April, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
40 1
Solubility of Nitrogen in Water at High Pressures and Temperatures' John B. Goodmanz and Norman W. Krase DEPARTMEKT OF CHEMISTRY.UNIVERSITY OF ILLINOIS, URBANA. ILL
A high-pressure gas-solubility apparatus has been in water and nitrogen-hydro8 PART of the prodeveloped which is suitable for the determination of gen mixtures in liquid ammogram for the collection of fundamental data gas solubilities at ordinary and elevated temperatures, nia show increasing solubility on compressed systems, this and at pressures from 100 to 1000 atmospheres. Cerwith temperature. Helium tain developments in solubility technic are presented, in water exhibits a minimum laboratory has begun a cornwhich are improvements over previous methods. solubility between 25" and prehensive study of the soluThe solubility of nitrogen in water has been measured 30" C. a t 1 atmosphere, and bilities of gases in liquids. at temperatures from 0' to 170" C. and at pressures hydrogen in water shows not Not only are such data inonly a minimum at 25" C. t e r e s t i n g in enlarging the of 100, 125, 200, and 300 atmospheres. The coefficient in Henry's law has been calculated for but a very unusual relationscope and testing the apthe several pressures and solubility theories tested ship with temperature. plication of existing theories These facts are safIicient to over wider ranges of temwith the aid of the new data. p e r a t u r e a n d pressure, call for further investigation but they are also necessary in the intelligent design and from a theoretical viewpoint a t least. The development of operation of many industrial processes employing high pres- high-pressure synthesis of ammonia and methanol and other sures. Practically no systematic measurements are re- processes where products are separated by condensation corded in the literature for even simple systems of industrial under pressure furnishes further pressing needs of solubility interest. The existing, fragmentary data, however, point data. The volume of gas dissolved in such condensed prodto this field as possessing special importance because of ucts is often very large. Most efficient operation calls for departures from the ordinary, ideal laws of gas solubility. an exact knowledge of such data. Since 1803 Henry's law has been generally accepted as Previous Work describing the effect of pressure on the solubilit'y of a gas in a liquid. Dalton'a amplification of this law has also served Investigation of the literature has shown the previous well for many purposes, although the number of exceptions seems to grow as more measurements are reported. Cer- published work on pressure solubilities to be very meager. tainly as 'ideal" laws, useful in cases where Polarity, association, chemical combination, and deviations from the perfect gas laws do not enter, they will continue to find a place in textbooks. The effect of temperature on gas solubility is usually connected with the thermal changes involved by means of tJhe Clausius-Clapeyron relation in such cases where it is considered valid. Below the critical temperature of the gas, therefore, this always indicates diminishing gas soh-. bility with increasing temperature. For gases above their critical temperature no satisfactoq generalizations are available, although it is cus-. tomary to reach the conclusion just stated for suck cases also. Travers (IO) makes the statement:
A
1
I'.,
i
1D
-,o
20
30
44
Ty.m+.n -C1"iq3. The curves representing the change of solubility with temperature, for gases under constant pressure, apFigure 1-Positive Temperature Coefficients of Solubility pear to indicate that for each gas there exists a point of minimum solubility and thaf the temperature corresponding to although many investigations have been carried out to test this is in some way related to the critical point for the gas. the validity of Henry's law. The systems considered usually
Very few data are offered to support this astonishing conclusion, and none involve common solvents such 11s water or gases considerably above their critical temperatures. It is not generally recognized that the existence of minimum solubility points is common. The logical interpretation of the phenomenon is difficult and will be discussed subsequently. For examples of systems showing radical departures from accepted theory, reference to Figure 1 is invited. Xeon Received January '16, 1931. Presented before the Division of Physical and Inorganic Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931. Submitted in partial fulfilment of the requirements f o r the degree of doctor of philosophy in chemistry in the Graduate School of the University of Illinois.
were treated a t low temperatures and a t but small tcmperature intervals. The pressures were low and seldom exceeding a few atmospheres. The solubility characteristics have been determined for a number of the inactive gases at ordinary pressures by numerous investigators. Just (5) determined the solubility of nitrogen and other gases in aqueous and non-aqueous solvents. Bohr and Bock (9)have shown data on the solubility of nitrogen, oxygen, hydrogen, and carbon dioxide in water a t temperatures from 0" to 100" C . Besides the work of Estreicher (4) and Von Antropoff ( I ) on the solubilities of helium! neon, and argon a t atmospheric pressure, there exist many notations of repeated work on nitrogen in water and in non-aqueous solvents.
