as “ice” and trihydrate or monoand trihydrate, the third phase, vapor, has for a given temperature a definite composition, and thus a unique vapor pressure for both H 2 0 and H N 0 3 is observed. As stated earlier, if only one condensed phase is present, it coexists with vapor over a range of pressures2and the compositions of the vapor and the compound are correlated.1° For the HN03/H20system, for example, the trihydrate composition changes as it is varied from three-phase equilibrium with ice to three-phase equilibrium with the monohydrate. When the trihydrate and vapor equilibrium is established, there is a unique H20 and H N 0 3 pressure for each value of the composition of the trihydrate, and the range of pressures is bound by the coexistence curves for “ice”/trihydrate and trihydrate/monohydrate. As mentioned before, the experiment is being improved, and the behavior of the pressures of H 2 0 and H N 0 3 over the trihydrate will be investigated. In addition, a better determination of the coexistence curves will be made. These results will then be applied to the Antarctic stratosphere.
Acknowledgment. This work was supported by a Grant from NASA’s Upper Atmosphere Research Program. Registry No. HN03, 7697-37-2.
