February 15, 1942
ANALYTICAL EDITION
TABLEVI. SOLUBILITY OF SULFUR DIOXIDEIN BENZENE Temperature of Bomb
c.
26 26 27 25
T o t a l Pressure Atmospheres (abs.) 1.20
Solubility
G. SO2 dioxide/lOO
8.
CaHs
24.7 37.6 84.8
1.75 2.40
125.5
2.65
Conclusions The results obtained show that the modified rotating bomb can be used to determine the Of gases at high pressures and high temperatures approaching the critical. The method applied to the determination of the solubility of methane in benzene indicates that pressure is the principal factor affecting the solubility. The effect of temperature is minor, becoming more noticeable a t higher pressures.
171
The solubility determinations of propane in benzene indicate that both temperature and pressure have a marked influence on the solubility.
Acknowledgment The authors wish to express their appreciation to Charles RohEng and Lee Fischer for assistance rendered on these experiments. Literature Cited (1) Ipatieff,V. V., Jr., Feodorovitch, V. P., and Levine, I. M., oil G ~ S J., 32,No. 20, 14 (1933). (2) Ipatieff, V. V., Jr., and Tichomroff, V. T., J . Gen.Chem. (U. S. S. R.), 1, 736 (1931). (3) Sage, Webster, and Lacey, IND.ENG.CHEM.,28, 1047 (1936). (4) Webb, G. M., Universal Oil Products Co., unpublished data, PRESENTED before t h e Division of Petroleum Chemistry a t t h e 102nd MeetCHEMICAL SOCIETY,Atlantic City, N. J. ing of t h e AMERICAN
Determination of Critical Temperatures by the Rotating Bomb V.
N. IPATIEFF
A>I)
G. S. &IONROE, Universal Oil Products Company, Riverside, Ill.
T
HE methods usually employed in determining critical temperatures involve the determination of the pressurevolume relation along various isotherms or the appearance or disappearance of the meniscus a t the phase boundary a t the critical temperature. These procedures, though capable of giving a high degree of accuracy, require special apparatus and skillful technique. In cases where an accuracy of only +2' C. is required, a method involving less elaborate apparatus would be desirable. It mas with this in mind that the authors developed a method for determining critical temperatures by the rotating bomb. A study was made of the pressure-temperature curves of various pure substances to ascertain if any change occurred in the pressure-temperature relation in the region of the critical temperature that could he employed in determining criti-
TABLEI.
COUPARISON O F POINTS O F
Substance Amount charged cc. Temperature of break, ' C. Critical temp. (literature), e
c.
Test samples: Above break temp., ' C. Liquid-phase sampling cock, cc. liquid/100 cc. gas Vapor phase sampling cock, cc. liquid/100 cc. gas Below break temp., O C. Liquid - phase sampling cock, cc. liquid/100 cc. gas Vapor pha?e sampling cock, cc. liquid/100 cc.
-
-
Curve 0
gas
Xethane, 27 mole % Benzene, 73 mole yo Temperature range, 62' t o 261' C. Benzene, 70 mole Propane, 30 mole % Temperature range, 94' t o 256' C. Hexane "practical", 32 weight Benzene, 68 weight Temperature range, 120' t o 288' C.
9
DISCOSTINUITY ON PRESSURE-TEMPERATURE CURVES OF PURE SUBSTANCES
'Determined b y t h e rotating bomb with critioal temperatures obtained from hterature) Hexane ProHexane "PractiCycloPropane "Practical" Cyclohexane pane H2 cal" Hz hexane Hz Benzene7 Benzene .H -r 2000 2200 1400 1400 1800 1800 700 1300 1500 700 1300 96-95 96-98 226-229 234-239 276-281 277-280 283-29OU 286-289 285-288.5 288-290.5 288.5-289.5
+
96.9
96.9
.. ,
101
...
7.7
...
...
7.8 88
..
10.3
...
-. D 2
,
1-A
1-B
234.8b ,
.
... ...
... 2-d
--
+
+
+
7 -
1500 286-288
234.88
281
281
288.6
288.6
288.6
288.6
...
283
288.6 ...
288.6
235
...
...
...
292
294
5.4
...
8.6
...
...
.,.
..
5.2
6.2
5.5
...
...
8.7 270
... ...
... ...
...
...
