DETERMINATION OF VAPOR PRESSURE OF LIQUIDS
329
STUDIES IN VAPOR-LIQUID EQUILIBRIA. I
A NEWDYNAMIC METHODFOR THE DETERMINATION OF VAPORPRESSURES OF LIQUIDS THEODORE T. PUCK AND HENRY WISE1 Department of Medicine, the Douglas Smith Foundation f o r Medical Research and the Bartlett Memorial .Fund, University of Chicago, Chicago, Illinois, and the Commission on Air-borne Infections, Army Epidemiological Board, Preventive Medicine Service, Ofice of the Surgeon General, U.S. Army Received March 28, IO&‘
No completely satisfactory method has been available for measurement of the vapor pressures of slightly volatile liquids. Each of the customary procedures involves certain inherent experimental difficulties which become more serious the lower the vapor pressure which is to be determined. Thus, in the static method, special manometric techniques are required in order to measure very low pressures. Moreover, significant error may be introduced through the failure to remove completely minute traces of air or other volatile impurities dissolved in the liquid or adsorbed on the walls of the containing vessel. Similarly in the airsaturation procedures, difficulties are encountered in handling and measuring the large volumes of inert gas which must be employed in order to evaporate an accurately weighable quantity of a non-volatile liquid. Finally, these so-called “phlegmatic” liquids (9) attain equilibrium with their vapors at so slow a rate as to make both of the aforementioned methods of vapor-pressure measurement time-consuming and laborious. The procedure to be described in this communicationwas suggested by the dew point apparatus which is used routinely to determine the water-vapor content of air (10). This latter device consists of a metal plate polished to mirror brightness, whose temperature can be accurately controlled. The plate is placed in an air stream, and gradually cooled until the point is reached at which a visible condensate forms on its surface. At this temperature the water vapor present is just sufficient to saturate the air. By reference to a table of values for the saturated vapor density of water a t every temperature, the moisture content of the test air can be computed. This method can also be used to determine the partial pressure of a single volatile component of a solution, provided the vapor pressure of the pure substance is already known over a wide range of temperatures (8). We have found it possible to adapt this principle to the determination of vapor pressures of pure compounds, by maintaining the metal mirror a t a constant temperature and varying instead the composition of the gas stream directed a t it. A visible condensate forms on or is removed from the metal surface, depending on whether the concentration of the vapor in the gas stream is supersaturated or Submitted by Henry Wise t o the Facultj. of the Graduate School of the University of Chicago in partial fulfillment of the requirements for the degree of Master of Science, September, 1944.
330
THEODORE T. PUCK AND HENRY WISE
unsaturated with respect to the temperature of the metal target. The vapor concentration at which a film just barely forms, or when once formed is just barely removed, is then taken as the saturation value for the substance a t the temperature of the target. If the vapor obeys the gas laws, the vapor pressure in millimeters of mercury may then be calculated. APPARATUS
The apparatus, in essence, consists of three parts: (1) A vaporizer which vaporizes the test liquid a t a constant, known rate. (2) A stream of an inert gas whose flow can be arbitrarily +regulatedand accurately measured, which carries away the vaporized material. The concentration of the condensable vapor in the issuing gas stream is controlled by an adjustment of the rate of flow of this SINCHRONOUS MOTOR AND
SlRlNOE
NEEDLE
I
VAPORIZER
\
THERYOREOULATOR
I
-WATER
SATM
DRI NITROGEN GAS
FIG.1. Apparatus for measuring vapor pressures of liquids
diluting gas. (3) A polished metal mirror whose surface temperature is carefully controlled and which serves as the target for the vapor-laden stream. The apparatus is shown in figure 1. The test liquid is contained in a syringe whose piston is advanced a t a constant rate by a synchronous motor furnished with a worm gear drive and a horizontal screw arrangement? The liquid contained in the syringe is forced through a % 20 hypodermiq needle (inner diameter 2 A ~-R.P.M.synchronous motor was used in the present study. Varying either the diameter of the syringe or the gear ratios i n the delivery mechanism made it possible t o secure any desired rate of vaporization. Thus, for a moderately volatile compound like water, a 50-cc. syringe was used and the piston was advanced at a rate of 0.0169 cm. per minute. For triethylene glycol, an extremely non-volatile substance, a t-cc. syringe was employed, and its piston was driven forward at a rate of 0.00105b cm. per minute. The feeding mechanism was calibrated by weighing the amount of mercury delivered during measured time intervals.
