uration described hy Chen (8) is not critical, as the relaxation constants of these methyl groups should be very close, because of the similarity of their molecular environments. Therefore, high radio-frequency power can be used to get high signal to noise ratios for the determination of low concentrations. For polyisoprenes containing all Sour structural units, the present method can be comhincd with the NMR method described previously (2) or one of the infrared methods, which are satis-
factory for the determination of the 1,2 and 3,4 Unit contents of polyisoprenes (1,S, 4 , 5 ) . Thus polyisoprenes contnining a wide range of 1,4, 1,2, and 3,4 repeating units can he analyzed with an estimated over-all error of about 2 to
3y0.
LITERATURE CITED
(1) Binder, J. L., Ransaw, H. C., ANAL.
CHEM.29,503 (1957). (2) Chen, H. Y.,lbid., 34, 1134(1962). (3)33, Corish, 975 (1960), P. J., Rubber Chem. & Technol. (4) Cunneen, J. I., Higgins, G. M. C., Watson, W. F., J . Polymer Sci. 40, 1
(1959).
ACKNOWLEDGMENT
The author thanks E. G . Pritchett and William Hoffman for synthesizing some of the polyisoprene samples used in this work.
( 5 ) Richardson, W. S., Sscher, A. J., Ibid., 10,353 (1953).
(6) Tobolsky, A. V., Rogers, C. E,, Rubber Chem. & Techml. 33,655 (1960). R~~~~~~ for review 25, 1962. Accepted October 8, 1962.
Determination of the Surface Population of Copper Oxide Whiskers by Electron Microscopy Techniques WILLIAM R. LASKO and WARREN K. TlCE Research Laborotories, United Aircraft Carp., East Hartford, Cann.
b Two electron microscope techniques are discussed for the characterization of unusual oxide whisker growths formed on copper. The direct transmission technique has been employed b y many investigators to determine the size, shape, and population of the growths. However, this technique is somewhat limited in affording an accurate population and shape estimation. The inability to make a true whisker count can be attributed in part to the masking error introduced b y the whiskers appearing in the foreground of the specimen surface and the inability to make a calculation of an exact areo. Other factors such as surface roughness and grain orientation, which also influence growth, cannat be observed b y this technique. In addition, the shape of the whisker is represented b y a silhouette which provides information in only two dimensions. On the other hand, the indirect or carbon replication technique employing selective etchantr permits a more quantitative estimation of the whisker population and reveals the true whisker shape.
ground of the specimen and because of the errors introduced by grain orientation (7) and surface roughness. In the proposed carbon replication technique employing selective etchants, a more accurate method of determining the surface population of whiskers is afforded. The technique also permits
/
,.. . ?
T
(8, 6) in the field of whisker technology, particularly in t.he attempts (S, 4) made to postulate the mechanism of growth and the relation of this growth process to the dislocation concept, has indicated the need for a new approach to the measurement of whisker population. In the past, these population counts have been made using the direct-transmission technique from whiskers emanating from wires, holes in disks, slotted disks, etc. These measurements are subject to error because of thr masking effect introduced by the whiskers originating in the foreHE RLXEWED ACTIVITY
lhl
Figure 1. Oxide whiskers formed on copper in air at 400' C. for 30 minutes
( 15,000 X )
(I. b.
Direct tranrmirrion Replication
the observation of the true whisker shape as well as providing basic informs, tion about the nature of the growth site.
SPECIMEN PREPARATION TECHNIQUES
In preparing the specimens for this study extreme care was exercised to ensure that the specimen surfaces examined by the transmission and replication techniques were similar. However, the area available for study using the replication technique was considerably larger than that afforded by the transmission method. Details of each of the techniques follows: Direct Transmission. I n the directtransmission techniaue disks 0.3 cm. in diameter were dnnched out of a 0.0152-cm. sheet of high purity (99.999'%j copper and a slot was prepared with a No. 6 jewelers' saw. Thc disks were degreased and annealed in argon at 4.50' C. for 4 hours t o eliminate the stresses induced during the preparation of the slotted disk for both techniques. In, addition, the disks were cleaned by etching with a 507, solution of HNOl followed by reduction in 507, HC1 and subsequent rinsing in demineralized water. This step was undertaken to ensure that all impurities and asperities introduced during cutting and annealing mere removed as well as to make the surfaces for whisker growth homogenous. Whiskers were grown in the area of the slot by containing the disks in a porcelain boat and exposing to air for 30 minutes at a temperature of 400' C. A Hitachi HU-11 electron microscope was used to examine the growth products, and normnl bright fieldimages, using the 75-kv. beam, wcre taken a t 3000X and enlarged optically to 15,000X. I n Figure IQ an clcctron micrograph of typical oxide growth products formed VOL. 34, NO, 1 3 , DECEMBER 1962
-
1795
whisker population counts obtained by the replication procedure, on the average, are somewhat greater than that typified by the direct transmission technique because the larger whiskers in the foreground of the specimen may completely obscure many of the smaller whiskers occurring in the background. In addition, the replication technique reveals quite clearly the true surface whisker population without the problems of depth of field, surface roughness, etc., as illustrated by the micrograph in Figure l b . From the carbon shells of the whiskers, the tubelike shape of the whiskers (shown by a tailed arrow) is clearly discerned. In some cases, depending on how the whiskers are tilted, the cross-sectional shape of the whiskers (shown by untailed arrows) can be
observed. The micrographs show also that the whiskers emanate a t some oblique angle rather than a t normal incidence. The effect of grain orientation on whisker growth is seen in Figure 3a, where the area in the center of the micrograph is relatively devoid of growth while other areas (shown by untailed arrows) show pronounced growth. At still higher magnifications, as shown by the micrograph in Figure 3b, some of these effects are more pronounced and reveal the background structure of the base oxide. The replication technique affords a more dependable method of determining whisker population. The technique should also have wide applicability in providing additional information on the over-all morphology of oxide whiskers,
particularly in attempts to correlate whisker population densities with dislocation counts. LITERATURE CITED
(1) Bradley, D. E., J. Appl. Phys. 27, 1399-412 (1956). ( 2 ) Eshelby, J. D., Phys. Rev. 3, 755-6 (1953). (3) Franks, J., Acta Met. 6, 103 (1958). (4) Gulbransen, E. A., Copan, T. P.,
Andrew, K. F., J . Electrochem. SOC.
