prove applicable t o many systems for the measurement of solubility and distribution as a function of temperature.
( 2 ) Baldwin, iJr. H.,Higgins, C. E., SoIdano, B. *'., J * PhlP. Chem. 63, 118
(1959).
~ L, L,, ~ u. s, ~ ~~~~i~ ~ E~~~~ ~ , Comm., HW-17822 (May 17, 1950).
(3) B
14) Zbid.. HW-23228( c l---a s s i----, fid) \
LITERATURE CITED
(1) Baldwin, \Ir. H., Higgins, C. E., Rainey, W. T., Jr., Higgins, R. W.,
Division of Industrial and Engineering Chemistry, 135th meeting, ACS;Bosto< Mass., April 1959.
I
(.Tan. \-
9.
1952)
\ -
(5) Burger, L. L., Wagner, R. AI., Znd. Eng. Chem., Chem. Eng. Data Series 3,310 (1958). ( 6 ) Davies. W.C., Jones. W. J.. J . Chem. SOC. 1929, 33. ( 7 ) Told, R. D., Vold, 11.J., "Solubility,"
in "Physical Methods of Organic Chemistry," Vol. I, Part I, p. 319, 2nd ed., "Technique of Organic Chemistry," A. Weissberger, ed., Interscience, Tiew I'ork-London, 1949. review July 6, 1959. Accepted Kovember 13, 1959. Based on work performed for the U. S. Atomic Energy Commission at the Oak Ridge Satighal Laboratory operated by the Union Carbide Corp. RECEIVEDfor
Effect of Centrifugation on Solution Temperature and Solubility of Tributyl Phosphate and Tributyl Phosphine Oxide in Water CECIL E. HlGGlNS and WlLllS H. BALDWIN Chemistry Division, Oak Ridge National laboratory, Oak Ridge, lenn.
b Many analytical procedures, and therefore the reported results, depend on centrifugation. This technique, however, can result in significant temperature disturbances which affect the solution equilibrium. Centrifugation at 25" C. with a laboratory clinical centrifuge raised the solution temperature of water saturated at 25" C. with tributyl phosphate and tributyl phosphine oxide as much as 14". The "equilibrium" solubility of these compounds dropped accordingly. On the other hand, this adverse effect when magnified by auxiliary heat from infrared lamps provided a rapid method for studying the temperature dependence of solubility over a moderate range. The speed of determination was limited by the speed o f the analysis itself.
I
THE LIGHT of the pronounced temperature dependence of the solubility of tributyl phosphate (TBP) in aqueous solutions (8),it appeared worthwhile to investigate in detail any phenomenon associated with the analytical separation procedure which could affect the equilibrium solution temperature. I n previous work (8) it was noted that centrifugation raised the solution temperature with consequent lowering of T B P solubility. Earlier, Wunderly and Smelo (9) had reported a rise in solution temperature when water samples were centrifuged. In this work water solutions of T B P and tributyl phosphine oxide (TBPO) were utilized. Centrifugation offers one of the most efficient and practical means for effecting clean separations in many tFvo-phase
N
235
O
ANALYTICAL CHEMISTRY
systems (4). The fact t h a t centrifugation can change the temperature in a system might help to explain the different reported values for T B P solubility in water ( I , I , 8). The application of centrifugation as an aid in the rapid determination of solubilities as a function of temperature was also investigated. Values from this method compared favorably with those obtained through the use of a constant temperature bath. EXPERIMENTAL
Materials. Labeled tributyl phosphate (TBP-P32)r a s prepared by ester interchange a t 206" C. between 54 mmoles of purified T B P and 1 mmole of anhydrous orthophosphoric acid-P3* (7). Tributyl phosphine oxide (TBPO) was prepared by the reaction of butyl magnesium bromide with phosphorus - oxshloride (6). Solubility Determinations. The solubility of TBP in water was measured with TBP-P32 (8). The solubilitv of TBPO in water (8) was so much greater than that of T B P that its concentration could easily be measured by a refractometer. Comparison was made with readings on water solutions of known TBPO concentrations (6). Centrifugation, Duplicate, balanced, 5- t o 10-ml. samples of mater saturated with TBP-P32 (417 mg. per liter) and water saturated with TBPO (55.4 grams per liter), both at 25.0" A 0.1" C., were centrifuged in 15-ml. glass tubes in a n International clinical centrifuge (International Equipment Co., Boston, Mass.). The relative centrifugal force at the tip was 1100 X G and at the free surface of the liquid was 300 t o 600 X G. Room temperature was 25" i 0.5" C. Unless otherwise