402
INDUSTRIAL AND ENGINEERING CHEMISTRY Comprehensive solubility measurements a t higher pressures are likewise 1a c k i ng . The solubility measurements of Larsen and Black (7) on the system nitrogen-hydrogen-liquid ammonia are the only instances of recent determinations of high-pressure solubility. These data, while a t rather low temperatures, included pressures up to 150 atmospheres. The work of Cassuto (3) includes the solubility measurements of nitrogen in water a t pressures up to 10 atmospheres along with solubility determinations of hydrogen, oxygen, and carbon monoxide in water. In the consideration of solubility m e a s u r e m e n t s a t higher pressures, the work of Sander (9) on c a r b o n dioxide in water might also be included. This work embraced measurements taken up to 170 atmospheres. Von Wroblewski (1.9))in his determination of the hydrates of c a r b o n dioxide, took measurements up to 60 atmospheres.
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has been developed which permits the use of this procedure in determining gas solubilities in water up to 1000 atmospheres pressure and from 0" to 170" C. Solubility Apparatus
The main portion of the apparatus is, of course, the solubility pipet (Figure 2). This was constructed of chromevanadium steel with R liquid chamber l/z inch (1.3 cm.) in diameter and 5'/2 inches (14 cm.) long, the capacity being about 19 cc. The whole pipet was machined to a smooth finish and then completely copper-plated inside and out. The inner surface of the chamber was further plated with silver. These protective coatings were used in order to reduce corrosion and to prevent contamination of the solutions with iron. The magnetic stirrer was placed on top of the solubility pipet. The present design of this stirring device is unique as applied in this problem, although the original idea wm derived from an all-glass gas-circulating pump described by Porter, Bardwell, and Lind (8). The cylinder of the stirrer was constructed from a bar of nichrome IV; this alloy was chosen after experience with a manganese-bronze alloy showed that electrolytic corrosion in this section was serious. In the cylinder there was placed a piston of soft iron care-
Experimental Method
The present method is quite satisfactory for liquidphase determinations and is briefly as follows: The gas is stored in cylinders at a pressure above that used in the experiments, and is led, after purification, to the solubility apparatus proper. It first enters the saturator, which contains water saturated with gas a t room temperature and a t the particular pressure of the experiment. A portion of the solution is then transferred from a connection in the bottom of the saturator to the solubility pipet, which is surrounded by either a copper thermostat for high temperatures or an ice thermostat. While in the solubility pipet, the liquid solution is s t i r r e d by means of a magnetic stirrer, at the same time being subjected to the desired presFiQure 2-Solubility - Pipet . sure. After sufficient time has elapsed to bring the solution to equilibrium, a sample is removed by means of a needle valve a t the bottom of the solubility pipet. The sample removed is separated by means of a cooled trap into the gaseous and liquid constituents. The usual corrections for gas volume are applied to reduce the results to standard conditions. A satisfactory apparatus
Fi$ure 3-Assembled
Apparatus
fully turned but fitting somewhat loosely, there being about l/TZ-inch (0.8-mrn.) clearance. To insure free passage of the gas past the piston, two longitudinal grooves were cut in the piston; these grooves also permitted any liquid carried up into the stirrer from the lower section of the pipet to drain down again. At the lower end of the piston was attached a small bronze rod, which passed through the connecting nipple between the stirrer and the solubility pipet. At its lower end two small paddles were attached to induce circulation of the liquid in the pipet as the piston was drawn up into the cylinder. To actuate the magnetic stirrer a motor-driven interrupter was designed. In this work it was noted that measurements at the higher temperatures were considerably affected by temperature changes. The operation of a high-temperature thermostat is often troublesome, for hot-oil, molten-salt, or metal baths are a considerable hazard. To remove entirely the hazards and to improve further the temperature control, a copper
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1931
thermostat was used. The authors have used this style of thermostat in previous problems and found it completely satisfactory. The present thermostat was a cast block of copper 5 by 6 by 7 inches (12.7 by 15.2 by 17.8 em.). Three holes were bored through the block to accommodate the solubility pipet, the electrical heating unit, and the ther-
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10
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so
40
60
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10
10
so
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403
solubility apparatus. By means of a needle valve any desired quantity of gas could be admitted to the saturator. This was a steel cylinder 2 feet (61 em.) long, 2l/2 inches (6.3 em.) inside diameter, with a wall thickness of 1 inch (2.5 cm.). The solubility operations were facilitated by filling this vessel with about 1.5 liters of distilled water and then bubbling nitrogen through this liquid at a higher pressure than at which the experiments were t o be run. This gave a saturated solution of nitrogen in water at room temperature. When this solution was run into the solubility pipet, it simply remained to add or remove nitrogen in solution to correspond to the temperature of the experiment. This presaturating of the solution shortened the time of stirring in the pipet. The pressure on the solubility apparatus was indicated by a calibrated Bourdon tube gage. Although constanbreading, the Bourdon gage was not sufficiently accurate for measuring the pressures. The actual total pressures were measured by means of the dead-weight piston gage which was connected into the pressure line when so desired. This piston gage was of the type developed at the Massachusetts Institute of Technology (6).
