Phase Relations of Nitric Acid at Physicochemical Equilibrium

The Physicochemical Properties of Pure Nitric Acid. S. Alexander Stern , J. T. Mullhaupt , and Webster B. Kay. Chemical Reviews 1960 60 (2), 185-207...
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Phase Relations of Nitric Acid at Physicochemical Equilibrium WEBSTER B. KAY AND S. ALEXANDER STERN The Ohio State University, Columbus, Ohio

I

T HAS been known for a long time t h a t nitric acid is ther-

mally unstable a t temperatures above its melting point and that it decomposes to form oxides of nitrogen, water, and oxygen, according to the over-all stoichiometric relation usually expressed by the equation 2HNOs = 2x02 HzO '/202

+

+

However, i t has not generally been known that when the liquid phase decomposition of nitric acid is allowed to take place in closed systems, considerable pressure may develop even a t room temperature. I n view of t h e wide use of highly concentrated nitric acid as a component of binary rocket propellants, a knowledge of the phase behavior of the acid a t physicochemical equilibrium with its decomposition products has become increasingly important. Reynolds and Taylor (8) were the first to report, in 1912, that pressures up t o 10 atmospheres were developed by the decomposition of nitric acid in sealed tubes a t room temperatures and that the magnitude of the pressure was dependent on the extent to which the tubes were filled with acid. These investigators did not follow the thermal decomposition to equilibrium and do not seem to have carried out their studies under controlled conditions. Apparently, no further work was done on this subject until recently, when Reamer, Corcoran, and Sage ( 7 ) determined the specific volume of nitric acid a t physical and chemical equilibrium between 71.1' C. (160" F.) and 171.1" C. (340' F.) and a t pressures up t o 5000 pounds per square inch.

Figure 1. Apparatus for preparation of pure nitric acid and transfer to experimental tube

procedure. The effect of the ratio of vapor volume to liquid volume on the rate of decomposition was also determined and, finally, the degree of decomposition of nitric acid was estimated. The results of this investigation are summarized in the present paper. PREPARATIOX OF PURE NITRIC ACID

Apparatus and Procedure. Pure nitric acid was prepared by the reaction of 100% sulfuric acid with anhydrous potassium nitrate in the all-glass apparatus shown in Figure 1. Sufficient amounts of reactants were charged to flasks A and B, respectively, each of 500-cc. capacity, to prepare stoichiometrically 100 grams of nitric acid. Previous to charging, the nitrate was finely powdered and heated a t 130" C.in an electric oven for a week. The I00 yo sulfuric acid was prepared by fortifying %yosulfuric acid with fuming sulfuric acid. All chemicals used were of reagent grade. During the preparation of the acid as well as later, during its transfer, the apparatus was covered with aluminum foil to prevent any possible photochemical decomposition of the acid vapors. The apparatus was evacuated by means of a mercury diffusion pump backed by a mechanical vacuum pump and the pressure was measured by a McLeod gage. When the pressure was less than could be read on the gage, or about 1 X mm. of mercury, the stopcock, F , was closed and by turning flask A around ground joint C, the acid was added, a few drops a t a time, to t h e nitrate in flask B. The nitric acid vapor formed was immediately transferred and collected in the solid state in container E, which was kept at the temperature of liquid nitrogen. Previous to this, both flasks were cooled to 0" C. by surrounding them with ice water in order to prevent possible thermal decomposition of the nitric acid formed. [From the recent data of Ellis and Murray ( 3 ) it appears, however, that the rate of decomposition of nitric acid vapor at room temperature is negligible.] Because the reaction was vigorous, bulb D,loosely packed with glass wool, was inserted in the line in order to prevent particles of powdered nitrate from being carried over into the receiver, E. When approximately 50 to 60 cc. of liquid acid had been produced, the reaction was stopped by cooling B with a dry iceacetone mixture and the acid-generating section was sealed off from the line at point G . The ground joint, C, as well as all other joints and stopcocks in the apparatus that were exposed to the acid vapors, was lubricated by an inert and nonvolatile perfluoro grease. CHARACTERIZATION OF ACID

I n 1951, a program was started in this laboratory to investigate systematically the factors affecting the stability of pure nitric acid. As a part of this program, the liquid phase decomposition of the pure acid was studied under isochoric conditions in the temperature range from 76" to 125" C. The reversibility of this reaction was established and the relations among the ratio of vapor volume to liquid volume, the equilibrium pressure, and the temperature were determined. The specific volumes of nitric acid at physicochemical equilibrium were measured a t pressures up to 1800 pounds per square inch for comparison with the data of Reamer, Corcoran, and Sage, who used a different experimental

