inlet connectors, for sample and for carrier, Cat. KO. 264-35; and column connectors, Cat. No. 344-8. Other commercially available equipment includes : therrial conductivity detector, RIodel KO. 9677-AEL with 8-K thermistors (Gowhlac Instrument Co., Madison, N. J.); 5-way valves, stainless steel with Teflon plugs (Conant Valve Co., hledford, hcass.). The following components were fabricated in the ORNL shops: cubicle, cast from Xo. 108 alumhum casting alloy; cabinet, U. S. standaid 18-gauge coldrolled sheet steel; fmeplate, 1/4 inch thick Micarta. The cubicle is 8 inches on each side; one side is open. The walls of the cubicle are 9/16 inch thick. Inside the cubicle, connecting lines are provided for the f l o ~of gas from the gas-sample inlet to a %way valve, from the 5-way valve to each column inlet, from each column outlet t o a second 5-way valve, and from this 5-way valve to the detector. I n order to minimize dead volumes, these connwtions are made with l/le-inch 0.d. stainless steel tubing. The cubicle may be hwted by means of cartridge heaters inser ed into its base a t each corner. Electrical connections from the selector switch to each column heater and to each thermocouple and from the selector switch to the cable connectors are also 13cated inside the cubicle. The detectcr oven with the T C cell is mounted on the bottom of the cubicle. The door 01 the side of the sheet-metal cabinet Flrovides access to the carrier-gas connections of the T C detector and to the cwtridge heaters of the cubicle. A chromatographic column
Table 1.
Results of Analyses of Synthetic Gas Mixture
With unmodified Kromo-Tog Component" found, Std. dev., vol. 7c vol. 7* 25.15 0.10
Component present Ethane 11.88 Ethylene Propane 8.42 Butane 20.31 Butene-1 34.24 Average of four determinations.
is connected to each of the four vertical faces of the cubicle. The columns are held securely in place by Transite standoff insulators with latches. Guards provide insulation and protection for the columns. The valves and switch on the removable faceplate are operated by means of extension handles through the n-alls of a shadow shield or, if simple adaptors are provided, by means of a master-slave manipulator. The vacuum-gas sampling apparatus is designed for measuring and injecting samples a t pressures in the range from 10 mm. to 2 atmospheres absolute.
A synthetic mixture that contained most of the principal components obtained from the hydrolysis of the carbides of uranium and thorium was used to compare the operating efficiencies of the unmodified Kromo-Tog and the described assembly. The results of the analyses are shown in Table I. The
0.07 0.25 0.09
0.16
With remotely controlled assembly Component" found, Std. dev., vol. yo vol. % 25.65 11.35 8.51 PO 21 34.28
0.25 0.34 0.34 0.31 0.55
identities of the constituents of the mixture were known but not the amounts. *4Disc Integrator was used to measure the area under each elution peak. The area of the peak for each component ivas compared with that for a known volume of a C.P.grade standard gas. The results obtained by means of the described assembly are in good agreement m-ith those obtained by means of the unmodified Kromo-Tog. The loss of precision with the former is caused primarily by the difficulty in reading the shielded manometer. The results were normalized to eliminate the contribution of approximately 1% contamination by air that was introduced during sampling. The effect of radiation on column packings and on electrical components has not been studied fully. The radioactivity of the samples that have been analyzed to date has not exceeded a few microcuries of gamma radiation.
A Simple Cryoscope for Molecular Weight Determinations Using a Thermel Detector and Phenyl Ether as Solvent R. H. Campbell,' A.
E.
Bekebrede, and B.
was needed for determining the average molecular weight of dark colored polyphenyl mixtures taken from organic-moderated nuclear reactors. The color of these samples rendered impossible an accurai e or reproducible visual detection of either the freezing or the melting point. This problem was overcome by the design and fabrication of a cryoscopic molwxlar weight apparatus incorporating an automatic stirring device and a thermel for temperature measurement. Phenyl ether was used as solvent. Although the use oj' thermocouples to measure freezing po nts in molecular n-eight determinations is not new, this paper describes a 1m-cost, efficient,
A in. our laborator:;
N ANALYTICAL METHOD
Present Address: IvIonsanto Chemical Co., Nitro, W. Va.
J. Gudzinowicz, Monsanto Research Corp., Boston laboratories, Everett 49, Mass. versatile molecular weight apparatus that can be readily assembled for use in any laboratory. EXPERIMENTAL
Apparatus. A schematic drawing of the thermocouple assembly and components is given in Figure 1. The potentiometer used for this investigation was a Model 2733 Rubicon instrument ( Minneapolis-Honeywell Reg. Co., Philadelphia 32, Pa.) containing a standard cell and reference galvanometer with a reported accuracy of 0.005 mv. A 30-inch) 4-junction therinel of 20 gauge iron-constantan thermocouple wire (A. S. Richards Co., Kewton Highlands, Mass.) was prepared according to Daniels et al. ( 2 ) . To prevent thermocouple short-circuiting, a Teflon spacer was placed 1 inch from the end of the thermel. With this arrangement, the thermel provided a senqitivity of
approximately 0.2 nil.. per degree centigrade. The automatic stirring assembly consisted of a belt-driven linear reciprocating stirring device (E. H. Sargent Co., Chicago, Ill.) driven by a 1/100 H.P., Type V-10, electric motor (Palo Laboratory Supplies, Inc., New York,
h-. Y.)
