Bromine again serves as a suitable illustration. There is evidence ( 5 ) for a molecular species Brl, with an absorption maximum near 205 nanometers ( E about 1200). We have applied the absorbance temperature correlation to bromine vapor between 190-250 nanometers with condensed phase temperatures in the range 267-92 OK. At each wavelength the plots were linear (six to nine observed points), but the slope increases steadily with decrease in wavelength reaching a maximum at about 200 nanometers with a slope equivalent to about 61 kJ mole-1 for the liquid-vapor transition. Although further calculations and interpretations from these results are possible, the important feature is that the absorption band in bromine vapor at 205 nanometers is, by
virtue of its log,, A us. l/T, behavior, not predominantly associated with the same molecular species as gives rise to the stronger absorption at longer wavelengths. Under these experimental conditions, the vapor is not behaving ideally, yet the absorbance-temperature correlations are still meaningful and amenable to simple interpretation. We have applied the absorbance-temperature correlation extensively in a study of about fifty nitro and nitroso compounds, many of which dimerize, and many of which are photochemically active. RECEIVED for review December 21, 1971. Accepted March 23,1972.
CORRESPONDENCE Determination of Low Surface Areas by the Continuous Flow Method SIR: The use of the continuous flow method ( I ) for measuring surface areas as low as 0.1 mZ/ghas been reported (2-4). However, anomalous signals have been seen (2), which merge with the true signal, thus limiting the accuracy of low surface area measurements. We have also observed anomalous peaks on a variety of materials and have found that they can be eliminated by proper cell geometry. EXPERIMENTAL Apparatus. All measurements were made with a Quantasorb (Quantachrome Corporation, Greenvale, N.Y. 11548), a continuous flow instrument for measuring the adsorption of gases on solid samples. Signals were simultaneously integrated with the built-in electronic integrator, and recorded with a potentiometric strip chart recorder. Two types of sample cells were used. One was of conventional design-U-shaped with an enlarged horizontal cylinder at the bottom. The arms were 4-mm i.d., 1-mm wall thickness, and 10 cm long. The horizontal region was 12-mm i.d., I-mm wall thickness, 3 cm long, and had a volume of about 3.4 cc. The other cell was a simple U-tube with these dimensions: 2-mm i.d., 1.5-mm wall thickness, 11.5 cm high, and having about a 3-cm separation between arms. Reagents. The adsorbent and carrier gases were J. T. Baker Company’s zero grade nitrogen and helium, respectively. Mixtures of these gases were used as supplied by Baker, who guaranteed their compositions to 1 relative. A sample of zinc oxide was obtained from New Jersey Zinc Co., who reported its specific surface area as 3.80 m2/g. Our three point B.E.T. analysis of this sample gave 3.83 m*/g, and our single point analysis (9,using a relative pressure of 0.2, gave 3.88 m2/g. Procedure. Samples were outgassed at 110 “C for one hour. Completeness of outgassing was checked by observing that repetitive adsorptions and desorptions gave reproducible (1) F. M. Nelsen and F. T. Eggertsen, ANAL.CHEM., 30,1387 (1958). (2) M. G. Farey and B. G. Tucker, ibid., 43, 1307 (1971). (3) E. Cremer and H. Huck, Glusreclr. Ber., 37, 511 (1964). (4) H,W. Caeschner and F. H. Stross, ANAL. CHEM.,34, 1150 (1962). ( 5 ) G. Sandstede and E. P. Robins, Chem. Ing.-Tech., 32, 413 (1960).