The nitric acid produced by t h e above method was a white crystalline solid which melted to form a colorless liquid. It was characterized by chemical analysis and by a determination of its freezing point and orthobaric density. Total acidity, nitrogen dioxide content, and sulfur trioxide were determined by conventional methods of chemical analysis. A special method of taking samples of acid for t h e analysis was developed which prevented any contamination by external moisture or impurities. Several small, thin-walled bulbs, I (Figure I), about 15 mm. in diameter with long, capillary stems attached, were blown from

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GREAT PRACTICAL IMPORTANCE should b e attached to these data

. . . in

problems associated with handling and storage of highly concentrated nitric acid

borosilicate glass tubing, weighed, and sealed inside flask H which, in turn, was sealed to the vacuum line. The system was evacuated, after which stopcock J was closed, stopcock F opened, and sufficient nitric acid distilled into H to fill the sample bulbs partially, by cooling flask H with liquid nitrogen and bringing the acid in E to 0' C. Stopcock F was then closed and flask H sealed off from the vacuum line. A drying tube, filled with phosphorus pentoxide suspended on glass wool, was attached by means of plastic tubing to stopcock K and the moist air between t h e cock and the drying tube was pumped out. As soon as the acid in flask H melted, K was cautiously opened to admit a small amount of dry air. The pressure of the admitted air forced the liquid into the evacuated bulbs. H was then cut open, the sample bulbs were removed, and the capillary tips immediately sealed by means of a torch. The bulbs were washed, dried, and weighed, and the weight of the acid was determined by the difference between the full and the empty bulb. Each sample amounted to approximately 1.5 grams of nitric acid.

MANOSTAT

JET

Figure 2.

@,o

Apparatus for measurement of decomposition pressure

B y analysis, the sample was found to be 100.00 f 0.05% nitric acid, with no trace of oxides of nitrogen or sullur trioxide. The freezing point was measured in a magnetically stirred apparatus of standard design, by means of a calibrated copperconstantan thermocouple. The apparatus mas loaded with a sample of the prepared nitric acid by distillation under high 0.05' C., vacuum. The freezing point was found to be -41.60 in good agreement with the value of -41.62" C. found by Dunning and Nutt ( 1 ) and -41.59 ' C., found calorimetrically by Forsythe and Giauque ( 3 ) . Under the prevailing experimental conditions, the property measured actually represents the triple point of nitric acid. The orthobaric density was measured by means of an appropriate pycnometer which, with the procedure used, has been

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described (IO). The orthobaxic density thus obtained was expressed for the temperature range 0' t o 33" C. by the equation: d(grams per cc.) = 1.5492

- 0.00182 t

(' C.)

in very good agreement with the values reported in the literature. MEASUREMENT OF DECOMPOSITION PRESSURES

