Table VI. Enhancement of Signal/ Noise Ratios by Carrier Distillation
- S/N Ratio
Without
Line T 2821
GeOz 26’3
0 340
Cr I1 ‘2677 150 Pb I 28-33 06‘3 Mn I 3070 266 &Io I 3170 347 XI I 29‘32 505
0 585 1 068 1 290 I 102 0.193
C‘ri
iVith GeOn
0 580 1.282 1 572 1 862 2 446 0 402
additional advantage of being adaptable to very small samples as well as t o corroded samples. Partly corroded water pipe circa Louis XIV as well as ancient British cannon have been analyzed by this procedure. The present technique was designed primarily for the elements and concentration ranges of current interest. However, the method could be expanded easily to other elements and concentration ranges for which NBS samples already exist. LITERATURE CITED
deviations (Table V). Relative standard deviations were determined on one s:imple per day over a period of several months to include a n y day-to-day variations in the analysis. During this investigation, covering 7 months, over 1500 analyses mere completed. All results fell within the limits of the relative standard deviations. Millings, drillings, and small chips were analyzed easily without resorting to pelletizing. The method had the
(1) Am. Soc. Testing Materials, Philadelphia, Pa., “Methods for Emission Spectrochemical Analyeis,” pp. 1-35, 1957. ( 2 ) Bardocz, A.,VarsBnyi, F., Acta Tech. Acad. Sci. Hung. 13,409 (1955). (3j Berta, R., Palisca, .4., Spectrochim. Acta 5 , 87 (1952). (4) Canody, L. J., Harris, T., Jr., Woodruff, J. E., J . Opt. Sac. Am. 43, 145 (1953). (5) Carlsson, C. G., Jernkontorets Ann. 129, 193 (1945). (6) Grikit, I. A., Izvest. Akad. S a d S.S.S.R., Ser. Fiz. 19, 171 (1955). (7) Iijima, H., Bunk6 K e n k y d 6 , 16 (195.7).
Kaiima. Yasuda. Y.. Kalima, J.. J., Yasuda, Y., Sanada. Sanada, K.. K., Japdn Anal& Japan Analyst 2, 108’( 108 (1953). 1953). (9) Koritskii, V. G., Izvest. Akad. N a u k S.S.S.R., Ser. Fiz. 12, 429 (1948). (10) Lament, A., Congr. groupe. -atlance. mBhodes anal. spectrog. prod. mkt. 18, 77 (1955). (11) Mathien, V., Spectrochim. Acta 4, ((8) 8) \-,
185 (1950).
(12) Mitsuhashi, T., Shiraishi, Y., Na-
kashima, T.. Tetsu to Haoane 39, 1277 I
,
(1953). (13) Moore, C. E., “Ultraviolet Multiplet Table,” Natl. Bur. of Standards ( U . S. ’I Circ. 488, Section I (1950), Section 11 (1952). (14) Nachtrieb, N. €I,, “Principles and Practices of Spectrochemical Analysis,” pp. 135-9, McGraw-Hill, New York, 1950. (15) Scribner, B. F., hfullin, 13. R., J . Research Natl. Bur. Standards 37, 379 (1946). (16) Weisberger, S , Pristera, F., Reese, E. F., A p p l . Spectroscopy 9, 19, (1955). (17) Yoshinaga, H., Minami, S., Fujita, S., Technol. Repts. Osaka Univ. 5 , 251 (1955).
RECEIVED for review July 5, 1961. Accepted December 4, 1061. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961.
Effect of Rate of Nitrogen Adsorption and Desorption on the Automated Determination of Pore Size Distributions E. V. BALLOU’ Gulf Research & Development Co., Pittsburgh, Pa,
P When pore size distributions of commercial petroleum processing catalysts, or other porous materials, are determined with an automatically programmed apparatus, it i s essential to set the rate of nitrogen adsorption or desorption to a value which yields meaningful daia for subsequent calculations. Several catalysts of different pore structures were studied, and adsorption and desorption data were taken at rates varying from apparent equilibration to markedly nonequilibrium conditions. The test results have aided in optimizing the use of an automatic apparatus to provide service from pore size data as a process research tool, Valid data on small pore catalysts-i.e., median pore radius less than 40 A.-can be obtained when 1.5% of the pore volume i s filled or emptied per minute, while valid data on large pore catalysts-i.e., median pore radius greater than 100 A.must b e obtained at rates of filling or emptying of less than 0.770of the pore volume per minute. Data for catalysts with pore size distributions between 40and 100-A, radius may be obtained at
a rate such that between 0.7 and 1.5% of the pore volume i s filled or emptied per minute, The sum of the equilibration times at all data points should range from 40 to 70% of the total time of the experiment.