5.1 288
5.9 282
8.9
...
9 , l C
...
...
...
5.1
8.7
0.5 2-B
3-8
9.26
... 4-A
4-B
1.4 5-B
0.4 5-C
22s
...
3-B
...
...
4-ij
..
...
5-A
Determination of additional points on pressure-temperature curve between 283' and 290' would reduce range of discontinuity.
b n-Hexane. C
cal temperatures. The investigation was also extended to two-component systems. The pressure-temperature curves were determined in the presence and absence of hydrogen for propane, cyclohexane, hexane "practical", and benzene. The pressure-temperature relationship was also studied for the following two-component systems over the temperature ranges indicated:
Apparently with a m o u n t of cyclohexane charged (1800 cc.), liquid level was still above vapor-phase sample line.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
172
Vol. 14, No. 2
Apparatus and Procedure
of the bomb momentarily and read the gage in a vertical position, thus giving greater accuracy in the pressure readings.
T h e Ipatieff rotating bomb used for the determination of the solubility of methane in benzene was also used in this work, modified as follows:
In studying the pressure-temperature relationship with pure substances the procedure was essentially as follows:
The vapor-phase sampling line ivas bent outward to within 0.31 cm. (0.125 inch) of the bomb wall. This gave the bomb a capacity of 3145 cc. up to the vapor-sampling line or 92 per cent of the total capacity of the bomb (3420 cc.). This change made possible the study of two-phase systems over a greater range of temperature than by the previous arrangement by which solubilities were determined (1). The gage was mounted a t approximately 120' to the axis of rotation of the bomb. This made it possible to stop the rotation
The bomb was evacuated and the charge drawn into the bomb by the vacuum from a graduated cylinder until the desired amount of liquid was charged. In case of propane the bomb was evacuated and the propane charged from a graduated pressure charger. When hydrogen was used, the bomb was charged with the substance to be investigated according to the procedure outlined above and enough hydrogen added t o increase the gage pressure about 10 atmospheres. With hydrogen present, it was possible to determine when a two-phase or one-phase condition existed in the bomb by comparing the ratio of the liquid to gas from the samples taken from the vapor-phase and liquid-phase sample lines.
In studying the two-component systems the procedure for charging was as follows: In case of benzene-methane, the benzene and methane were charged as described above when hydrogen was used with pure substances. When the propane-benzene system was studied, the benzene was charged t o the bomb after evacuation, and propane was charged from a pressure charger in the desired amount. The hexane-benzene was mixed in the desired proportions outside the bomb, and charged to the bomb after evacuation. The heat input to the heating furnace was controlled to raise the temperature approximately 1" C. per minute. The procedure for drawing samples when the bomb charge consisted of two-component systems has been previously outlined (1). In studying the compositions of samples from the hexane benzene system, the weight per cents of the two constituents were determined by refractive index curves, the compositions of sam-
215 220 225 230 235 240 245 250 255 260 265 270 275 280
TEMPERATURE
'C
FIGURE I
TEMPERATURE
FIGURE 2
'C.
ANALYTICAL EDITION
February 15, 1942
173
ples from the other systems investigated being determined by the ratio of liquid t o gas.
PRESSURE-TEMPERATURE RELATION OF PURE SUBSTANCES. Table I and Figures 1 and 2 summarize the results obtained with pure substances alone and in the presence of a small amount of hydrogen (10 atmospheres, initial). In all experiments the pressure-temperature curves became discontinuous in the region of the critical temperature, the part below the critical temperature being curved and the section above the critical temperature being practically straight. Variations over a relatively wide range in the amount of material charged did not noticeably affect the temperature of break on the curve. I n case of propane (curves 1-A and 1-B), the break in both curves occurred over a range of 96" to 98" C., the hydrogen having no noticeable effect on the temperature of the break in the curve. Bomb samples drawn from the vapor-phase and liquid-phase sampling *lines when hydrogen was used showed that a t a temperature of 100-101 " C. the bomb charge was a single phase, the hydrogen content of the two samples being the same. When samples were taken a t 88" C., the hydrogen content indicated the existence of two phases in the bomb. Curves 2-A and 2-B, using hexane "practical" from petroleum, showed the characteristic breaks in the pressure-temperature curves a t 228-229" and 234-239" C., respectively. The hydrogen content of samples drawn (curve 2-B) indicated the presence of a single-phase region a t 235" C. and a two-phase region below 228" C. The critical temperature for n-hexane is 234.8" C. Closer agreement with the critical temperature of n-hexane cannot be expected, as the sample used probably had hexane isomers. The experiments with cyclohexane (curve 3-6) and with hydrogen (curve 3-B) gave points of discontinuity on the pressure-temperature curves at 276-281 " C. The critical temperature for cyclohexane according to the literature is 281" C. The presence of hydrogen did not noticeably displace the break on the curves. I n this case samples drawn from the bomb when runs were made with hydrogen did not indicate a two-phase region below the critical temperature. This may be due to the possibility that the level of the liquid phase was above the vapor-phase draw-off line and samples of liquid phase were obtained from both sampling cocks. This suggests the difficulty of obtaining satisfactory samples when
TABLE11.