Experimental Method and Apparatus. For the measurement of the decomposition pressure of pure nitric acid, a small sample of the acid was maintained at a constant temperature and constant total volume in a glass tube until physical and chemical equilibria were established. The apparatus employed was similar to that described in connection with measurements of vapor pressures of organic liquids u p to their critical points (4,6). Referring to Figure 2, a small air-free sample of pure nitric acid was confined in the thick-walled glass tube, 1, over a high boiling fluorocarbon liquid (Fluorolube oil S, Hooker Electrochemical Co.) which was immiscible and chemically inert to the nitric acid. Tube 1, the experimental tube, was immersed in a Fluorolube oil bath which was contained in jacket 2. The oil was heated by the vapors of a series of organic liquids vaporized in the side-arm boiling flask, 3, by the electric heater, 4. The pressure on the boiling liquid, and therefore the temperature of the vapor, were controlled b y means of a pressure regulator connected through a condenser, 5, to the side arm a t the top of the constant temperature jacket. I n this manner, the temperature of the Fluorolube oil surrounding t h e experimental tube was kept constant to within 0.05' C., as measured b y a calibrated mercury thermometer marked in 0.1 ' C. divisions. The experimental tube was fastened in the compressor block, 6, which was partially filled with mercury. This, in turn, was connected by copper tubing, through the manifold, to the compressed gas tank, 7 . Pressure adjustments on the sample in the tube were made b y the addition of high pressure gas from the tank or by releasing the gas through the exhaust valve, 8. The surge tank, 9, served as a pressure stabilizer by increasing the volume of t h e system. The pressure was indicated by the precision, Bourdon-type gage, 10. I t s 16-inch dial covered the range from 0 to 2000 pounds per square inch and was handmarked in 2-pound divisions. The gage was tested a t 20-pound intervals with a precision dead-weight gage tester. Pressures were read to 1.0 pound per square inch. T o assure equilibrium between t h e liquid and vapor phases, the sample was stirred b y a glass-covered iron stirrer of dumbbell shape which was moved up and down by means of a cylindrical permanent magnet, 11, around the outside of the experimental tube. The magnet was suspended by means of a fine Chrome1 wire and a string over a pulley, and was kept in continuous up and down motion by an automobile windshield wiper, 12. The constant motion of the magnet in the Fluorolube oil bath also served to stir the bath and to produce a uniform temperature in the jacket. The dimensions of the experimental tube are shown in Figure 3. The tube was constructed of borosilicate glass. The upper 150 mm. constituted the working section wherein the acid sample was confined. The total volume of this section to an etched reference line, as well as the volume of the tube in terms of the distance from a chosen mark, was carefully determined by calibration with mercury. By using a cathetometer reading to 0.05 mm. to measure this distance, the volume of the separate phases could be determined accurately to within 0.0005 cc. I n order to reduce t h e diffusion and solution of the nitric acid into the Fluorolube, a section of capillary 15 cm. long and 1.0mm. bore with an outside diameter of 8.5 mm. was interposed between t h e sample chamber and the lower end of t h e tube. Losses of the sample due to diffusion, which were measured by determining the change in the equilibrium pressure with time,

INDUSTRIAL AND ENGINEERING CHEMISTRY

July 1955 were found t o be insignificant even under the most severe conditions. For example, a decrease of only 2 pounds per square inch in an equilibrium pressure of 1700 pounds per square inch absolute a t 125' C. was noted in 6 hours when the nitric acid was subjected t o a heating cycle similar to that shown in Figure 4, in the temperature range from 105" to 125' C. Loading Sample in Experimental Tube. The experimental tube was loaded with a sample of acid calculated to give the desired vapor-liquid ratio a t the reference temperature of 0' C. The loading o p e r a t i o n w a s carried out by means of the allglass high vacuum system used for the generation of pure nitric acid (Figure I).

o^

) (z T I 0

.'- ..f f

9,OMM. 0.0. 3MM. I.D.

+ I 0

P-

5

1.0MM. 1.D Y 8.5 MM. O.D.

REFERENCE LINE FOR A C I D - O I L INTERFACE

.-d .-c

.-c

Onoe the tube was filled with the desired amount of sample, stopcock F was closed and the acid was frozen. Bulb M was then rotated around joint N , so t h a t the Fluorolube oil flowed into the experimental tube and filled its capillary section to a mark indicated in Figure 1. The tube was then sealed off from the line and the tip kept immersed in liquid nitrogen until ready to start the decomposition run. Transfer of Loaded Tube to Compressor. In preparation for the transfer of the experimental tube to the pressure apparatus (Figure 2), the acking assembly for holding the tube was put in place on the tuge and the compressor block filled with mercury. Next, the constriction in the tube, just below the sealed end, was scratched with a file and the oil in this section of the tube frozen with liquid nitrogen using a specially designed cooler. The tube was inserted in the leg of the compressor block, and broken off a t the constriction, under the surface of the mercury. The Fluorolube oil in the tube was thus brought in direct contact with the mercury. The tube was fastened in place and the mercury in the back leg of the compressor block was adjusted to the operating level. The back leg was then closed, the copper tube attached, and a pressure of a proximately 5 pounds per square inch applied to the sample. T i e acid sample was melted, care being taken that the melting progressed from the oil-acid interface upward to the tip of the tube.

B

Before transfer of a sample to the experimental tube was begun, the bulb, M , was charged with about 5 cc. of Fluorolube GLASS COVERED S. Stopcock L was opened and the Fluorolube oil in M was degassed by pumping on the li uid Figure 3. Experimenuntil the pressure was less %an 1 X 1 0 - 6 m m . of m e r c u r y . tal tube Stopcock J to the vacuum pump was then closed, stopcock F opened, and the liquid nitrogen flask removed from around the acid storage bulb, E. The acid was allowed to melt and an icewater bath brought up around flask E to hold the temperature a t 0" C. A sample of nitric acid was then transferred by distillation and frozen in the tip of the experimental tube by immersing the latter in a liquid nitrogen bath. The amount of sample transferred was measured by determining the volume of the acid in the experimental tube a t 0" C.