s a n industrial catalyst research tool, nitrogen adsorption and desorption experiments yield data which characterize the pore structure of many experimental materials. An automatic apparatus for this purpose has been described ( 1 ) . A mechanically timed cycle n-as then suggested, for data matching that from manual equilibration for the catalysts used. However, the suggested timing cycle was limited in two respects; its application was only confirmed for small pore catalysts, and the maximum rate a t which adequate data could be obtained was not indicated. A more detailed study on both these points was needed to service process research programs properly with automatic equipment. Consideration was, therefore, given to the factors which could be varied
to obtain nitrogen adsorption data in minimum time on materials of both large and small pore diameters. The total time spent on a n experimental run was a function of the following interrelated factors: the number of data points; the gas dose size; the sample size; the orer-all rate of vapor addition or removal; and the relative amount of time spent in equilibrating a t data points, compared to the amount of time spent in gas flow into or out of the sample and vapor system volume. For the purposes of these experiments, 10 points were considered adequate to define an isothermal adsorption or desorption curve. In practice, this number varies viith the complexity of the curre and the degree of definition desired. The gas dose size was limited n-ith the apparatus used to bet1veen 1.0 and 2.5 cc., a t standard temperature and pressure, per dose. It was not experimentally convenient to reduce or increase this amount, and still retain the 1 Present address, Lockheed Missiles and Space Co., Sunnyvale, Calif.
VOL. 34, NO. 2, FEBRUARY 1962
233
necessary precision in the dose size value. The simplest parameter to vary was the sample size, with a lower limit imposed by sampling technique, and by the relation of adsorbed gas volume to dead space. A study of the critical dosing rate factors was then carried out by varying either the sample size, or the time of the dosing cycle, or both. To facilitate the general application of the results of this study to porous systems, the rate of adsorption or desorption n a s expressed as the per cent of total pore volume of sample filled or emptied per minute. The 300-A. radius pore volume determined a t 0.967 relative pressure, as given in the Barrett, Joyner, and Halenda method ( 8 ) , was the definition of total pore volume. The percentage of time spent in equilibration at each data point was the same in a given test isotherm, since it was fised during a single test by the doser timing cycle.
240
1
R A T E OF P O R E F I L L I N G OR EMPTYING
I
f 2
I8O 160
ADSORPTION
c
0
- 0 57%/MlN (EQUILIBRIUM)
0
X
-0769./MIN - I 04X/MIN -I45%/MIN - I79%/MIN
0
- 0 40%/MIN (EOUILIERIUM)
d X
DESORPTION - 0 6OX/MIN
A -0 89Y./MIN
+
-I
A
- I 57%/MIN
25%/MIN
r-
APPARATUS
The apparatus has been described previously ( I ) . Two units were in operation, and a portion of the experimental work was done with each. The doser mechanism introduces or withdraw a fixed number of moles of gas per unit cycle, regardless of the pressure conditions prevailing in the vapor in contact with the sample. The nulling pressure gage operates without change in volume.
01
0 2
03
04
These gas increments v\ ere applied in series of four successive doses, followed by equilibration a t the data points. Three samples were used in this study-a small pore catalytic alumina, and two large pore catalytic aluminas. They are referred to as catalysts 1, 2, and 3, respectively. The physical characterizations of the samples, from both equilibrium adsorption and equilibrium desorption isotherm branches] are given in Table I. The median radius of the pore size distribution is taken as the radius value such that half the pore volume below 300 A. is in pores of greater radius and half is in pores of lesser radius. Experiments for the small pore alumina catalyst 1 were carried out on a 0.182-gram sample; experiments for the large pore alumina catalyst 3 were done with a 0.075-gram Sample.
