Volumetric and Phase Behavior of the Nitric Acid-Nitrogen Dioxide System W. H. CORCORAN, H. H. REA-MER, AND B. H. SAGE California Institute of Technology, Pasadena, Calif.
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XPERIMENTAL information concerning the behaviw of mixtures of nitric acid and nitrogen dioxide is not abundant. Klemenc and Rupp ( 6 ) established the effect of composition in this binary system upon the specific volume a t atmospheric pressure. Klemenc and Spiess (6') determined the composition of the liquids in the three-phase region. A critical mixing temperature of 142' F. was found for the two liquid phases and the corresponding composition was 0.686 weight fraction nitrogen dioxide. At 40" F. two liquid phases coexisted between 0.53 and 0.95 weight fraction nitrogen dioxide. Data for the electrolytic conductance for the liquid phases of the nitric acid-nitrogen dioxide system were obtained a t 32" F. and atmospheric pressure a t compositions containing less than 0.50 weight fraction nitrogen dioxide (12). Many of the physicochemical properties of pure nitric acid have been investigated. Yost and Russell (19) reviewed many of the earlier data. Veley and Manley ( 1 7 ) measured the electrolytic conductance and specific weight of nitric acid a t atmospheric pressure. Taylor ( 1 6 ) determined its fugacity in aqueous solutions, and Forsythe and Giauque ( $ ) reported several other therniodynamic properties of the acid. The volumetric behavior of nitric acid was investigated a t physical and chemical equilibrium for temperatures between 160' and 340" F. (9). Equilibrium bubble point pressures of this compound were determined a t seven temperatures in this range. The cited measurements serve to establish the properties of nitric acid with an accuracy adequate for the present investigation. The volumetric and phase behavior of nitrogen dioxide mas investigated in some detail (11, 16). The data obtained appear adequate to describe the behavior of this compound for the present study and are in good agreement with earlier measurements of vapor pressure ( 1 4 ) and specific volumk (8, I!?). Throughout this discussion the term nitrogen dioxide is used to designate equilibrium mixtures of nitrogen dioxide and nitrogen tetroxide. Available information concerning the volumetric behavior of commercial samples of red and white fuming nitric acid (10) indicated that small quantities of nitrogen dioxide markedly lowered the bubble point pressures of mixtures of nitric acid, nitrogen dioxidr, and water a t chemical and physical equilibrium. The present work relates to studies of the volumetric and phase behavior of four binary mixtures of nitric acid and nitrogen dioxide. The measurements were carried out a t temperatures from 160" to 340" F. and for pressures up to 5000 pounda per square inch absolute. All the data were taken a t both physical and chemical equilibrium. M E T H O D S AND APPARATUS
The equipment and methods employed in this study have been described (9). I n principle, the apparatus is similar to that used by Beattie ( 1 ) for the study of the compressibility of gases, except that provision is made for the agitation of the sample. In order to avoid chemical reaction between the metal parts of the container and the nitric acid, the sample was confined within a glass pistoncylinder combination ( 8 ) which in turn was surrounded by a fluorinated liquid hydrocarbon. The glass piston-cylinder combination and the fluorinated hydrocarbon were confined within a stainless-steel pressure vessel, and the effective volume available
to the sample under investigation was varied by the introduction or withdrawal of the fluorinated hydrocarbon by mean6 of a mechanical injector. Experience with this equipment indicated that the specific volume of the sample may be ascei tained with a probable error of 0.6%. This measure of error includes the determination of the weight of the sample introduced into the glass vessel, which was small in comparisoq to the measurement of the total volume of the system. The sample of nitric acid was introduced into the evacuated vessel by a volumetric procedure (9). Nitrogen dioxide was added to the nitric acid by conventional weighing-bomb techniques (13). The composition of the sample introduced was known within 0.002 weight fraction nitrogen dioxide. Earlier measurements of the specific weight of mixtures of nitrogen dioxide and nitric acid a t atmospheric pressure (6) were in agreement with values obtained with the present equipment. Pressures were measured by means of a balance ( I S ) which wa8 calibrated