SUMMARY O F DETERMINATION O F CRITICAL TEhfPERATURES OF TWO-COMPONENT SYSTEMS BY THE ROTATING
BOMB
System Benzene, mole yo Methane, mole % Propane, mole % Hexane "practical", weight % Amount charged, liquid, cc. Benzene Benzene hexane "practical" Methane, atmospheres (initial) Propane Temperature of break, C. Test samples: Above break temperature at, O C. Liquid-phase sample cock, cc. liquid/100 cc. gas
+
Vapor-phase sample cock, liquid/100 cc. gas
io
.. ..
30 .,
1600
1415
..
600 214-220
27
.. 55
240-244
.. *.
..
Vapor-phase sample cock, cc. liquid/100 cc. gas
68 a t . %
.. ..
32
1500
..
..
255-260
246
222
266
1.0
0.9
6 8 . 5 w t %CsHa 3 1 . 5 Wt. 70csH14 68 5 w t . %CsHe 3 1 . 5 wt. 7 0 CaHl4 256
CC.
Below break temperature at, C. Liquid-phase sample cock, cc. liquid/100 cc. gas
Curve
73
1.0
0.9
238
216
1.1
1.0
69.5wt.%CeHs 3 0 . 5 wt. 70C ~ H H
0.1
0.4
66.0wt.%CeHs 3 4 . 0 wt. 7 0 CsHir
6
7
S
TEMPERATURE
"C.
FIGURE3
working in the region of the critical if the liquid level of the charge in the bomb is high. However, the presence of hydrogen was beneficial in that the temperature of the break on the curve was more clearly defined. The results of determinations of the critical temperature of benzene in the absence of hydrogen (curves 4-8, B, and C) and in the presence of hydrogen (curves 5-6, B, and C) were on the whole satisfactory. The hydrogen did not noticeably displace the break on the pressure-temperature curve. The critical as determined by the break on the pressure-temperature curve (in the presence and absence of hydrogen) ranged from 286" to 291" C. (critical temperature of benzene according to the literature is 288.6" C.). In the case of benzene, samples drawn from the bomb, when runs mere made with hydrogen, indicated a two-phase region below the break on the temperature-pressure curve and a one-phase condition above the break. I n the case of benzene the presence of hydrogen again gave a sharper break on the pressure-temperature curve a t the critical than did runs made in the absence of hydrogen. In the runs with benzene, variations in the amount charged (700 to 1500 cc.) produced no noticeable effect in the temperature of break on the curve.
INDUSTRIAL AND ENGINEERING CHEMISTRY
174 PRESSURE-TEhlPERATURE
RELATION OF TVO-COMPONENT
SYSTEMS.The pressure-temperature relationship of the following two-component systems was studied over the temperature ranges indicated: Benzene 1600 cc. Methang, 55 atmosphere initial (gage) Temperature range, 62" t o 261' C.
(73 mole 7) (27 mole '%)
Benzene, 1416 CC. Propane, 610 cc. (liquid) Temperature range, 100' t o 250' C.
(70 mole (30 mole
Benzene, 1440 cc. Propane, 605 cc. (liquid) Temperature range, 94' t o 256' C.
(70 mole 70) (30 mole %)
7)
2)
Hexane "practical", 32 weight Benzene, 68 weight 70 Temperature range, 150' t o 288' C.