DETERMINATION O F VAPOR PRESSVRE O F LIQUIDS
.
33 1
0.051 em., length 10 em.) onto a heater where it is vaporized. The heating element consists of an "Ohmite" resistor (75 ohms, 10 watts) across which voltage is applied from a variable transformer or resistance. I n order to effect rapid vaporization a t the lowest temperature possible, the delivery needle is made to rest on a glass-cloth wick which serves to disperse the liquid over the heated surface. The tip of the needle is inserted underneath several strands of the glass thread (not shown in the figure) so that $he needle's orifice is in direct contact with the glass fibers. In this way, droplet formation which would produce an intermittent flow of liquid to the vaporizer is a ~ o i d e d . ~ The heater is located in a glass T-tube (23 mm. in diameter, 200 mm. long) closed a t one end by a cork stopper through which the delivery needle passes. A stream of inert gas, whose rate of flow is controlled by a pressure valve and measured bya calibrated flowmeter, enters through the vertical arm of the T-tube. This stream impinges on the heater so that it carries away the volatilized material. Either nitrogen or air was found to be suitable for use as the carrier gas, although the former is preferable wherever danger of oxidation exists. The gas was dried by phosphorus pentoxide, and its temperature was measured before it entered the calibrated flowmeter. The volume of gas at the temperature of the target is calculated by means of the gas I ~ w s . ~ The gas mixture leaving the T-tube is directed a t the condensation surface, which consists of a thin, chromium-plated, copper disc (0.05 em. thick, 4.5 em. in diameter) polished to mirror brightness and placed about 2 em. from the exit of the T-tube. This copper plate is soldered over a hole in a wall of a thermostated water bath, so that its back face is in direct contact with the water. The bath, well insulated except for the area surrounding the condensation surface, is supplied with a mercury thermoregulator which controls an immersion heater. The temperature of the bath is kept at 25.00"C.f. 0.04", as read by a long-stem mercury thermometer calibrated by the U. S. Bureau of Standards. Very vigorous agitation of the water in the bath must be maintained a t all times by an efficient stirter in order to insure that the temperature of the copper mirror be the same as that of the water in the bath. Heat-transfer calculation shows that if the experimental conditions here described are maintained, the temperature of the outer surface of the copper mirror on which the condensation occurs is within 0.01"C. of that registered by the thermometer immersed in the bath (vide infra). The most convenient and reliable method of detecting the appearance or disappearance of a condensate on the polished target is by photoelectric means. A beam of light is focused by a lens onto the polished metal target and reflected therefrom into a photocell (see figure 1). The presence of a condensate on the target scatters the light beam, so that the light intensity reflected into the photocell is greatly diminished. A single-stage amplification circuit with a meter arrangement is satisfactory for detecting the variations in the photocell output corresponding to formation and removal of the condensed film. a The operation of such a system t o produce quantitative vaporization of minute quantities of liquids at a constant rate has been previously described by one of us (20). The volume of gas was sometimes adjusted directly to the desired temperature by passage through a copper coil immersed in a thermostated water bath.