The cell was prepared by inserting a 20- x 150-mm. borosilicate glass test tube through a rubber stopper into a 40- x 100-mm. tube. This arrangement optimized the cooling rate. The solvent used in this method was phenyl ether (Matheson, Coleman and Bell, East Rutherford, K. J.). Procedure. The technique employed is very similar to the steadystate freezing point depresqion methods (I). One end of the thermel is placed in a reference ice bath while the other is inserted into the cell containing 7 to 8 grams of weighed phenyl ether solvent, VOL. 35, NO. 12, NOVEMBER 1963
a
1989
Table I.
Determination of Freezing Point Depression Constant for Phenyl Ether
Kf
Molecular
Solute o-Temhenvl . .
wt.
Molality
(millivolts/ molal)
230.3
0.0727 0.1260 0.0416 0.0939 0.1635 0.2140 0.0693 0.1214 0.0629 0.1489
1.68 1 .TO 1.73 1.70 1.65 1.69 1.63 1.61 1.67 1 .75
m-Terphenyl
230.3
Naphthalene
128.2
Di-tert-butylbenzene
190.3
0.0818 0.1475
Table II. Molecular Weight Determinations of Known Compounds
Molecular weight TheoError, 5 retical Found
Compound Diisodecyl phthalate
446
Tricresyl phosphate Biphenyl
457 468 Av. 462 368 366 368 Av. 367 154 158 161
$3.6 -0.4
Deviation +0.01 $0.03 $0.06 +0.03 -0.02 +0.02 -0.04 -0.06 0.00 SO.08
1.64 -0.03 1.59 -0.08 Av. 1 . 6 7 i= 0.038
the temperature of which is adjusted to approximately 32" C. Cold water (-18" C.) is added to the bath container to provide a slow cooling rate for the ether and automatic stirring is begun. T h e n the temperature (as recorded in millivolts) drops to n-ithin 1' of the anticipated freezing point of the solvent, a small crystal of the phenyl ether is added to prevent uncontrolled super-cooling. The solvent's point of maximum rise in millivolts after the onset of crvstallization is recorded. To determine the molecular weight of a n unknown sample, two consecutire additions of n-eighed Folute or
sample are made to the solvent and the above procedure is repeated after each addition. DISCUSSION A N D RESULTS
Durand and Rouge (3) first determined the molal freezing point depression constant of phenyl ether and found it to be 8" C./molal. This value is approximately twice that of benzene and the solubility properties of the ether in many cases are preferable to those of benzene. Furthermore, its freezing point (26.9" c . ) is more convenient, since melting or crystallization can be brought about simply by using either hot (40" C.) or cold 118" C.) water in the bath. Any common laboratory potentiometer having approximately 0.005-mv. sensitivity n-ill give satisfactory results when used with this apparatus. This sensitivity provides a theoretical detection accuracy of 0.025' C. *in instrument such as the Leeds and Korthrup 7653, Type K-3, provides a sensitivity of 0 0025" C., which compares favorably with that given by a Beckmann thermometer. The molal freezing point depression constant, Kf,for phenyl ether was determined empirically by using a number of recrystallized compounds (Eastman Kodak Co.) aq shown in Table I. The results of the molecular weight determinations of some standard compounds are shown in Table 11. This method has several additional advantages over other cryoccopic methodq. Since visual detection of the freezing point is not required, highly colored solutes may lie determined n-ith no loss of accuracv. The thermel has a wide temperature range so that other qolrents can be used. Its sensitivity t o temperature rivals that of the Beckmann thermometer, while its response to temperature change is more For a compound with a rapid. molecular weight near 200, as little as 80 mg. of sample is required for a determination. Using this apparatus, the relative error obtained is C575. LITERATURE CITED
Figure 1. Drawing of thermocouple assembly and related components A.
B. C.
D.
E. F. G. H. 1. J.
K. I.
1990
Electrical leads to Rubicon potentiometer from 30-inch, 4-junction thermel W i r e stirring rod attached to reciprocating motor Glass sleeve assembly for thermel Rubber stopper Sample side thermel Reference side thermel Cold W a t e r (-1 8" C.) Teflon spacer 20- X 150-mm. test tube 4 0 - X 1 00-mm. test tube Cold water container Dewar flask ice bath
ANALYTICAL CHEMISTRY
(1) Bonnar, R.
I*., Dimbat, hl., Stross, F. H., "Sumber-.kverage Molecular Weights," p. 20, Interscience, Sew York, 1958. (2) Daniels, F., Mattheas, J. H., William, J. W., et al., "Experimental Physical Chemistry," 4th ed., p. 403, McGrawHill, New York, 1949. (3) Durand, J. F., Rouge, E., Bull. SOC. Chem. France 3 7 , 6 9 7 (1923). Work supported in part by the United
States Atomic Energy Commission under Contract No. AT (11-1)-705.