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EXPERIMENTAL PROCEDURE

Referring t o Figure 2, the constant temperature heating jacket was placed around the tube with only the working section of the tube in the heated zone. Mercury was added to cover the rubber stopper a t the bottom and Fluorolube oil added to fill the jacket to about 3 inches above the tip of the experimental tube. The boiler was charged with a suitable boiling liquid through the side tube a t the top of the jacket and the condenser and pressure regulator were attached to the side tube. The electric heater was turned on and the Fluorolube oil bath brought up to the desired temperature. After a short period of time, depending on the temperature, a gas phase started to form in the sample due to the decomposition of the acid, and the increasing decomposition pressure caused the total volume occupied by the sample to expand rapidly. When the acid-oil interface reached the reference line etched on the narrow capillary, just below the sample section (Figure 3), the expansion was stopped by counterbalancing the decomposition pressure with gas pressure from the gas cylinder (Figure 2). The interface was held on the line by

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3000

3400

3800

4200

Time i n M i n u t e s

Figure 5.

Effect of stirring on decomposition pressure of

constantly adjusting the applied pressure. The magnetic stirrer was then put into operation. Pressure and temperature readings were made a t regular intervals of time until the pressure became constant. T o be certain t h a t a constant pressure had been attained, the readings were continued for a period of time, a t least equal t o t h a t necessary for t h e constant pressure t o be established. T h e time measurements were made using a 10-inch dial mechanical clock with a sweep second hand and the intervals were measured within 3 seconds of the appointed time. At intervals during the run and a t the end of t h e run the length of the tube occupied by the vapor phase was carefully measured, corrected for the height of the meniscus, and converted into volume by means of the volumetric calibration of the tube. As the total volume occupied by the sample was known, the ratio of the vapor volume t o the total volume, V G / V ,for the sample was calculated. T h e absolute decomposition pressure was obtained by correcting the recorded gage pressure for: (1) deviations of the gage readings from the true pressure; ( 2 ) the barometric pressure; 0.800 (3) the pressure equivalent due t o the difference in height between the Fluorolube oil-acid interface and the mercury level in the back leg of the compressor block. For this correction the 0.6CO density of the Fluorolube oil was determined and its relation to the temperature expreased by the following equation: d(grams per cc.) = 1.996

-

+

1.89 X t 3.5 x 10--6t2("C.)

for the temperature range 12' t o 21" C. The vapor pressure of the Fluorolube was negligible. Upon attaining a constant pressure, the run was either terminated or the temperature was changed and the run continued a t the new temperature, the total volume of- the sample being kept constant. T h e procedure employed in taking the data as well as the performance of the apparatus is clearly illustrated by the curve in Figure 4. The sample was subjected, without interruption, to

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the heating cycle shown; sufficient time was allowed a t each temperature, except a t room temperature, for physical and chemical equilibrium to be attained. The rate of reaction a t room temperature was too slow for chemical equilibrium t o be reached in a reasonable period of time. The reversibility of the decomposition reaction is shown by the fact t h a t the equilibrium pressure was the same regardless of whether it was approached from a lower or higher temperature. The importance of thoroughly mixing the liquid and vapor phases is illustrated by the curve in Figure 5, where it is seen t h a t when the stirrer was stopped and the experimental conditions were changed, the observed pressure exceeded the equilibrium pressure by approximately 250 pounds per square inch. The rate of re-establishment of equilibrium was very slow, probably being limited by the rate of diffusion of oxygen back into solution. When the stirrer was started again the previous equilibrium state was rapidly restored. Tests were made t o find whether the measurenitric acid ments were subject to a scaIe-effect-Le., whether the equilibrium pressures depended on the size of t h e sample used. For this purpose the volume of the tubes used was varied by a factor of 8 without any such effect becoming apparent. The equilibrium pressure was found to be dependent only on the VG/Vratio and temperature for an initial sample consisting of pure nitric acid. Tests were also made t o determine the chemical inertness of the Fluorolube oil to concentrated nitric acid. A sample of the Fluorolube oil was heated for 3 hours with concentrated nitric acid (90% "01, 10% NOz) at 125" C., the highest temperature a t which decomposition studies were conducted, after which the oil layer was separated from the acid layer a t room temperature and its refractive index measured. No difference in the refractive index of the treated and untreated Fluorolube oil was found. The acid layer was analyzed for fluorine which might have re-

0.400

0.200

0 0

400

I200

1600

2000

EQUILIBRIUM PRESSURE, PSIA

Figure 6.