332
THEODORE T. PUCK AND HENRY WISE
The formation and disappearance of the condensate can also be observed by the naked eye. The contrast can be rendered more prominent if a wetting agent is initially applied to part of the mirror surface (25). EXPERIMENTAL
Procedure 05 a delemnimtion
A syringe of suitable size is filled with the liquid whose saturated vapor density is to be determined, and inserted into place in the syringe holder. The flow of the inert gas through the apparatus is then begun, after which the heater and the synchronous motor are started. The heater voltage is adjusted to produce the lowest temperature a t which no accumulation of 'liquid droplets on the heater surface can be observed. This temperature, at which complete vapori~ationis secured, lies below the normal boiling point of the liquid, because evaporation is materially hastened by the swiftly flowing gas stream which bathes the heater. Thus, determinations may even be carried out on some liquids which would decompose a t the temperatures of their normal boiling points. The flow of dried air or nitrogen is first adjusted to a high value, so that the mixture impinging on the target is very dilute with respect to the condensable component; hence the surface of the mirror remains shiny. The volume of flow of the diluent gas is then gradually decreased by means of the regulating valve until the point is reached a t which a condensate forms on the mirror. It is most efficient to determine this saturation composition approximately, a t first, and then with greater accuracy by means of successive, small adjustments. Experience has shown that even for compounds with vapor pressures as low as 0.001 mm. Hg, a period of 3-5 min. is sufficientto decide whether a gas stream containing a given composition of vapor will produce a condensate on the shiny mirror. If no visible film appears within this interval, the vapor is too dilute. Equilibrium may be approached from either side, since a gas stream undersaturated with vapor will remove a condensate already formed on the mirror. Hence the practice was adopted of alternately undersaturating and supersaturating the gas mixture by means of the reeulating valve. The interval between the formation and disappearance of the condensate is progressively narrowed until two successive flowmeter readings are obtained, within 2 to 5 per cent of each other, for the lower of which a deposit is present on the mirror, and for the higher of which no condensate is detectable. The midpoint between these two readings is taken to represent the gas flow required to produce equilibrium? The rate of vaporization of the liquid and the rate of gas flow necessary to produce a satu5 The evaporation rate of a condensed film of a phlegmatic substance like triethylene glycol is a much slower process than that of more volatile compounds. Hence, it is necessary t o permit only a thin film of material to deposit, or else the removal of the condensate by a slightly undersaturated gas stream will require an excessive interval of time. With the use of a recording milliammeter to make more definite the small changes in the photocell output corresponding to formation and removal of such thin films, it was possible even with triethylene glycol to approach equilibrium from both sides with an interval no greater than 5 per cent.
'
333
DETERMINATION OF VAPOR PRESSURE OF LIQUIDS
rated composition having thus been determined, the saturation vapor density a t the temperature of the target can be calculated. TABLE 1 Boiling points of liquids used
I
UTERIAL
.
BOILING POINT
BOILING POINT GIVEN IN TEE LITERATURE
'C.
.
.
.
Water. . . . .. . . . . . . . . . . . . . . . . 1-Propanol, . . . . . . . . . . . . . . . . . 1-Butanol.. . . . . . . . , . . . . . . . . . . , Nitrobenzene. . . . . . . . . . . . . . . . . 1,2-Propanediol. . . . . . . . . . . . Triethylene glycol. . ...... . . . .
..
..
.
.
"C.
loo. 0 97.2 117.7 210.9 187.4 176.5
100.0 96.0-96.2 (740 mm. Hg) 116.3 (740 mm. Hg) 209.2 (747 mm. Hg) 185.0-186.0 (750 mm. Hg) 176.5-178.0 (22.5 mm. Hg)
(3) (3) (12) (22) (22.5 mm. Hg) (7)
TABLE 2 Experimental determination by the condensation method of saturated vapor density of six liquids at 86'C. RATE OF GAS FLOW AT EQUILIRBIUM '
COldPOUND
liters X sec.-I
OFLIQUID
1 I 250j40
mt. x sec.-l
EQUILIRRIUM VAPOB DENSITY
VAPOB PRESSURE
grams per liier
mm. Hg
Water.. .. . . . . . 0.0769 f 0.0002*
76.8
X
0.99707 0.0229
1-Propanol. . . . 0.0202 f 0.0004
76.8
X
0.7925
1-Butanol.. . . .
0.0582 f 0.0005
76.8
X
0.8050
Nitrobenzene..
0.0689 f 0.0007
11.04 X lod5 1.1931
1,2-Propanediol?. . . . . . . . Triethylene glycol. ... . .