specified, all centrifugations were performed n i t h the lid closed. Solution temperatures were observed from small thermometers (ABA, 3?/4 inches, 0" t o 50" C. or LaPine, 4l/2 inches, 10" to 40" C.) immersed in the solution throughout the centrifugation. This eliminated errors r h i c h could result from insertion of a cold therniometer into the liquid after centrifugation. Immediately after the solution temperature was noted, samples for analysis were taken with pipets drawn out t o a fine capillary. Validity of the solution temperature indicated bythe small included thermometer !vas confirmed by addition of a long-stemmrd thermometpr t o a centrifuged solution (1 hour, t = 36" c.). The rrsultant solution temperature registered the same on both thermometers. RESULTS
The effects of centrifugation on the temperature of the solution and the resulting T B P concentration are shown in Figure 1. Heat from the motor (perhaps also from air friction) caused a rise of 12' above the initial temperature over a 2-hour period. Over half of this rise occurred during the first 20 minutes. Reproducible results were obtained by allowing the centrifuge to cool several hours between tests, and by running the experiments when room temperature was the same as the bath temperature. Centrifugation with a recently used centrifuge displaced the solution temperature curve upwards. Samples a t room temperature which were centrifuged for 5-minute periods a t 30-minute intervals progressively
warmed to a maximum temperature by the third centrifugation. This maxinium was 1" higher than the temperature rise (At) resulting from the first ccntrifugation period. The solution temperature curve (consequently, the solubility curve) shifted also with changes in room temperature. Solution temperature could be maintained essentially a t room temperature during centrifugation if the centrifuge were kept open, thereby providing air cooling. The maximum rise was < l o during a 4-hour period. However, if the samples were not immediately rcmoved, the solution temperature was considerably affected. Samples allowed to stand in the open centrifuge after a 30-minute centrifugation with the lid u p showed a temperature rise of 0.7" C. in 5 minutes to a maximum of 3 " above room temperature in a half hour. Results varied between centrifuges. Another centrifuge of the same type produced greater changes in solution temperature (At's of 10" and 14" in 0.5- and 2-hour periods, respectively, compared with 8" and 12" for equivalent centrifugation times in Figure I ) , hence lower solubilities. The rotor speed was slower, however (2100 r.p.m. DS. 2600 r.p.m.)l, indicating that motor heat is the main source of thermal rise in the solutions. Variation in sample size had little effect on solution trmperature until the volume became large (> 15 ml.). However, 40-ml. samples had At's of only 1.4", 6", and 9" after 10-minute, ao-rninute, and 2-hour periods, respectively, whereas the temperature of 10-ml. samples centrifuged simultaneously rose 2.4", So, and 12". Larger
Table I. Dependence of Solution Temperatureu on Rotation Rate
Time, Minutes R.P.M. 600 1000
10 25.5 25.6 26.5 28.5
Table 11. Temperature and Concentration of Tributyl Phosphine OxideW a t e r Solutions after Centrifugation"
30 27.2 28.4 30.3 33.0
60 120 29.2 31.0 31.0 32.8 1800 32.0 34.2 26OOb 35.5 36.7 Initial temperature 25" C.; volume
Table 111. Rapid Determination of Solubility of TBPO in W a t e r as a Function of Temperature"
O
J25 120
Figure 1. Solution temperature and solubility of TBP in water samples subjected to centrifugation
50.2 40 0 46.4 44.1 43.1 41.3
39.7 33.4 26.3 23.2 19.5
0 Measurements made in one afternoon using two 10-ml. samples, centrifuging t o temperature listed. b Heating augmented with infrared lamp.
.-
60
;
I
1
I
I
1
?
