1100 Table IX.
A N A L Y T I C A L CHEMISTRY Comparison of Gamma Spectrometer Energy Measurements to Standard Values
0 364 hZ.e.v., 1 - 1 3 1 0 513 M.e.v. Ru-Rh-146 0 771 M.e.v., Nb-95 (hleasured Measured Measured ,energy, Error, energy, Error, energy, Error, E h m ,e.Y. % m e . Y. % m.e.v. 70 -0.8 0.520 0.361 +1.3 0.760 -1.5 $1.1 0.755 +1.3 0.520 0.368 -2.1 -0.6 0.766 0.516 +0.6 -0.7 0,362 +4.0 +3.9 0,777 0.533 +0.8 0.380 +1.7 0.762 -1.2 $1.3 0.520 0.370 fl.1 -0.4 0.768 +1.3 0.520 0.368 +1.7 0.752 -2.5 -1.0 0,508 0.370 +1.7 0.510 -0.6 0.740 -4.1 0,370 -0.6 0.518 $1.0 0.758 -1.7 0.362 Std. dev. of 27 measurements 2~ 1.8%.
The experimental energy assignments have a standard deviation of =!=1.8%,and the emitter responsible for each photopeak is easily determined. The calculated results are compared to known compositions in Table VIII. The nine triplicate measurements have an average precision within 3 ~ 7 %(%yo confidence limits) over a range of concentrations 73.3 to 13.3% of the total gamma activity. .4s could be expected, minor constituents of loner energy, such as iodine-131 in solution 11, are determined with the least accuracy, and major constituents with the highest energy, such as niobium-95 in solution 11, are determined with the highest accuracy. It is estimated that in a single energy scan for the more unfavorable cases a minor constituent present to the extent of only 5y0 of the total gamma activity can be determined within 3 ~ 5 0 %of the true value, and for the more favorable case mentioned above, a major constituent can be determined to within =!=2%of the true value. DISCUSSION
The most significant advantages of the gamma scintillation spectrometer are its speed and reliability. Studies involving mixtures of radionuclides, which v-ould normally be unreasonably tedious, are thereby facilitated. This is well illustrated by the
fact that 27 measurements were obtained on the standard solutions in a total instrument time of 4.5 hours. Other workers will undoubtedly find applications of the method -for example, in the determination of trace impurities by neutron activation, wear and corrosion studies of alloys, the investigation of chromatographic separations using radioactive tracers, and the study of any system involving several radionuclides. LITERATURE CITED
(1) Christian, D., et al., Sucleonics, 10, S o . 5 , 41 (1952). (2) Cook, T. B., and Haynes, S. K., Phys. Rev., 86, 190-5 (1952).
(3) Coryell, C. D., and Sugarman, N., “Radiochemical Studies. The Fission Products,” Xational Nuclear Energy Series, 1‘01. 9, Div. IV, Xew York, McGraw-Hill Book Co., 1951. (4) Hofstadter, R., and McIntyre. J. A . , .Vucleonics, 7, S o . 3, 32 (1950). (5) Hopkins, J. I., Rec. Sci. Instr., 22, 29 (1951). (6) Johansson, S. A., Yature. 166, 794-5 (1950). (7) Jordan, W. H., “ilnnual Review of Nuclear Science,” Vol. I, Stanford, Calif., Annual Reviews. Inc., 1952. (8) Kelley, G. G., Nucleonics, 10, No. 4, 35 (1952). (9) Maeder, D., and Wintersteiger, V., Phys. Rev., 87, 537-8 (1952). (10) Afaienshein, F. C., Oak Ridge Xational Laboratory, Oak Ridge, Tenn., Document ORNL-1142 (1952). (11) Parsons,, J. H., Atomic Energy Commission, Docuneiit AECD1827 (1948). (12) Rider, P. R., “Statistical Methods,” London, John Wiley & Sons, 1939. (13) Schwendiman, L., Hanford Works, Richland, Wash., Documenf HW-18258(1950). (14) Strickler, T. D., and Wadey, I$‘. G., Rev. Sci. Inst., 24, S o . 1 , 13 (1953). (15) Taylor, C. J., et al., P h y s . Rev., 84, 1034 (1951). (16) U. S. Atomic Energy Commission, Isotopes Division, Oak Ridge, Tenn., “Isotopes Catalog and Price List No. 4.” (17) Van Rennes 4 B., Nucleonics, 10, No. 8, 22 (1952). (18) Way, K., et hi.,’“Nuclear Data,” National Bureau of Standards, C~TC 499 . including Supp. 1, 2, and 3 (1950). BEC documents may be obtained from U. S. Atomic Energy Cornmission, Reference Branch, Technical Information Service, P. 0. Box 62, Oak Ridge, Tenn. RECEIVEDfor review January 13, 1953. Accepted April 6, 1953. Presented before t h e Division of Analytical Chemistry at t h e 123rd Meeting of t h e AMERICAN CHmiIcAL SOCIETY,Los Angeles, Calif.