0.066 f 0 . 0 0 0 0.207 f 0.003
1.0328 3.47 X 11.04 x 10-6
.
'
RATE OF DELIVEBY
0.802 f 0.002
0.770 X
13.68 f 0.0000 i 0.06 21.5 0.0694 A 0.0014 f 0.4 6.14 0.0244 f 0.0002 i 0.05 0.291 0.00192 f o.oo00 i 0.003 (54.8 A 1.0) 0.133 x 10-5 f 0.003
1.124
(1.079 f 0.02)
x
10-8
0.00134 f O.ooOo3
* The average value and the average deviation from the mean are given for each set o'f measurements. t Syringes of two different sizes were used. Experimental measurements Vapor-pressure determinations a t 25OC. were performed by this method on the followkg liquids: water, 1-propanol, 1-butanol, nitrobenzene, 1 ,2-propanediol, and triethylene glycol (2,2'-ethylenedioxydiethanol) . The materials were dried when necessary, and purified by fractional distillation. In table 1 the boiling points of the fractions selected are compared with those given in the literature. Four to eleven vapor-pressure measurements were made for each compound.
334
THEODORE T. PUCK AND HENRY WISE
The experimental results are listed in table 2. From the values of the saturated vapor density, the vapor pressures in column 6 were calculated by means of the gas laws. In order to evaluate these results it was necessary to compare them with vaporpressure measurements performed on the same liquids by other methods. For the more volatile of these compounds it was possible to obtain satisfactory data from the literature. Reliable published data on vapor pressures of liquids in the region of 0.lmm.Hgwerenot found,however. Hence,in order tocheck thevalidity of the condensation method in this range, a special study mas made on 1,2propanediol. Its vapor tension at 25°C. was determined by the air-saturation method and by extrapolation of static pressure measurements obtained at higher temperatures. Similar measurements with triethylene glycol were not feasible because of its extremely small volatility. For this latter compound, therefore, the only comparison possible was with the extrapolated result of static pressure measurements performed at temperatures above 130°C.
V a p o r pressure of l,%propanediol by the air-saturation method For these measurements, an apparatus similar to that devised by Krauskopf (14) was constructed. Air dried by,calcium chloride and phosphorus pentoxide was saturated with 1,2-propanediol vapor by passage through a series of vessels wherein intimate contact between the gas and liquid phases at 25°C. was promoted. The propylene glycol evaporated by the air stream was then absorbed in water and determined by analysis (18). The volume of air employed was measured by displacement of water from a calibrated aspirator bottle. It was found that varying either the rate of air flow or the total volume of air passing through the saturator had no appreciable effect on the results obtained. This was taken to indicate that equilibrium was reached between the gas and liquid phases in the saturator. The results of these determinations are recorded in table 3.
V a p o r pressure of 1,W-propanediol by the static method Static measurements of the vapor pressure of 1,2-propanediol are reported in the literature, but not below the region between 80°C. and 13OOC. (22). Direct extrapolation of these data by means of a plot of log P vs. 1/T results in a value of 0.21 mm. Hg at 25°C. Other studies, however, have shown that this value must be too high (19). Apparatus was not available which would enable us to obtain an accurate, static measurement of the vapor pressure of 1,2-propanediol at 25°C. Hence, additional measurements were performed between temperatures of 64°C. and 80°C. (table 4) and these results were combined with those obtained at the higher temperatures (22) in order to shorten the range of extrapolation. Furthermore, heat-capacity data were secured6 in order to compute the correction due t o the change of AH over the temperature interval involved. AC;for the process 6
We are indebted to Prof. T. B.Young for supplying the heat-capacity data.