Isothermal relation between V 5 / V ratio and equilibrium pressure of nitric acid

=

INDUSTRIAL AND ENGINEERING CHEMISTRY

July 1955

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Table I. Equilibrium, Pressure Lb./Sq. In'ch 200 300 400 600 800 1000 1200 1400 1600

Smoothed Values of V G / V and Specific Volume of Nitric Acid at Physicochemical Equilibrium

76 0' C. Spec. vol., cu. ft./lb. 0.03428 0.02388 0.01986

VQ/V 0.690 0.558 0.460 0.319 0,229 0.169 0.125 0.092 0.066"

0.01612

0.01419 0.01309 0.01246a

... ...

85.0' C. Spec. vol., cu. ft./lb. 0,03855 0.02588 0.02118 0,01667 0,01459 0.01342 0 , 01277a

VQ/V 0,726 0,590 0.487 0,340 0,245 0.180 0.133 0.098 0 , 06ga

... ...

95.0' C. Spec. rol cu. ft./lb.' 0,04837 0.02909 0.02289 0,01767 0,01826 0.01389 0.01303 0.01254'

VQ/V 0.780 0.637 0.525 0.368 0.264 0.193 0.143 0.105 0,074=

105,OOC. Spec. vol., CU. ft./lb. 0.06763 0,03587 0.02588 0,01875 0.01442 0.01594

VQ/V 0,846 0.694 0.573 0.401 0.288 0.209 0.153 0.111 0.078

0.01348 0.01292

...

115.0°C. Spec vol cu. it./lb.' 0.1148 0,03048 0.04571

W/V 0.934 0,774 0.642 0.445 0.319 0.234 0.171 0.123 0.08V

0.02048 0.01695 0.01511 0.01400 0.01329 0.01282a

125 0°C. Spec. vol., cu. ft./lb.

VQ/V

0 : 867 0.726 0.511 0.364 0.266 0.195 0.141 0.098

0.O8i28 0.03886 0.02327 0.01833 0.01603 0.01468 0,01380 0.01320

' Extrapolated values.

sulted from a possible reaction between the oil and the nitric acid. No fluorine could be detected to within 1 part in 10,000. I n a second test the Fluorolube oil which had been in contact with nitric acid in the experimental tube during a decomposition run at 1.25' C. was removed from the tube and the tmro layers were separated in a centrifuge. The Fluorolube oil layer was analyzed by means of an infrared spectrophotometer. The spectrum of this layer was found to be identical with that of the untreated oil. As the results of these tests were completely negative and the experimentally measured decomposition pressures were perfectly reproducible and consistent, i t was concluded that the nitric acid did not react with the Fluorolube under the conditions covered in this investigation. VOLUMETRIC EQUILIBRIUM RELATIONS

perimental techniques very different from those employed in this laboratory. For a given specific volume and temperature, the equilibrium pressure of Reamer, Corcoran, and Sage is somewhat lower than that obtained in this investigation. The deviation, however, is relatively small and is made less significant by the fact that their acid may have contained, as stated by the authors, up to 0.5% by weight of impurities. TIiME REQUIRED FOR PURE XITRIC ACID TO REACH EQUI LIBRI UM

The rate curves, shown in Figure 8 for 76" C. and different values of V G / V are , typical of the pressure-time data obtaired in the course of this investigation. From these curves the time required for equilibrium to be attained-that is, when the pressure became constant-was computed. Zero time was obtained by extrapolation to the vapor pressure of the pure acid a t the temperature concerned. The dependence of tEe equilibrium time on the V G / Vratio is clearly shown. From constant temperature plots similar to that for 76" C., data were collected for the construction of the curves (Figure 9) showing the relation between equilibrium time and temperature a t constant V G / Vratios. The effect of the VG/Vratio on the equilibrium time is appreciable a t the lower temperatures but decreases with increasing temperature and becomes relatively insignificant a t temperatures above 125" C. Below 70" C. the rate of decomposition was so slow that i t was