Metal Column for Distillation of Corrosive Gas Mixtures at low Temperatures N. C. ORRICK AND J. D. GIBSOY’ Carbide and Carbon Chemicals Corp., K-25 Plant, Oak Ridge, Tenn.
A
METHOD of analysis was needed for complex multicompo-
nent gaseous mixtures containing corrosive materials such as fluorine, chlorine, and hydrogen fluoride. The identity of some of these components was not known and others were desired in the pure form for further study. Fractional distillation could, in many instances, give a complete analysis as well as pure components and was consequently chosen as a suitable method for the analysis of these mixtures. Even in cases in which distillation has a limited application, because components have boiling points a t about the same temperature or azeotropes are formed, this column is useful in producing distillate samples with a smaller number of components than the original sample. These less complex mixtures are then more readily analyzed by other methods of gas analysis, such as infrared absorption, mass spectrum, and gas density measurements (8). h number of low temperature distillation columns are described 1
Present address, Engineering Staff. Ford Motor Co., Dearborn Mich.
in the literature (1, 3-6); houever, none is suited for corrosive mixtures. The glass in the column, the mercury in the manometer, or the metal packing will react with one or more of the gases present. The corrosion-resistant apparatus described herein is made entirely of nickel, bronze, and chlorotrifluoroethylene polymer (Kel-F, made by AI. 1%‘. Kellogg Co., Jersey City, N. J,). The design is based on that of Podbielniak ( 4 ) with certain modifications. DESCRIPTION OF APPAR4TUS
The apparatus consists of the fractionation column and pot assembly, the temperature- and pressure-measuring instruments, the distillate withdrawal unit, and the automatic coolant supply system. Fractionation Column and Pot Assembly (Figure 1). The fractionation column proper was a 30-inch section of 5 / ~ i n c h nickel tubing packed with 3,1a2-inchnickel helices. The upper
V O L U M E 25, N O . 7, J U L Y 1 9 5 3
1101
The analysis of multicomponent mixtures of corrosive gases is greatly simplified if the individual gases can be isolated prior to determination. To make this separation, a low temperature distillation column is desirable, but no column had been described whose material of construction was resistant to corrosire gases. The distillation column described is suitable for qualitative determination of constituents present in corrosive gas mixtures in the temperature range of -150" to $25' C. when the individual components
portion of this column served as a reflux condenser when cooled with liquid nitrogen under dry air pressure. A brass block 1.5 inches in diameter and 1.5 inches i n height! placed around the condenser, served as a heat reservoir to minimize temperature fluctuations. To the top of the column were attached a gaseous distillate nithtirawal line and a l/lsinch thermocouple well, the latter extending down the center of the column to about 0.5 inch below the condenser. A nickel still pot of 20-ml. capacity was welded to the lower end of the column. Other nickel joints in the apparatus were silversoldered. The still pot was heated by an external, manually controlled, 25-watt heater and was filled through a 3/lo-inch nickel charge line. The all-metal column construct,ion increased the problem of obtaining adiabatic conditions in the fractionation section. This problem was minimized by using a coolant passagen-ay, a vacuum jacket, and 2 inches of fiber glass as insulation for the column. The coolant passageway was used in three applications: ( 1 ) for rapid cooling of the colunin during the transfer of a sample to the apparatus. (2) for prevention of heat inleakage during distillation TO D I S T I L L A T E RECEIVERS, CONTACT WIRES -PRESSURE TRANSMITTER, FOR COOLANT AND VACUUM PUMP CONTROL I F P R E S S U R E TRANSMITT
\
~
FIBERGLASS 'INSULATION
COOLANT PASSAGEWAY COLUMN
II :
CHARGE LINE
COOLANT E X I T
REMOVABLE DEWAR
have boiling points separated by 10" o r more. Quantitative determination or purification is also possible with substances whose boiling points are separated by 30' or more. Comparative results are given for three column packings. This apparatus follows the general design used in the Podbielniak apparatus for low temperature distillation analysis, except that nickel, bronze, and chlorotrifluoroethylene polymer (Kel-F) are used as construction materials and still and distillate pressures are measured indirectly.