'
335
DETERMINATION OF VAPOR PRESSURE O F LIQUIDS
C3Ha(OH)2(liquid) C3He(OH)2 (vapor) was found to be -17 cal. mole-' deg.-', and was assumed to be constant over the temperature range employed. When corrected in this manner, all of the points fell on a straight line, whose extrapolation, to 25°C. yielded a value of 0.14 mm. Hg as the vapor pressure of 1,2-propanediol. In table 5 , the vapor-pressure measurements by means of the condensation method are compared with the values obtained by other methods. The close agreement of the vapor-pressure data in the case of water is particularly signifiTABLE 3 Vapor pressure of 1 ,R-propanediol at 25°C. by the air-saturation method SATURATED VAPOR DENSITY
EXPERIMENT NUMBER
VAPOR P R E S S W E
mg. p e v liter
mm. H g
1
0.35 0.53 0.47
0.093 0.130 0.115
2
0:45 0.44 0.58
0.110
3
0.40 0.50 0.50
0.097 0.121 0.121
4
0.49 0.50 0.61
0.119 0.123 0.151
0.488 & 0.05
0.12 f 0.01
Average. , . , . . . . . . . . . . . . . . . . . . . . . . . . .
P
1
"C.
64.20
72.00 79.80
I
0.106 0.141
mm. Hg ,
3.0 5.4 8.7
336
THEODORE T. PUCK AND HENRY WISE DISCUSSION
The accuracy of the condensation method depends upon three factors: constancy of the mirror temperature; the intimacy of contact between the gas-vapor mixture and the metal mirror, i.e., whether a true equilibrium is achieved a t the surface of the target; and the accuracy of the gas-flow measurements and the syringe delivery. TABLE 5 Comparison of vapor pressures obtained by condensation method with those obtained by other methods VAPOR PRESSURE AT
comom
25'c.
BY CONDENSATION " OD
VAPOR PRESSUPE AT
mm. Hg
Water. .................. 23.68
1-Propanol.. ............ 21.5
1-Butanol. ..............
6.14
j=0.06
f 0.4
f 0.05
25°C. BY OTHER KETRODS'
mm. Hg
23.76 23.71 23.75 23.752 23.772 23.69 23.763 21.76 21.5 20.2 6.96 6.44 6.44 6.78
(A) (15) (A) (14) (A) (6)
(A) (17) (S) (11) (S) (S)
(26) (21)
(A) (5) (S) (S)
(23) (16)
(A) (S) (S) (S)
(5) (1)
(13) (4)
Nitrobenzene. . . . . . . . . . . 0.291
f 0.003
0.340 (S) (2) 0.2 (E)t (13)
l12-Propanediol.. . . . . . . .
f 0.003
0.12 f 0.01 (A) (data of this paper) 0.14 f 0.05 (E) (data of this paper)
Triethylene glycol..
....
0.133
0.00134 f 0.00003
0.0014 (E) (7)
* S, static method; A, air-saturation method; E, extrapolation of static measurements performed at higher temperatures. t Extrapolated from measurements performed at temperatures of 53-81'C. The temperature constancy of the metal mirror on which the vapor-laden gas stream impinges is adequately maintained by the rapidly circulating mater of the thermostated bath. Calculation shows that under the conditions here employed the maximum temperature rise of the outer face of the target, due to heat transferred by the gas mixture issuing from the vaporizer, lay between 0.0006"C. and O.0loC., a quantity within the allowable error.7 7 C,, the molal heat capacity of dry, carbon dioxide-free air at atmospheric pressure, is 6.98 cal. mol-1 deg.-l The gas velocities employed in these vapor-pressure measurements
DETERMINATION OF VAPOR PRESSURE OF LIQUIDS
337
The validity of the method depends directly on whether equilibrium is actually achieved between the vapor and the mirror surface. That such is indeed the case mas demonstrated in a number of ways. One of these consisted in repeating measurements of the vapor pressure of a given liquid, using various rates of vaporization. This necessitated corresponding changes in the rates of gas flow. In spite of these variations, the value obtained for the saturated vapor density of the test liquid remained constant (table 2,1,2-propanediol). An even more convincing proof that equilibrium is achieved is the fact that, in every case, the same saturation point was observed regardless of whether the end point was approached from the undersaturated or the supersaturated side. The necessity for accurate flowmeter calibration is