The equilibrium decomposition pressure of pure nitric acid was measured at 76", 85', 95O, 105', 115", and 125" C. over a range of ratios of vapor volume to total volume, V G / V ,from approximately 0.07 to 0.95. The experimental results are shown graphically in Figures 6 and 7 . From large scale plots values of the V G / Vratio and the specific volume were read at even pressures for the temperatures mentioned above and are tabulated in Table I. The original experimental data are available (6). The points on each of the curves were obtained by starting either with an undecomposed sample or with a sample which had previously been decomposed a t a higher or lower temperature, as has been described. The degree to which they fall on smooth isothermal curves, regardless of the manner in which equilibrium was reached, is indicative of the consistency of the data. The great sensitivity of t h e pressure to the VG/V ratio (Figure 6) is due to the nature of the heterogeneous mixture which results from the decomposition of the nitric acid. The mixture, under the pressure and temperature conditions, consists of a liquid phase composed of water and oxides of nitroREAMER, CORCORAN, S A gen dissolved in nitric acid, while the vapor phase is made up, principally, of oxygen which has a very small solubility in the liquid phase. Hence, a decrease in the volume of the gas phase is accompanied by a large change in the pressure. I n Figure 7 the experimental P-V-T data have been plotted as a series of P-V isotherms, where V is the specific volume of the heterogeneous system. Included on the graph are some of the data on nitric EQUI L I B RlUM PRESSURE ( P S I A ) acid recently p u b l i s h e d b y R e a m e r , Corcoran, and Sage ( 7 ) . The latter were Figure 7. Isothermal relation between specific volume and equilibrium pressure of nitric acid obtained with an apparatus and ex-

Vol. 47, No. 7

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10,000

8,000 6,000

4,000

I

20 -

Figure 8.

I

I

1

I

Decomposition rate curves of nitric acid at

76.0" C.

not feasible to determine the equilibrium time by the experimental methods employed. ESTIMATION O F DEGREE OF DECOMPOSITION O F PURE NITRIC ACID

Using the data obtained in the present study, the degree of decomposition, CY, of pure nitric acid was estimated on the basis of the following considerations: If the stoichiometry of the heterogeneous reaction studied is taken as

2HXOa = 2N02

+ HzO + '/zOz

noz = n&

+ n& = 7

I

I

-1

TEMPERATURE OC

Figure 9. Relation between time required to reach physicochemical equilibrium and temperature

VQ = volume of vapor phase a t equilibrium ko, = C o ~ / p o Henry's ~, law constant for solubility of oxygen in liquid phase of equilibrium composition and temperature T Now, the initial number of moles of undecomposed nitric acid is

(4)

(1)

where NO2 designates an equilibrium mixture 'of NOz and NrO4, the oxygen balance in the system a t equilibrium can be written a8 n2a

I

where Cy: = initial concentration of nitric acid

(2)

!vhere the symbols have the following meaning : no2 = total number of moles of oxygen in

the system a t equilibrium, a t pressure P and temperature T n& = number of moles of oxygen in liquid phase n;, = number of moles of oxygen in gas phase n; = initial number of moles of acid Equation 2 may be rewritten as

where

%&,

concentration of oxygen in liquid phase PO, pressure of oxygen in vapor .- = partial phase Zo2 = compressibility factor of oxygen a t pressure PO, and temperature T V L = volume of liquid phase a t equilibrium =

Figure 10.

Relation between reciprocal of equilibrium pressure and V a / V Lratio for nitric acid

July 1955

INDUSTRIAL AND ENGINEERING CHEMISTRY

VLo = initial volume of undecomposed acid d: = density of pure acid of volume VLo Ma = molecular weight of nitric acid = 63.02

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Table 11. Calculated Values of Degree of Decomposition of Pure Nitric Acid and Henry’s Law Constant for Oxygen in Nitric Acid a t Physicochemical Equilibrium

Substituting Equation 4 in Equation 3 and rearranging, one obtains

The compressibility factor, Zo,, was found from generalized compressibility charts to be very close to unity for the conditions ~ be considered as equal to the total presinvestigated. p o can sure, P, of the system, as the partial pressures of all constituents except oxygen are relatively small. This approximation will be better the higher the total pressure. However, this equation cannot be used directly for the calculation of a:, because ko, is not known for experimental conditions. Writing V L / V L o= r, and rearranging Equation 5 with 20,taken as unity one obtains