I "
o
30 40 TEMPERATURE,
I 50 "C
60
Figure 2. Comparison of tributyl phosphine oxide-water solubilities resulting from centrifugation with those from use of a constant temperature bath 0
Room temperature = 25' i 0.5" C.; samples (10 ml.) centrifuged a t 2 6 0 0 r.p.m. in clinical centrifuge
c.
27.0 27.8 29.0 30.2 31.1 32.0 32.5 36.0 42.6b 46, Ob 50. 45
2C
i
TBPO Concn., G./ Liter Solution
Temp.,
F m
1
55.4 52.0 47.7 45.0 38.2 33.8 31.6 28.4 22.3 20.6 17.4
TBPO suspension, Centrifuged 20 seconds, centrifige open, to clear. 0 Thermal rise aided with infrared lamp. d Thermal rise aided with two infrared lamps.
x 1 I ' I . I 20 40 60 80 400 CENTRIFUGATION T I M E , minutes
c.
25.0 i b 26.8 28.7 1O b 15 29.8 33.2 30 36.4 60 36.7 120 20c 40.0 20d 45.0 23d 49.7 30d 55.0 Room temperature 25' C.
c 3
420-7
O
TBPO Concn., G./Liter
0
10 ml. * Curve, Figure 1.
volume solutions (100 ml.) in a large centrifuge (size 2) had At's of 3" and 8" in 0.5- and 2-hour periods a t 1500 r.p.m., and 3" and 9.5" for the same periods at 2000 r.p.m. Solution temperatures as a function of rotor speed in the clinical centrifuge are listed in Table I. Substantial At's resulted even a t low rotation rates. The rise of solution temperature as a result of centrifugation is immediately reflected in a decrease in T B P solubility (Figure 1). The speed with which the heat is translated into solubility drop suggests t h a t centrifugation might offer a rapid means of obtaining fairly reliable solubilities as a function of temperature. The method of centrifugation for controlling solution temperature has therefore been utilized in the measurement of the solubility of tributyl phosphine oxide in water. The temperature range was 25" to 55" C. (Table 11). Additional heat from infrared lamps was applied to obtain the higher temperatures listed. The application of centrifugation for a rapid determination of T B P O solubility in water as a function of temperature is
Solution Temp. after Centrifuging,
Centrifuge Time, hlin.
A
Constant temperature bath values ( 6 ) Centrifugation, Table II Centrifugation, Table 111
VOL. 32, NO. 2, FEBRUARY 1960
237
demonstrated in Table 111. The values in the entire table were determined in less than half a day. All TBPO solubilities in water obtained as a function of temperature b y centrifugation are plotted in Figure 2 for comparison with solubilities obtained using a constant temperature bath (6). The points fall rather well along the line drawn through the constant temperature bath values. Most values are within experimental error (=t2%); the largest deviation was +5%. Any laboratory operation involving a phase separation would appear to be accomplished most conveniently and satisfactorily b y centrifugation with air cooling (open centrifuge), if the tubes are removed from the cups immediately after centrifugation. For systems whose equilibrium is disturbed b y temperature changes, the air temperature must necessarily be the sxne
as the equilibrium solution temperature. However, centrifugation in :t closed centrifuge with external heat from infrared lamps can also be utilized for a quick and convenient method for determining solubilities of substances that become less soluble as the temperature rises. If centrifugation is used for any system which is temperature-sensitive, a knowledge of the final temperature after centrifugation is important. ACKNOWLEDGMENT
The authors express appreciation to
B. A. Soldano for suggesting a more detailed study on the effects of centrifugation on solution equilibrium. LITERATURE CITED
( 1 ) Alcock, IC., Grimley, S. S., Healy, T. V., Kennedy, J., &Kay, H. A. C., Trans. Faraday SOC.52,39 (1956). (2) Burger, L. I,., Forsman, R. C., U. S.
Atomic Energy Comm. HW-20936 (.\pril 2 , 1951, declassified March 2, 1957). (3) Burger, L. L., Wagner, R. M., Znd. Eng. C h e w , Chem. Eng. Data Series 3, 310 (1958).
(4) Craig, L. C
Craig, D., “Extraction and DistribuGon” in “Separation and Purification,” Vol. III, 2nd ed., pp 227, 300, “Technique of Organic Cheniistry,” A. Weissberger, ed., Interscience, New York-London, 1956. (5) Davies, W. C., Jones, W. J.. J . Chein.
