pwcm length of the plate and about 3 an (45 reflections) can
be employed ifnecessary. There are many applications of the Internal Reflection Probe. As originally illustrated, it eliminates the need of special cells for studying liquids and powders. It is also useful for viscous media and turbid solutions where suitable cells cannot be made. The spectra of acetone and quartzkaolin powder were recorded by dipping a few millimeters of the end of the IRP into the sample and are shown in Figure 3. Spectral contrast can he controlled by adjusting the depth to which the IRP is immersed into the sample. Of particular interest is the use of the IRP for reaction monitoring as re-
quired in profess control. The Internal Reflection Probe can readily be combined with a low cost commercially available instrument for this purpose. Although the use of the internal reflection probe has been demonstrated only for the IR, it can be adapted to any spectral region from the W to the far IR.
N. I. HARRICK Harrick Scientific Corporation Ossining, N. Y.10562
R m m for review May 11,1971. Accepted lune 22,1971
Changes of Dropshapes on Freezing Sm: Davis and Bartell (I) determined the surface tension of molten materials by measurement of the shapes of solidified pendent drops, obtaining reasonable results for a variety of substances, including metals. Their method requires that the shape of the drop not change during “cooling.” This is at least a two-stage process, except for glasses-namely freezing, followed by cooling of the solid to room temperature. They suggest that for isotropic substances the second stage should cause no trouble, hut they do not comment on the first stage. Unless customary precautions are taken, metal castings, on solidification, commonly form depressions at the free surface, or even deep pipes into the body of the casting, owing to shrinkage on freezing. The reverse phenomenon occurs with water and materials such as type-metal, which expand on freezing. The effect of this on drop-shape can invalidate the method unless solidification occurs superficially first, and the interior is frozen in such a way that the volume change is in some way vented. Cheng (2) has reponed the ejection of microdroplets during freezing of a supercooled water drop. An experimental arrangement to allow this venting in a more controlled way can be imagined hut it is difficult to see that this was achieved in the method as originally described. A clear example of the distortion that can occur may be seen in the photographs, Figure 1. Three drops of water are shown, each resting in a shallow depression on top Of an aluminium rod, the bottom of which dips into freezing mixture. As each drop is frozen from below, a change of shape occurs, leadm ultimately to the formation of a cusp. In other experiments, when water drops were placed directly in depressions in pieces of Lhy Ice, the form of the frozen drop resembled (in cross section) a Gothic arch. These more Byzantine forms were always obtained when the aluminum rod was used. The general features of the distortion of a drop on freezing in this way may he understood hy assuming that the original shape of the unfrozen drop is spherical, and that the solid advances in a front of some particular form. If the advance creates a volume do of solid, the amount of liquid removed will not be do, hut do‘ = do p./p,. For water do‘ < do, so a small extra volume do‘ = do(l p,/p..) is generated. If this is accommodated in the liquid, its volume is slightly increased over that contained in the original envelope. If this envelope
-
(1) J.K.DavisandF.E.Bartell,A~*~.~~.,u),llSZ(1948). (2) R. 1. Cheng, Scfenee. 170,1395 (1970).
I Figure 1. Three water drops being frozen from below Io each sequence (from top to bottom) it m a y be seen that the extra i o l m e generated on freezing accumulates at the top of the drop (scale in millimeters)
.
is more than a hemisphere, its radius of curvature R will be increased. It it is less than a hemisphere, R will he decreased. Thus the lower portion of an originally spherical drop should become more gently, and the upper portion more sharply curved. Attempts to cany out this calculation showed that the outer form depends on the shape of the advancing front in a critical way. When it was taken to be flat, the predicted form for the frozen drop was egg-shaped. with the small end up. Assumption of a spherical interface meeting the free surface at right angles led to a cusp, hut no reverse curvature. When freezing was intempted by blowing off the liquid with a puff of air, exposing the interface, it was found to be saucer-shaped, with the center flatter, and the outer portion more curved, than a spherical surface. We did not attempt to use an interface of this form in the calculations. In all versions, the assumption was made that the free surface of the
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
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liquid was continuous through the intersection with the interface, and that no change in slope occurred at this line. This appears to be apprOximatelY justified by the Photo@aPhs, though we expected that the requirement Of equilibrium of the surface and interfacial tensions would require a change of slope. It is clear from these considerations that the calculation of surface tensions from the form of frozen drops is fraught with dangers. The established method (3) involving photography of liquid sessile or pendent drops is to be preferred. (3) W. D. Harkins and A. E. Alexander, in “Physical Methods
of Organic Chemistry,” A, Weissberger, Ed., 3rd ed., Interscience, New York, N. Y., 1959,pp 805-809.