110 110 I 3 0 lka IS0 160 I70
T.y&..-c."+"p&
Figure 4-Ahsorption
Coefficient-Temperature Isohars for Nitrogen-Water
mometer. The mass (of copper left after the necessary holes were bored was approximately 40 pounds (18.2 kg.). For measurements at 0" C. an ice thermostat was used, It was simply a large galvanized iron container, heavily lagged with hair felt and filled with cracked ice and water. The sampling valve connected to the bottom of the solubility pipet was of the usual design of high-pressure valve. The exposed interior sections of the valve body and stem were plated as much as was possible with copper to reduce corrosion. A11 the high-pressure tubing carrying either liquid solutions or gas was of chrome-molybdenum steel with copper lining. Auxiliary Apparatus
Figure 3 shows all necessary apparatus. Compressed nitrogen was expanded from the commercial gas cylinders down to about 5 pounds (0.35 atm.) pressure and led into the first stage of a Rix three-stage, water-cooled compressor, in which it was compressed to 4000 pounds (272 atm.) into the low-pressure storage cylinders or into the low-pressure cylinder of the high-pressure storage system. The compression mas continued in the high-pressure storage cylinders using a water piston and hydraulic pump. These cylinders were used to store the compressed nitrogen a t a pressure always above that used in the experiments. The connecting tubing used in the system is all of chromemolybdenum alloy steel, copper-lined. All gages used were calibrated against the laboratory piston gage taken as a standard. Ordinary hydraulic gages calibrated up to 10,000 pounds per square inch (680 atm.) were used for all low-pressure readings. The gas was purified as it was drawn out of the highpressure cylinders by passing over heated copper wire a t 450" C. It was then cooled by passing through a water-jacketed tube before entering the purifying tower. The absorbent used with nitrogen was soda lime, because this material removes any water in the gas as well as small traces of carbon dioxide. A line from the high-pressure storage system ran to the
Figure 5-Absorption
Coefficient-Pressure Isotherms of Nitrogen- Water
The solution sample, removed by means of the needle valve a t the lower end of the pipet, was collected in a weighed and cooled glass trap. As the dissolved gas was evolved it passed through a weighed calcium chloride tube and was collected over mercury in a buret. Several gas burets were used, the size depending on the volume of gas to be measured. All the necessary measurements, such as barometric pressure, gas temperature, etc., were recorded. Results
The experimental results, showing the solubility of nitrogen in water a t 100, 125, 200, and 300 atmospheres and at temperatures up to 169' C., are presented in Table I. Table I-Solubility of Nitrogen in PRESSURE 0' C. 25' C. 50' C. Afm. Cc. Cc. Cc. 1.07 1.003 100 1.46 1.24 1.44 125 1.76 2.49 2.76 200 3.19 2.99 3.25 300 3.60 l/T,'K.
Water a t Various Temperatures 80' C. 100' C. 144' C. 169' C. Cc. Cc. Cc. Cc. 1.025 1.08 0.934 0.954 1.30 1.52 1.15 1.17 3.29 2.25 2.68 2.27 3.46 3.83 2.86 2.91
0.00366 0.00336 0 . 0 0 3 1 0.00283 0 . 0 0 2 6 8 0 . 0 0 2 4 0 . 0 0 2 2 6
The data are shown graphically in Figure 4, where the absorption coefficient, cubic centimeters of gas at standard
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conditions per gram of water, is plotted against temperature. Figure 5 presents the solubility isotherms a t several temperatures. Figure 6 is t,he usual representation of the thermal characteristics by plotting the logarithm of the solubility coefficient against the reciprocal of the absolute temperature. T a b l e XI-Henry’s Law Constant ABSORPTION FUGACITY TEMP. PRESSURECOEFFICIENT K v Aim. Afm. c. 100 1.46 0.0146 97.43 0
Kf
200 300
3.19 3.60
0.0160 0.0120
195.4 302.6
0.0150 0.0163 0.0119
50
100 200 300
1.003 2.49 2.99
0.0100 0.0125 0.0150
101.0 207.0 326.5
0,0099 0.0120 0,0092
80
100 200 300
0.934 2.27 2.86
0.00934 0.0114 0.0143
102.0 212.5 334.1
0.00915 0.0107 0.0086
100
100 200 300
0.954 2.25 2.91
0.00954 0.0113 0.0145
102.8 214.3 338.7
0.0094 0.0105 0.0086
169
100 200 300
1.08 3.29 3.83
0.0108 0.0185 0.0128
104 217.8 344.0
0.0104 0.0151 0.0111
viscosity and suggested also that the thermal expansion of the solvent as measured by its specific volume change with temperature was a factor opposite in effect to viscosity. As a result these opposing factors a t some point neutralized each other and a minimum solubility point resulted. Careful study of these properties has convinced us that, while qualitatively the idea seems sound, the quantitative application to a particular system doubtless involves other factors also.