SOC. 1929.33. (6) Higgins; C. E., Baldwin, ANAL. CHEY.32,233 (1960). . ( 7 ) Higgins, C. E., Baldwin, J . Org.,Chem. 21,1156 (1956). . (8) Higgns, C. E., Baldwin, Soldano. B. A,. J. Phus. Chem . 63, (1959). (9) Wunderly, H. L., Smelo, L. S., ISD. ENQ.CHEM.,ANAL.ED. 12, 754 (1940).
w. w w
RECEIVEDfor review July 6, 1959. Accepted Sovember 13, 1959. Based on work performed for the U. 6 . Atomic Energy Commission a t the Oak Ridge Xational Laboratory operated by the Union Carbide Corp.
Radioassay of Finely Divided Solids by Suspension in a Gel Scintillator SAMUEL HELF, C. G. WHITE, and R. N. SHELLEY’ Explosives and Propellanfs laborafory, Picafinny Arsenal, Dover,
b The counting of radioactive materials as suspensions in a gel scintillator is described. The radioelements included in the suspended samples are strontium90-yttrium-90, chlorine-36, sodium-22, barium-1 33, nickel-63, carbon-1 4, and hydrogen-3. Factors affecting suspension counting efficiency are discussed and a comparison is made with homogeneous solution counting for some specific cases.
T
more iibua1 methods of sample preparation for liquid scintillation counting entail making a homogeneous solution of the radioactive sample and the liquid scintillator. Because aromatic solvents, toluene in particular, are most commonly used for liquid scintillators, samples soluhle in such media present no problem. The difficulties associated \T-ith counting materials insoluble in toluene or other pure aromatic solvents can often be overcome through the use of mixed solvent systems containing polar organic liquids and/or other additives to increase solubility. Davidson and Feigelson (2) HE
1 Present address, Food & Drug Administration, Dept. of Health, Education, and Welfare, Washington, D. C.
238
ANALYTICAL CHEMISTRY
N. J.
have thoroughly reviewed and discussed many of these practical solvent systems for the homogeneous solution counting of a wide variety of radioactive materials. Homogeneous solution counting, using complex solvent systems and/or additives to increase solubility, may impart two disadvantages. First, the addition of other solvents and additives may dilute or quench the scintillation process, thereby reducing counting efficiency ( 2 ) . Second, at best, only very small amounts of tagged material can be dissolved. Thus, where lom specific activity is involved, sensitivity is limited. Moreover, i t is not unlikely that for some materials, particularly inorganic salts, a suitable solvent system for homogeneous solution counting may be impossible t o obtain. To overcome the above disadvantages for very insoluble compounds, a suspension counting technique can be employed to good advantage. The radioactive material in finely divided form can be suspended directly in a simple liquid scintillator (5) or in a gel scintillator containing either aluminum stearate (3) or the thixotropic agents, Thixcin (9) and Cab-0-Si1 (8). The gel media are preferable where no settling and good reproducibility of samples are
desirable ( 7 , 9). By preparing the radioactive samples in a sufficiently finely divided state, self-absorption effects can be eliminated for all but the very low-energy beta emitters such as tritium ( 5 ) . As much as 1 gram of some carbon-14-labeled compounds can be suspended in a 20-ml. volume of a gel scintillator without appreciable effect on counting rate due to opacity (9). Previous references on suspension counting dealt primarily with tolueneinsoluble carbon-14 compounds. This paper compares the counting characteristics of a wide variety of suspended beta emitters, ranging from very low to very high emission energy, in a gel scintillator medium. A positron and a conversion electron emitter are also included among the radioactive sources. I n addition, the relative advantages of suspension over homogeneous solution counting are illustrated for certain applications. EXPERIMENTAL
Radioactive Samples. T h e following radioactive solids were used for this study: SrBOS04-Y%304(in equilibrium), Ni63Cz04, NaC136, Ba133C12, Na22C1, 1,3,5,7-tetranitro-l,3,5,7-tetracyclo-octane (HMX)-C1‘ (uniformly