The results obtained are given in Table I1 and Figure 3. As in the case of pure substances, the pressure-temperature curves exhibited well-defined regions or points of discontinuity, the section of the curve below the break being curved and the section above the break being practically straight. Samples drawn from the liquid-phase and vapor-phase sampling cocks a t temperatures above the break had approximately the same composition, indicating a one-phase condition, while samples drawn from the liquid-phase and vaporphase cocks below the temperature of break on the curve had different compositions, indicating a two-phase region. The points of discontinuity evidently correspond to the critical temperatures of the two-component systems studied, since analyses of samples taken above and below the temperature of discontinuity showed the existence of one-phase and two-phase regions, respectively.
Vol. 14, No. 2
Conclusions The results obtained with pure substances in either the presence or absence of hydrogen indicate that the critical temperatures may be determined with a fair degree of accuracy and by determining a sufficient number of points in the range of discontinuity the accuracy should be + 2 O C. The amount of material charged to the bomb may vary from 20 to 50 per cent of the total capacity of the bomb without any appreciable shift in the temperature of discontinuity. It is probable that a bomb of smaller size (200 to 300-cc. capacity) might serve equally well if no samples are removed. The smaller bomb could be easily rotated in a liquid heating bath and the temperature determined with greater accuracy than in case of the larger bomb used in these experiments. The pressure-temperature curves of two-component systems exhibited points of discontinuity similar to those shown by individual substances, and the critical temperatures of the systems probably lie within the break on the curves. Up to the present time, however, check determinations have not been made on two-component systems of known critical temperatures. Acknowledgment The authors are indebted to Lee Fischer and Charles Rohlfing for valuable assistance rendered during the course of the experiments. Literature Cited (1) I p a t i e f f , V. N., and Monroe, G. S., IND.ENQ. C H ~ M ANAL. ., ED., 14, 166 (1942).
PRESENTED before t h e Division of Petroleum Chemistry a t t h e 102nd MeetCHEMICAL SOCIETY, Atlantic City, N. J. ing of t h e AXERICAN
Glass Electrode Measurements of the pH of Chromic Acid Solutions WINSLOW H. HARTFORD, Research Laboratories, Mutual Chemical Co. of America, Baltimore, Md. The effect of temperature, purity of acid, and water on the pH of aqueous chromic acid solutions has been measured and shown to be negligible, except for a slight variation with temperature in dilute solutions. A considerable variation in the readings obtained at pH below 2 with different types of measuring instruments exists. It increases as the pH of the solution decreases, and may amount to 0.2 pH at pH 0. A large portion of this discrepancy results from various liquid junction potentials, while a residual portion cannot at present be explained. Calculations show that the correct pH value is probably not far from that obtained with certain of the instruments used. All instruments are self-consistent and may be used for control purposes, if standardized against known solutions.
T
HE pH of chromic acid solutions has been measured by means of the glass electrode by Thompson (f7),Buzzard and Wilson (2), and Neuss and Rieman (f6),and by means of the hydrogen electrode in fairly dilute solution by Britton (1). (The term "chromic acid" is used in this paper to apply to both solid chromium trioxide and its aqueous solutions.) It is only under unusual circumstances that the latter method is satisfactory, and with the development of
modern equipment for glass electrode measurements, the glass electrode is practically universally used for such determinations, and has become of industrial importance in the control of baths for the anodic oxidation of aluminum (2), It is the purpose of this paper to show the effect of varying apparatus and conditions on the pH readings obtained with chromic acid solutions, and to present certain calculations which corroborate data obtained with one type of apparatus.
iipparatus The equipment used for these measurements consisted of six commercial-type instruments of three different makes with various types of electrode assemblies, as shown in Table I. All instruments were provided with a temperature compensator and used a saturated calomel electrode as reference half-cell, except Nos. 2A and 5A, in which a silver-silver chloride reference electrode was substituted. No difference was discernible between setups 1 and 2 and these were used unless otherwise specified.
3Iaterials The material used in most of the work was technical flake chromic acid, analyzing 99.8 per cent chromium trioxide, 0.03 per cent sulfate, and 0.017 per cent chromic oxide. Chromium trioxide was determined by electrometric titration according t o the method of Kelley (9), sulfate ravimetrically as barium sulfate after reduction of the hexavafent chromium by alcohol and hydrochloric acid, and chromic oxide by cerimetry according to