against the vapor pressure of carbon dioxide a t the ice point. The pressure within the working section was known within 1 pound per square inch or 0.2%, whichever was the larger measure of uncertainty. Temperature was determined by means of a platinum resistance thermometer which was compared with a similar instrument t h a t had been calibrated a t t h e National Bureau of Standards. Throughout the range of conditions investigated, the temperature of the sample was known within 0.1" F. of the international platinum scale. The attainment of equilibrium was checked in a number of instances by the approach to a specific state with an increase or decrease in the total volume of the sample. In each case the properties of the sample a t equilibrium were found to be independent of the approach to the state. The details of these experimental techniques were the same as those used in the investigation of nitric acid ( 0 , I O ) . ,MATERIALS
Xitric acid was prepared from recrystallized potassium nitrate and pure sulfuric acid in accordance with the method described by Forsythe and Giauque ( 4 ) . After condensation it was redistilled a t pressures less than 0.01 inch of mercury. Samples of the acid employed yielded a specific weight of 94.232 pounds per cubic foot a t 70" F., whereas the value reported by Veley and Manley ( 1 7 ) was 94.250 pounds per cubic foot. Specific conductance of samples of nitric acid prepared in the same fashion reciprocal ohm-em. ( 1 2 ) . These measurements was 3.77 X indicate that there wag less than 0.05 weight per cent of material other than nitric acid in the samples employed. Nitrogen dioxide was obtained from the Allied Chemical and Dye Corp. It was fractionated in a glass column provided with I6 plates and operated a t a reflux ratio of approximately 12. Initial and final 15% portions of the overhead were discarded. The central fraction was dried over phosphorus pentoxide and frozen at liquid air temperatures. To aid in the removal of volatile impurities, the pressure was kept below 0.001 inch of mercury. After purification the nitrogen dioxide was stored in stainless-steel containers until use. EXPERIMENTAL RESULTS
I n a number of instances the approach to chemical and physical equilibrium was followed in order t o determine semiquantitatively the effect of the composition of the system upon the rate of attainment of chemical equilibrium. It was found that physical equilibrium between the phases was attained in 3 or 4 2541
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INDUSTRIAL AND ENGINEERING CIIEMISTRY
Vol. 46, No. 12
dioxide n-as typical of the transient behavior found for t'his system. No effort was niade to interpret data such as are shown in Figure 1 = o 0 1 4 8 7 c uF T / L B in terms of tlie kinetics of the probable chemical -n = o 2622 wr FR N I T R O G E N D I O X I D E reactions. The equilibrium relations of pressure to 220 specific volume were independent of the history of the sample. Their dependence upon z a only temperature and composition is considered to be an indication of the attainment of both _I physical and chemical equilibrium. 200 The volumetric behavior of four mixtures w U of nitrogen dioxide and nitric acid was investi3 YI m gated a t pressures up t o 5000 pounds per square U a inch in the temperaturc int,erval between 160' and 340" F. I n most cases the specific volume . was determined as a function of presmre a t 180 chemical and physical equilibrium a t six temperatures above 160" F. Experimental measurements upon two of the four mixtures are presented in Figures 2 and 3. The results I I I presented are typical of the experimental I I 5000 ib,ooo 13,000 data obtained for the other two mixtures. I n order to show the behavior near bubble TIME SEC Point' in great'er detail, the data for large Figure 1. Approach to Equilibrium under Isochoric Condjtions at 190' F. specific volumes have not been included. Curves drawn through tlie experimental point8 resulted from smoothing with respect t o minutes of agit,ation after a change in state a t a constant tenipressure, temperature, and composition. The standard and perature. The observed gradual approach over a period of average deviations in specific volunie for all the experimental data points are recorded in Table I. These comparisons indicate hours a t 190" F. toward a final equilibrium \vas attributed to the that the standard deviation of the smoothed specific volume from rearrangement of the molecular species in the system. Rates the experimental data was approximately 0.9%. The experiof attainment of physical equilibrium \