ab5
Discussion of Results
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3lb
Figure 6-Logarithmic
The variation of these results from the ideal law of Henry is shown in Table I1 and Figure 5. The Henry constant calculated both for pressures in atmospheres and for fugacities shows wide variations and no significant trends. In attempting to explain the peculiar properties of this system, many theories have been tested. I n general it seems best to attribute depaxtures from ideal behavior to some property of the solvent rather than to look for explanation in some peculiarity of nitrogen. I n this work the effects of solvent density, viscosity, internal pressure, surface tension, association, and compressibility were investigated with the help of such other data as were available. Interesting theories have been proposed from time t o time correlating one or the other of these solvent properties with solubility. Only one, however, has definitely recognized the existence of minimum solubility points for gases a t constant pressures. Winkler ( I 1 ) developed a relation connecting solubility and
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e
70s
$.
215
26s
2tF
226
to+
P l o t of Isobars for NitrogenWater
The results, in so far as fundamental solubility theory is concerned, lead one inevitably to the conclusion that many more data are necessary for complete understanding of these compressed systems. Literature Cited (1) Antropoff, v., 2. Elektrochem., 25, 269 (1919). (2) Bohr and Bock, A n n . phys. chem., 44, 319 (1891). (3) Cassuto, Nuooo cimcnfo, 6; 1903 (1913). (4) Estreicher, 2. physik. Chem., 31, 176 (1899). (5) Just, I b i d . , 37, 342 (1901). (6) Keyes and Dewey, M. I. T. Publication 179 (1927). (7) Larsen and Black, IND. END. CREM.,17, 715 (1925). (8) Porter, Bardwell, and Lind, Ibid.. 18, 1086 (1926). (9) Sander, 2. physik. Chcm., 7 8 , 513 (1911). (10) Travers, “Experimental Study of Gases,” p. 255, Macmillan, 1901 (11) Winkler, Z . p h y s i k . Chem., 9, 171 (1899). (12) Wroblewski, v . . W i d A n n . , 4, 268 (1879).
X-Ray Study of the Copper End of the Copper-Silver System’ Roy W. Drier DEPARTMBNT OI MEIALLUROP.MICHlQAN COLLBQE OF MIXINGAND TECHNOLOGY, HOUGHTON. MICH.
URING an x-ray investigation of the plastic deformation of copper, it was noticed that both the Hull and the Laue types of spectra contained lines other than those of copper. Upon analysis these were found to be lines from the silver spectrum. Inasmuch as the copper under investigation did not contain enough silver to be beyond the accepted solid-solution range for silver in copper, the appearance of the silver lines seemed quite worthy of investigation. Constitution diagrams (3, 4) of the copper-silver system all show that silver is soluble in copper up to about 5 per cent by weight. The diagram in the International Critical Tables has the silver-in-copper solubility line dotted from * Received June 26, 1930; revised paper received February 26, 1931.
D
the eutectic temperature, 778” C., down to room temperature, indicating that such solubility is questionable. It is generally accepted t h a t there are two types of solid solution. In the most common type the solute atoms replace the solvent atoms on the lattice of the solvent. Opinions differ as to whether the replacement is statistical or random, but whichever condition obtains the resulting solid solution is of the substitutional type. For a continuous series of solid solutions i t is necessary that the two pure components A and B crystallize on the same type of lattice. According to the additive law of Vegard (1) the lattice constant changes linearly with the composition in atomic per cent between the limiting values of the pure components. Copper and silver both crystallize on the face-centered