below -%', and ( 3 ) for provision of supplementary insulation for distillation a t higher temperatures. These effects were accomplished by directing all. a portion, or none of the coolant exit gases through the coolant passagen-ay. Temperature- and Pressure-Measuring Instruments. A recording potentiometer (llicromax), activated by a copperconstantan thermocouple, !vas used to obtain still-head temperatures. A pressure transmitter (manufactured by the Taylor Instrument Co., Rochester, N. Y., Model 206 RA-0, Mahi KO. 102) was attached to the distillate withdrawal line to prevent corrosive gases from coming in contact with mercury in the manometer, as indicated in Figure 1. The transmitter nae so constructed that the distillate gases Rere confined by a bronze bellom. The still pressure determined the position of the bellows in the transmitter which. through a mechanical lever arrangement, controlled an air leak in a constant pressure air line such that the output pressure of the transmitter n as linearly proportional to the pressure in the still. The still preqsure was then read on a mercury manometer attached to the output side of the transmitter. Another pressure transmitter, not shown, n as installed on the distillate withdrawal unit. Distillate Withdrawal Unit. The distillate withdraw4 unit was made up of two manifolds, three product receivers of 1-liter capacity, one product receiver of &liter capacity, n ithdrawal and vacuum connections, and appropriate valves. One nianifo!d served to distribute the distillate gases to a selrcted receiver. This manifold v a s constructed of '/rinch tubing to minimize the distillate holdup volume. The second manifold permitted distillate gases to be transferred from distillate receivers to storage cylinders TT hile distillate withdrawal continued into another receiver and also permitted evacuation of distillate receivers for further w e m ithout interfering with the diptillation. A11 valves used had vacuum-tight bronze bellows seals and all had Kel-F seats, except one needle valve used to control the rate of gaseous distillate withdrawal Automatic Coolant Supply System. An autoniatic coolant supply system is essential on a system of this kind. if constant pressure and consequent equilibrium conditions are to be maintained in the distillation column. The automatic coolant system was controlled b j the position of the mercury in the U-tube still manometer 15 hich, through electrical contact wires and electronic relays, operated solenoid valves in air lines to the liquid nitrogen coolant, or a pot heater switch as shown in Figure 2. The 10 r.p.m. motor-driven cam operating a microswitch in the output circuit of the lower contact relay served to give intermittent cooling, thereby achieving better control by avoiding supercooling. A manual coolant switch was also included. The automatic coolant system supplied more coolant to the condenser as pressure built up in the still and less as the pressure decreased; this operation maintained the still pressure at zkO.5 mm. of any set pressure in the range of 50 to 800 mm. As a safety measure, mercury contact with the center wire 2 mm. above the first gave continuous cooling and the upper wire turned off the pot heater when the pressure in the manometer increased about 3 mm. beyond the center wire. PROCEDURE
-4sample \vas vapor-transferred to the evacuated still pot and 2 5 W A T T POT H E A T E R
Figure 1. Distillation Column for Corrosive Cases
cooled by liquid nitrogen, and the sample charge line valve was closed. The still pot was then heated until the desired operating pressure was reached in the distilling column, a t which time the automatic cooling controls were adjusted to maintain this pressure. The column usuallv reached equilibrium conditions in 10 to 20 minutes, as indicated by a cessation of pressure fluctuations. Temperature and pressure in the still head were recorded, and dip-
ANALYTICAL CHEMISTRY
1102