obvious. With the apparatus described in the present communication, the error in any flowmeter reading did not exceed 1.75 per cent. The maximum error in the delivery of any of the syringes used was 1.3 per cent. The presence of impurities will not produce the same effect on bapor-pressure varied from 0.02 t o 0.80 liters per second. The temperature of the issuing gas stream (AT,) was 30.0"C. above that of the mirror at the lowest rate of flow, and 13.0%. at the highest velocity. Even if i t is assumed that all this heat is transferred t o the mirror, then, for the low rate of flow, the rate of delivery of heat to the mirror, Q / t , is given by:
Q / t = C, X (rate of gas flow) X ATg
- 6.98 X
0.02 X 273.1 X 30 22.4 X 298.1
= 0.17 cal. per second
Similarly, at the high rate of flow Q / t = 2.97 cal. per second. The rise in the surface temperature of the mirror can be calculated from this value by virtue of the fact that the back face of the mirror, in contact with water at 25"C.,is a t constant temperature. Hence, the amount of heat Q / t is transferred across the mirror every second. Knowing the thermal conductivity of copper, one can calculate the temperature rise of the front face of the strip (AT,) which is necessary in order that the aforementioned rates of heat conduction occur, across the thickness of the strip. Q/t =
KA*AT, ___ d
where K equals the thermal conductivity of the copper, A is the area of the copper mirrors and d its thickness. Hence
1
For the copper plate K = 0.92 cal. cm.-' sec.-I deg.-l; d = 0.05 cm.; and A = 15.9 om2. Therefore, for the slow rate of flow, AT, = O.O0058"C.,while for the rapid flow rate, AT = 0 -01"C. This calculation neglects any interfacial barriers which might tend t o hinder heat transfer across the copper plate. However, with the experimental conditions employed, Le., very rapid agitation of the water inside the thermostat and high rates of flow of the gas stream, these effects will be quite small. Furthermore, the assumption that all the heat of the gas stream is transferred to the copper mirror should more than compensate for any such surface effects.
338
THEODORE T. PUCK AND HENRY WISE
determinations by the condensation method as occurs in the static or air-saturation methods. Completely non-volatile impurities, such as inorganic salts, will have practically no influence on the results of the condensation method, since such materials would remain behind on the vaporizer. Only if such a foreign substance were present in quantities large enough to cause an appreciable change in the density of the liquid would it affect the vapor-pressure determination. The presence of volatile impurities, however, will disturb the measurements, but in this case the effect is to destroy the sharp definition of the end point. This phenomenon was observed when an unpurified commercial sample of 1-propanol was used for vapor-pressure measurement in the condensation apparatus. No sharp delineation of the end point was obtained, but instead a flickering film of condensate appeared and disappeared over a fairly wide range of vapor concentrations. When redistilled material was substituted in the syringe, this phenomenon disappeared. It should be obvious that if the method described in this paper is to be employed a t temperatures considerably different from room temperature, it will be necessary to shield the metal mirror from radiation which might otherwise affect its surface temperature. SUMMARY