108, 119-23 (1961). (5) Lasko, W. R., ANAL. CHEM. 29, 784-6 (1957). (6) Lasko, W., R., Tice, W. K., J . Electrochem. SOC.109, 211-15 (1962). (7) Martius, U., Discussions Faraday SOC.28,220-1 (1959).
RECEIVEDfor review July 27, 1063. Accepted October 16, 1962.
The UIt raviolet Spectrophotometric Determination of Benzene in Air Samples Adsorbed on Silica Gel HERVEY B. ELKINS, LEONARD
D. PAGNOTTO, and ELISE M. COMPRONI
Division o f Occupational Hygiene, Massachusetts Deparfmenf o f labor and Indusfries, Boston, Mass.
b This paper describes the analysis o f air for benzene vapor when it i s present as a component of petroleum naphtha with toluene and other higher boiling aromatics which may interfere. Samples are collected on silica gel and desorbed b y cyclohexane-n-heptane, ethylene dichloride-tetrachlorodifluoroethane, or isopropyl acetate-n-propyl acetate solvent systems. After fractionation, the benzene i s recovered in the lower boiling component of the solvent system used, and determined b y ultraviolet spectrophotometry. Virtually 100% efficiency of collection, at a rate of 1 to 3 liters per minute with a 50- to 100-liter air sample (adequate for measuring benzene below 10 p.p.m.), i s obtained when employing a U-shaped sampling tube, each arm holding 10 grams of silica gel. When longer samples are collected, or when higher sampling rates are used, some benzene may be lost, especially at high relative humidities.
B
BENZENE is one of the most toxic of the common organic solvents, its determination in workroom air has long been an important industrial hygiene procedure. The first major studies of benzene exposure were made by adsorption of the vapor on activated charcoal, followed by gravimetric determination ECAUSE
(6). This method was, of course, subject to interference from other organic vapors. Smyth developed a nitration procedure, followed by reduction of the dinitrobenzene, after steam distillation, with standard titanium trichloride (IO). Schrenk and coworkers determined the dinitrobenzene colorimetrically after addition of methyl ethyl ketone and strong alkali (9). Modifications of this procedure are in use (2, 3). The color developed by reaction with sulfuric acid and formaldehyde has also been used for determining atmospheric benzene (6), Hubbard and Silverman adapting this reaction to a directreading indicator tube ( 7 ) . Aliphatic solvents do not as a rule interfere in these methods, but toluene and other aromatic vapors do in varying degrees. The devices employed to overcome such interference have been only partially successful. The availability of reliable ultraviolet spectrophotometers has provided a new tool for the determination of benzene vapor in air. illaffett and coworkers adsorb the benzene on silica gel, followed by desorption with water and extraction with iso-octane, and determination of the absorbance a t 254.4 mp (8). Fahy, in this laboratory, extracted the benzene directly from the silica gel with isopropyl alcohol
(4).
While the characteristic ultraviolet absorption spectrum of benzene makes its determination in the presence of most solvents of the aliphatic series relatively easy, other aromatics, such as toluene, interfere. Since the absorption spectra of toluene and benzene are not identical, both compounds can in theory be determined by making measurements a t two or more wavelengths and solving simultaneous equations. A formula based on this principle is employed in the ASTM method for benzene in petroleum naphtha (1). In practice, the measurement of benzene in the presence of small amounts of toluene by this procedure presents no difficulty. As the relative amount of toluene increases, the accuracy of the benzene determination is diminished and with a large excess of toluene is quite unsatisfactory. In a study of benzene exposures occurring chiefly in plants handling naphthas with varying aromatic content, the presence of toluene was frequently encountered. Higher aromatics, and other components of unknown identity, which also absorbed ultraviolet light in the range of interest (240 to 280 mp) were also present in some cases. For such situations, a procedure involving separation of the benzene from higher aromatics by fractional distillation in a carrier solvent was developed. VOL. 34, NO, 13, DECEMBER 1962
0
1797