The ratio r in the equation was found to vary by no more than 1% at pressures above 400 to 600 pounds per square inch, depending on the temperature. Since an inspection of the data indicated t h a t a: is constant or changes very little in the same ) be linear for a pressure range, a plot of 1 / P vs. ( V G / V Lshould given temperature. This appears to be the case in the high pressure range, as shown by Figure 10. Therefore, Equation 6 may be considered as that of a straight line for the conditions stated and the degree of decomposition and Henry’s law constant can then be calculated from the slope and the intercept, respectively. These calculations have been made and the results are summarized in Table 11. The calculated values of Henry’s law constant were found to be consistent with the experimental values of Robertson ( 9 ) , in the temperature region in which the data overlap. The possi, satisfy the bility t h a t a: and ko, are functions of V G / V Lwhich condition

(7) required for the linearity of a plot of l / P us. ( V c / V L ) ,cannot be completely excluded a t present. The nature of the apparatus precluded it quantitative determination of the various species at equilibrium from which the degree of decomposition could have been determined rigorously. ACKNOWLEDGME1T

This paper is based upon work performed under Contract AF33( 038)10381 with Wright Air Development Center, WrightPatterson Air Force Base. The assistance of the following is gratefully acknowledged: Robert Wilson, who helped in the early exploratory work; Chris Elenniss and Michael Tallarico, who made many of the observations; Manoj Kumar Sanghvi, who helped with the calculations and observations; and James Watters for spectroscopic analyses. The authors acknowledge with pleasure the cooperation of the Hooker Electrochemical Co. in making available samples of Fluorolubes for test. The high vacuum perfluoro stopcock

T, C.

a

koa, Mole/ Liter Atm. x 103

grease was kindly supplied through the courtesy of E. I. du Pont de Nemours & Co., Inc. Thanks are due the Indiana Steel Products Co. for assistance in connection with the magnetic stirrer used to stir the acid. NOMENCLATURE a: = degree of dissociation of nitric acid C,”, = concentration of oxygen in liquid phase Cg = initial concentration of nitric acid d9 = density of pure nitric acid ko, = Henry’s law constant for solubility of oxygen in liquid phase of equilibrium composition a t total pressure and temperature M A = molecular weight of nitric acid nF: = initial number of moles of nitric acid = total number of moles of oxygen in system a t equilibrium = number of moles of oxygen in gas phase = number of moles of oxygen in liquid phase = total pressure on system = partial pressure of oxygen in gas phase = ratio of liquid volume at equilibrium to initial liquid volume = universal gas constant = temperature, C. = total volume of system = volume of liquid phase = volume of vapor phase = initial volume of liquid phase = compressibility factor for oxygen

LITERATURE CITED (1) Dunning, W. J., and Nutt, C. W., Trans. Faraday SOC.,47, 15 (1951). (2) Ellis, W. R., and Murray, E. C., J. A p p l . C h e m . , 3 , 318 (1953). (3) Forsythe, W. R., and Giauque, W. F., J . Am. Chem. SOC.,64, 148, 3069 (1942); 65, 2479 (1943). (4) Kay, W. B., Ibid.,69, 1273 (1947). (5) Kay, W. B., and Rambosek, G. M., IND.ENG.CHEM.,45, 221 (1953). (6) Kay, W. B., and Stern, S. A., Am. Doc. Inst., Library of Congress, Washington, D. C., Doc. 4507. (7) Reamer, H. H., Corcoran, W. H., and Sage, B. H., IND. ENG. CHEM., 4 5 , 2 6 9 9 (1953). (8) Reynolds, W. C., and Taylor, W. H., J . C h e m . Soc., 101, 131 (19 12). (9) Robertson, G. D., thesis, California Institute of Technology, Pasadena, Calif.. 1953. (10) Stern, S. A., and Kay, W. B., J . Am. C h e m . Soc., 7 6 , 5 3 5 3 (1954). RECEIVEDfor review September 3, 1954. ACCEPTED December 17, 1964. Material supplementary to this article has been deposited a s Document No. 4507 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured b y citing the document number and b y remitting $1.25 for photoprints or 31.25 for 35-mm. microfilm. Advance payment is required. Make checks or money orders payable to Chief, Photoduplication Senrice, Library of Congress.