The distillate withdra.rva1 or take-off rate was maintained at about 20 cc. per minute as long as the temperature remained conMaximum Purity stant, was reduced to about 5 cc. per minute as the distillate teniBoiling Gas Take-Off oi Plateau hIateria1, perature rose, and was maintained a t this rate until the teniperaR~~ Compounds point, Charged, Rate, ~~~~k Xo. Charged C. Cc. Cc./Min. Point 70 ture again became constant. A center-cut sample was taken from 1 Freon 12 - 2 9 . 4 441 20 441 cc. 99 + each constant boiling plateau and purities n-ere determined by Freon 114 3.0 452 Freon 12 99+ the gas density method with results as shown in Table I. The 2 Freon 22 -40.8 427 20 4 2 i cc. 83 Freon 12 -29.4 388 Freon 22 91 purity of the plateau material was higher when the boiling points of the components were 30" apart than prhen boiling points Table 11. Comparison of Column Packings hIaximunl Break Purity were only 11' apart. -1nalysis was possiCompoiinds Chargi,il, Gas Take-off Rate, % ::$, ble to within 1 cc. for Freon l 2 a n d Freon Packing Charged Cc. Cr./IIin. Frron 22 yo 114 bvdrawing fractionation curvesof tem8 / 8 2 inch S i helices l'rron 22 417 20 427 83 perature us. volume distilled and selecting Freon 12 388 91 the break point as the mid-point on the S i Heli-Pak. 0.033 X 0.070 X 0.070 inch Freon 22 401 20 5Oi i4 Freon 12 850 97 curve between t'wo constant boiling 543 84 plateaus. One cubic centimeter in 800 to S i Hell-Pak. 0.050 X 0.100 X 0.100 inch Frron 22 466 20 Freon 12 1080 9.5 900 cc. gives an accuracy of about 0.1 yoof the total sample or about 0.2% of the individual component. The Freons having tillate withdrawal was started through the distillate rate valve boiling points separated by only 11" gave a similar analytical into a 1- or 5-liter, evacuated product receiver, whose exact accuracy, a fortuitous result for this particular distillation, as volume had been determined previously. -4 record was kept of indicated by the purity of the plateau material the pressure in the receiver as well as the distillation temperature A curve of volume per cent distilled us. dlstillation temperature In an effort to improve the efficiency of the column, the helice. was made when an analysis of knomn constituents was desired. originally used for column packing were replaced n-ith Heli-Pal, Khen one or more component8 were unknown. center cuts were (manufactured by the Podbielniak Co.: Chicago). Two differtaken from each conqtant temperature plateau for identification ent sizes of this packing were tried. Results from test runs made by other means. using Freon 12 and Freon 22 with each of t h e v packings are shown EXPERIM EYTA L in Table 11. .Isit was not practical to install sight glasses or other means of The Heli-Pak packing, 0.050 x 0.100 X 0.100 inch, was sedetermining reflux a i t h corrosive gases over such a wide range of lected as the most efficient packing. The column was brought to temperature, two test runs were made to determine the operatequilibrium in about the same time as required by the helices ing characteristics of the column, using ( a ) a known mixture of and gave a greater purity of plateau material. The smaller size and Freon 12 Freon 114 (1,2-dichloro-1,1,2,2,-tetrafluoroethane) Heli-Pak packing did not allow maintenance of equilibrium condi(dichlorodifluoromethane) and ( b ) a k n o m mixture of Freon 12 tions durinK distillation and consequently could not be expected and Freon 22 (chlorodifluoromethane) Data from t h e v disto fiix e reliable results. Corrosive gas samples analyzed in this laboratory by the low tillations are shown in Table I
Table I. Results of Fractionations of Freons
-
"?,?'I
Y
Figure 2.
Coolant Supply System
1103
V O L U M E 25, NO. 7, J U L Y 1 9 5 3 SU31311RY
h metal and plastic distillation apparatus n w designetl. con-
Table 111. Corrosive Gas Mixtures
structed, and operated successfully in the analysis of corrosive _ Sxmnll= Samwle 2 . ~ - - --1 _ _ ~~
~
Boiling point,
rolllyollcnt
c.
Compo ne n t
~~
Boil$
wiiit, C.
gas mixtures and in the purificattion of sonic of the components. This apparat,us did not have the nccurary and precision of an analytical low temperature gas distillation apparatus for noncorrosive mixtures, a situation resulting from the inherent tllermal conductivity of the metal required for coristruction. ACKNO'(VLEDG3lENT
a 3IasP s p r c t r o i n e k r sc~ansindicatc the presence of C102. ClOa. ClOal'. and C10sF. Infrared analysis s h o w t h a t t h e mai,or constituent in this material is identical with ClOzF obtained b y synthesis.