ACKNOWLEDGMENT This work was performed while the author was a guest of the Chemistry Department, University of California, Berkeley. For their hospitality, and for the generous assistance of paul L.Chambrb with the calculations, he is most grateful, R. A. STAIM Department Of Chemistry Trent University Peterborough, Ontario Canada RECEIVED for review May 3, 1971. Accepted June 5, 1971.
I AIDS FOR ANALYTICAL CHEMISTS New Use for a 0.5-Nanometer Molecular Sieve Gas Chromatography Column W. A. McAllister and W. V. Southerland
.
Department of Chemistry, East Carolina University, Greenville, N . C. 27834
THEUSE OF 0.5-nm molecular sieve columns in gas chromatography for separation of diatomic gases is routine. At this laboratory, however, it has been used for the separation of COS and NzO at 250 “C. This has been a valuable asset, allowing the use of a simple gas chromatograph for the separation and detection of CO, NP,NO, COZ,and NPOwithout a column change or modifications to the chromatograph. The literature (1-6) and commercial firms all propose using series columns, parallel columns, or elaborate column switching devices to separate this group of gases. These particular gases result from our studies of some catalyzed oxidationreduction reactions between CO and NO at 1 atmosphere. The detection limit is satisfactory in that a 0.5-ml sample provides a detectable limit of about 1 part per thousand on the 3-ft X l/,-in. column. EXPERIMENTAL
The gas chromatograph used in these measurements was a Gow-Mac Model 69-500 with a 1-mV recorder. Helium flow gas was used with a flow rate of 60 ml per minute and the detector current was 150 mA. The 60-80 mesh 0.5-nm molecular sieve was obtained from Fisher Scientific Company. It had been refined by Coast Engineering Laboratory and was from their lot 81170. It was packed manually in ‘/,-in. copper tubing (approximately 4 grams per foot) and was conditioned for 1 hour at 250 “Cby purging with helium. RESULTS AND DISCUSSION
One can only obtain complete separation of the diatomic portion of the gaseous mixture at 250 “C by using an inordi(1) W. M. Graven, ANAL.CHEM.31, 1197 (1959). (2) E. Heftmann, “Chromatography,” Reinhold Publishing Corp., NewYork,N.Y., 1961. (3) P. G.Jeffery and P. J. Kipping, “Gas Analysis by Gas Chromatography,” The Macmillan Company, New York, N. Y.,1964. (4) E.W. Lard and R. C. Horn, ANAL.CHEM. 32,879 (1960). (5) R. Stock and C. B. F. Rice, “Chromatographic Methods,” Reinhold Publishing Corp., New York, N.Y., 1963. (6) D. H. Szulczewski and T. Higuchi, ANAL.CHEM.,29, 1541 (1957). 1536
Table I. Retention Times for OS-Nanometer Molecular Sieve GC Columns Column length Temp, 3-ft 6-ft “C Gas Retention time, min:sec 0 1 0:30 0:42 N2 0:53 1:12 100 NO 1 :20 2:00
co
250
NzO
con
2:25 2:50 4:oo
3:45 4:05 5:35
nately long column. However, complete separation is not absolutely necessary for our reaction studies because the ratios of peak heights for the diatomic species on the threeand six-foot columns are sufficient to give an approximate indication of how the reaction is proceeding. The temperature can be lowered to 100 “C occasionally to obtain accurate diatomic concentration from peak area measurements. The data for typical operation are given in Table I. An unsuccessful attempt was made to separate SOZ and NOz. Ballistic temperature programming was also unsuccessful, apparently because of the slow rate at which the temperature of the column increases. A more sophisticated instrument or a modification of this instrument oven with an additional heater should work if the temperature rise is rapid enough. The 250 “C temperature seems to be the minimum for separation of the triatomic gases and must be reached fairly quickly while still allowing time for greater separation of the diatomics at lower temperatures. Six feet seems to be the minimum column length for this mode of operation. RECEIVED for review May 13, 1971. Accepted June 14, 1971. This work was supported in part by the National Air Pollution Control Administration, Consumer Protection and Environmental Health Service, Public Health Service Grant Numb& AP01085-01,
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