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Figure 1. Desorption of a small volume of nitrogen
results. All determinations on the above zinc oxide sample were made using the single point method ( 5 ) with a relative pressure of 0.2. Volume gas flow rates were always 12.0 cc/min. Calibrations of the gas volumes were made with nitrogen, or for very small signals, with mixtures of nitrogen and helium (6). RESULTS AND DISCUSSION
Table I shows the results of determinations of the areas of varying quantities of a zinc oxide sample using the “conventional” cell, and compares these results with the actual areas calculated from the specific surface area of the sample. The last column of Table I shows the development of the anomalous signal as the sample size is decreased. The height of the positive anomalous peak on the last entry in Table I is approximately half as high as the actual peak. Figure 1 shows a complete desorption tracing where the anomalous signal is fully developed. As a result of the merging of the peaks, and of the positive-negative nature of the anomalous signal, accurate integration of the true peak is impossible when these signals are of the same magnitude. The results in Table I were acquired by commencing integration at the start of the initial rise off the base line, and were continued until the final return back to the base (6) S. Karp and S . Lowell, ANAL.CHEM., 43, 1910 (1971).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
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Table 11. Data Obtained Using U-Tube Cell Wt ZnO, g Actual area, mZa Measured area, mzb 0.228 0.230 0.059 0.105 0.102 0.027 0.0175 0.O16lc 0.0045 a Calculated from specific surface area of 3.88 m2/g. * Average of duplicate determinations. Repeatability was within 3 relative. c This value is less 15 cm2 for the cell wall area, as estimated from the cell dimensions. Desorption peaks from an empty U-tube cell gave areas of 12-17 cm2.
Table I. Data Obtained Using “Conventional” Cell Signal shape at start of Actual Measured Deviation, desorption Weight, g area, m Z a area, m Z b peak
z
1.305
5.07
5.07
0
0.739
2.87
2.87
0
0.378
1.47
1.45
-1.3
0.177
0.687
0.686
-0.14
0.089
0.345
0.327
-5.5
0.049
0.190
0.166
-12.6
0.0190
0,0730
0.0481
-34.1,
0.0101
0,0394
0.0192
-51.3
z
Calculated from specific surface area of 3.88 m2/g. of at least two determinations. Repeatability was within 3 relative. a
* Average
line. Attempts a t commencing integration at any other point gave similar or larger deviations compared to those in Table I. It is apparent that the anomalous signal limits the minimum surface areas measurable. The anomalous desorption signals shown in Table I and Figure 1 are typical of what we have observed on measuring low areas with other adsorbents including alumina, charcoal, silica, and several organic substances. With few exceptions, adsorption tracings show anomalous signals as bad as or worse than the desorption tracings. The shape of the anomalous desorption signals, with their positive (nitrogen-rich) and negative (nitrogen-poor) regions, is due to the separation of the mixed gases which results from thermal diffusion. Thermal diffusion is to be expected under the extreme temperature gradients involved in this method (3). It was, therefore, reasoned that if the linear gas flow velocity through the cell were increased, and the void volume in the cell were decreased, then the separation of the gases would be minimized. This reasoning was tested using the simple U-tube cell described above. Areas measured with the U-tube cell agreed with the calculated areas (Table 11), and exhibited no anomalous peaks. This
cell geometry eliminated the anomalous signals on all other materials tried, e.g., alumina, charcoal, silica, and some organic substances. Farey and Tucker (2), using the continuous flow method, reported that slight shoulders appeared on desorption peaks which corresponded to volumes of adsorbate not sufficient for completion of a monolayer, and that these shoulders disappeared when the gas volume exceeded that required for monolayer coverage. The implication that appearance and disappearance of these peaks are related to monolayer and multilayer absorption cannot be valid for the following reason. In the continuous flow method, desorption of nitrogen results from removing the liquid nitrogen bath from the sample cell, causing the adsorbed nitrogen to rapidly leave the surface at a rate controlled essentially by heat transfer. Farley and Tucker’s reference to Kuge and Yoshikawa’s work (7) is not applicable in this instance since the latter were actually observing the development of “frontalgrams,” according to the method of frontal analysis (8). The authors believe that, by using the U-tube cell, the continuous flow technique, with nitrogen as the adsorbate, can be used for the low area measurements heretofore only measurable by the more cumbersome krypton adsorption method.
SEYMOUR LOWELL STEWART KARP C. W. Post College Long Island University Greenvale, N.Y. 11548
RECEIVED for review December 17, 1971. Accepted May 2, 1972. (7) Y. Kuge and Y. Yoshikawa, Bull. Chem. SOC.Jap., 38, 948 (1965). (8) S. J. Gregg, “Surface Chemistry of Solids,” Reinhold Publishing Corp., New York, N.Y., 1961, Chap. 8.
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