1. A new dynamic method, the condensation method, for the measurement of saturated vapor densities of liquids is presented. It may be conveniently applied to liquids with vapor pressures a t least as great as 23 mm. Hg and a t least as small as 0.001 mm. Hg. 2. Vapor-pressure deterkinations a t 25'C. are presented for six liquids: water, 1-propanol, 1-butanol, nitrobenzene, 1 ,2-propanediol, and triethylene glycol. These values are compared with the corresponding vapor-pressure measurements obtained by other methods. In general, excellent agreement is obtained. We wish to thank Prof. T. F. Young of the Department of Chemistry, University of Chicago, for many valuable suggestions. REFERENCES ALLEN,B . B., LINQO,S.P., AND FILSINQ,W. A,: J. Phys. Chem. 43,425 (1939). BRUCKNER, H . VON: Z. anorg. Chem. lM, 91 (1931). J. L., AND TOBIN, E . :J. Am. Chem. SOC.43,561 (1921). BRUNEL, R . F., CRENSHAW, BUTLER,J. A. V., RAMCHANDANI, C. N., AND THOMSON, D. W.: J. Chem. SOC. 1936, 280. (5) BUTLER, J. A. V., THOMSON, D. W . , A N D MACLENNAN, W. H.: J. Chem. SOC.1933,674. (6) DERBY,I . R., DANIELS, F., AND GUTSCHE, F. C.: J. Am. Chem. SOC.36,793 (1914). H . :J. Am. Chem. SOC.69,2521 (1937). (7) GALLAUGHER, A. F., AND H~BBERT, (8) HEPBURN, J. R. I . : Proc. Phys. SOC.40, 249 (1928). (9) HICKMAN, K., AND WEYERTS, W.: J. Am. Chem. SOC.62,4714 (1930). (10) HIXSON,A. W., AND WHITE,G. E . : Ind. Eng. Chem., Anal. Ed. 10,235 (1938). (11) International Ciitical Tables, Vol. 111, p. 212. McGraw-Hill Book Company, Inc., New York (1928). (12) Reference 11, p. 343. (13) KAHLBAUM, F . W. A . : Z . physik. Chem. 26, 577 (1898). (1) (2) (3) (4)
e
b
URANYL-IODIDE
(14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
SYSTEM
339
KRAUSKOPF, F. C.: J. Phys. Chem. 14, 489 (1910). LINCOLN, A. T.,AND KLEIN,D.: J. Phys. Chem. 11,318 (1907). M ~ N D E C. L , F.: 2. physik. Chem. 86, 457 (1913). PEARCE, J. M., AND SNOW,R. D.: J. Phys. Chem. 31, 231 (1927). PUCK, T.T.:Science 96, 178 (1942). PUCK, T.T.,ROBERTSON, 0. H., AND LEMON,H. M.: J. Exptl. Med. 78,387 (1943). 0. H., BIGG,E., PUCK, T.T.,AND MILLER,B. F.: J. Exptl. Med. 76,597 ROBERTSON, (1942) SCHEEL, K., AND HEUSE,W.: Ann. Physik [4] 31, 715 (1910). SCHIERHOLTZ, 0. J., AND STAPLES, M. L. : J. Am. Chem. SOC.67, 2709 (1935). SCHMIDT, G. C.: Z. physik. Chem. 8,628 (1891). TORAL,M. T.,AND MOLES,E.: Anales SOC. espafi. fis. quim. 31, 735 (1933); Chem. Abstracts 28,945 (1934). VANDEN AKKER,J. A., AND WINK, W. A. :Science 97,494 (1943). VON HOLBORN, L., AND HENNING, F.: Ann. Physik [41 26,833 (1908).
.
A PRELIMINARY STUDY OF THE PHOTOCHEMICAL SYSTEM URANYL-I OD1DE JOHK J. McBRADYl AND ROBERT LIVINGSTON School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received March 22, 1946
Preliminary to the study of the formation of tetravalent uranium during the uranyl-sensitized decomposition of oxalic acid ( 5 ) , a few measurements were made on the photochemical system uranyl-iodide. Although this latter investigation is far from complete, the results obtained lead to several unexpected, and therefore interesting, conclusions. EXPERIMENTAL METHODS
The photometer, light source, filters, and reaction vessel have been described by McBrady and Livingston in a recent paper ( 5 ) . The uranyl sulfate-potassium iodide solutions were prepared as follows : 10 ml. of uranyl sulfate solution of known concentration was pipetted into the cell. A weighed crystal of potassium iodide was placed in the head of the hollow stopper, which was then inserted into its lightly greased, ground-glass seat. The stopper was turned to connect the cell to the side tube, and dissolved gases were removed by evacuation (allowing a small amount of the water to distil off). The stopper was then turned to seal off the vessel, and the cell tilted to drop the crystal into the solution. After the crystal had dissolved, the cell was again evacuated to remove any gases which might have been trapped in the crystaL2 This paper is based upon an excerpt from a dissert,ation submitted by J. J. McBrady t o the Graduate School of the University of Minnesota in partial fulfilment of the requirements for the degree of Doctor of Philosophy, April, 1945, This technique and the design of the cell are adapted from the standard procedure of Thunberg (8).