The authors wish t o thank FI.'H. Lunn, ~ l i o scni1)ly of the apparatus. LITERZTCRE ClTED
tcnipc~r:tture distillation method xere priinaril). products ol' several cheniicd reactione, each carried out under various eonditions. The constituents \?-ere t,he same for a series of samples resulting from the same reaction, varying only in coiiceiitration. ;Zfter the constituents were identified, further samples in the mine series could usually be analyzed by distillation alone or in csonjunc-tiori with one or two molecular weight determinations. &\s an indicat,ion of the typc or niisturcs an:tlyzed, tlie components found in two s : i m p I ~arc ~ listcti in Table 111.
C:tliEomia Satural Gasoline Association, "'l'critative Standard Nethod for Xiialysis of Natural Gas and Gasoline by Fmct,ional Distillatlion." .~ . ~ . BUZZ.TS-411 ~~-(1941). ( 2 ) Sash, L. K., ANIL. CHEM.,22,108 (1950). ( 8 ) Oberfell, G. G., and -4ldei1, R. C., Oiland Gris J . , 27, 142 (1928). 1
I
~
(4)
~
~
~~~
~~
Podbielniak, w. J., I K D .
I
~
~I
EiYC. CHEhi., A N A L . E D . ,
13, 639 (1941).
(5) Shepherd, M.,J . Research AVntZ.Bur. Standards, 26, 227 (1941) R E C E I V E for D review Sol-rntber 17, 1932. Accepted A p r i l 17, 1033. Preiented brforc the Southeastern Rraionai Xcetinp, A M E R I C ~ X Ciri:aiic.~~ SOCIETY. Atlanta, Ga., October 1 7 . 1950. I3ased on work ~ierforiliedfor the Atomic E n e r q Commission.
Use of. Thermistors in Precise Measurement of Small Temperature Differences Tlierinometric Determination of Molecular Weights i u L m H. MCLLERI
AND
Iriss J . s'roLrm2
W h s l i i n g t o n Square College of Arts and Sciences, .Vew l b r k L'niversity, ,\eiv
1 orli 3, ,V.Y.
So extensi,c data are available 0 1 1 the reliabilil) and strict reproducibiiiky of thermistors. Because every thermistor possesses several variable physical properties, specific information is needed concerning the selection of any one of these for use. The selection of a high resistance thermistor and circuitrj applping to the precise measurement of small temperature differences is described. Tables of data concerning thermistor characteristics, their reliability and reproducibility, have been obtained. The resulting apparatus is used for the thermometric determination of the n~olecularweight in solution of several nonvolatile solutes in various solvents. The magnitude of the signal produced by thermistors permits analytical determinations with a precision exceeding 1%.
S
I- thermal sciisitig clement have recently been descrilwti (2, 3.6, V c 9 ) . Thcsc? use thermistors whose resistance is of the order o f 5000 ohms a t 25" C. In this iiivrstigation a thei~nistorof approximately 100,000 ohms resistariw nt 25" C. with a riegativc tcniperaturc coefficient of resistance of :ipl)roxiniately 4.6% pcr degrw ceritigrade n m srlected. This thrrmistor is the 14.4, produced by the Western Electric (10. I t is used for tempei,ature measurement, control, and compensation. The physical diiiirusions arc illustrated in Figure 1. The thcrniistor clement, which is a small brad, is enclosed in the slightly eiilargcd end of a solid glass cj-linder having two tinned wire terminals brought out axially at t,he opposite end from the scnsitivc clement. The nominal ratings given for this type nppcar in Table I. The "cold resistance" R, is the resistance of the thermistor, nic:i.wreti with a power small enough so as not to heat it appreci1
:tl)ly, at n spccifietl :mbic,nt telil~Jc~:ltUr~, Tlic: resist:tiiue-trniperaturr function is :rpproxininted over a range of several huiithcd dcyrees 1))- t h c relation
n
=
xo Pxp B (1, T
- li7'0)
where K is thc resistance a t any Kelvin temperature 2'; Ito is the resistance a t tlie reference trniperature 7'0; exp is 2 . l i 8 . . the Snperinti basc; and B is :rl)prosimately a constant v l ~ o s e value dcyeiids on tlic nature of the soniconductor. Tht. tenip r s t u r e coefficirnt
a = 1,'R X dI?, dl' is related t,o B by the approsim:itc. rrlatioii n
=
- R . 7'?
The di,5sipatioii eoiistniit
Present addrms, The Los .4i:inios Scientific Laboratory, Los Alamos,
s.AI. 2
Present address, Central R e s e a d l Laboratory. General Aniline & Film
Corp., Easton, Pa.
is thc proportionality constant between the po\vcr (libsipation