I. Physical Data from Nitrogen Adsorption Isotherms on Catalyst Samples BET Area, sq, Meters/ Pore Volume, &./Gram Median Pore Radius A. Catalyst Gram Adsorption Desorption Adsorption Desorption
Table
Catalyst No. 1 KO.2
No. 3
234
0.20 0.54 0.69
0.23 0.55 0.81
-4verage Radius 2 V / A , A. Adsorption Desorption 40 114 3 03
ANALYTICAL CHEMISTRY
07
08
09
Figure 1 . Experimental nitrogen isotherm points on small pore alumina catalyst no. 1.
Samples were pretreated by evacuation a t 200' C. for 2 hours prior to the first nitrogen adsorption run and were allowed to adjust to the temperature of the liquid nitrogen bath before the adsorption operation. Subsequent adsorption runs on the same sample were made after 2-hours evacuation a t room temperature. The nitrogen adsorbate was Matheson prepurified grade, used directly from the tank. The dead space was determined with helium. Individual dose sizes of 1.6 cc., a t standard temperature and pressure, for adsorption, and 1.4 cc. for desorption were used rvith one apparatus, and 2.2 cc. for adsorption and 2.0 cc. for desorption, with the other apparatus.
100 95 134
0 6
PIP0
EXPERIMENTAL
No. 1 KO.2 KO.3
0 5
46 116 121
38 138 133
33 102 99
Sum of Pore Areas, Sq. Metere/Gram Adsorption Desorption
-
105 90 133
123 114 177
The sample size of catalyst 2 !vas varied from 0.236 to 0.952 gram. RESULTS
Adsorption and Desorption Isotherms. Figures 1, 2, and 3 shorn t h e isotherms obtained Tvith t h e three samples a t various rates of adsorption and desorption. T h e evperimental points are indicated as far as possible. I n the case of overlapping data points, that point representing the slon er rate is designated. The over-all rates of pore filling or emptying, corresponding to each experiment, are given in the keys to the figures. A second major variable, the percentage of time spent in equilibration a t each data point, also affected the results. Although some pressure equilibration was continually taking place while a doser was operating, the term is used in reference to the time in which the vapor pressure in the volume around the sample n a s not being mechanically increased or decreased by connection to a gas doser. I n the experiments with catalysts 1 and 3, 25 to 70y0 of the total time of the mechanically set adsorption and desorption runs was spent in such equilibration] with the percentage increasing from the fastest to the slowest rate. I n the experiments with catalyst 2, the percentage of equilibration time was 67 for the four slower rates, and 27 for the fastest rate. The following
600 R A T E OF P O R E F I L L I N G OR EMPTYING
4
A OSORPTION - 0 17 % / MIN. 0 - 0 32 % / MIN A - 0 5 0 % / MIN A -0 60% / MIN X-070% /MIN 0
4 50
DESORPTION - 0 1 5 % / MIN I - 0 2 7 % / MIN A - 0 4 3 % / MIN
400
W
2 350 0
9
ADSORPTION
!j
-0.69% A -0.95 % A - I,33 % - I,5 7 %
I
2
400
; .
~-
*
0
* - 0 2 % /MIN.(EOUILIBRIUM) rn -0.70 % / M I N .
a 300
r-0.86%/MIN
R
250
/MIN. / M I N. / M IN, / M IN.
DESORPTION
v,
B
9
R A T E OF PORE F I L L I N G OR E M P T Y I N G o -0.5 %/MIN. (EQUILIBRIUM)
+ - 0 5 l % /MIN x -0 60% / MIN
300
~-
0
4
L
2
500
+ - 1.05%/MIN x - I 5 3 % / M I N.
4 W
5
$
200
200
I50
100
100
50
0.1
0.2
03
0 4
05
06
07
OB
01
09
02
03
PIP0
All rates of adsorption and desorption used on catalyst 1, the small pore alumina, yielded data close to equilibrium values. All rates, except the fastest, yielded data close to equilibrium on catalyqt 2 , a large pore alumina. Only the slomest rate of adsorption and desorption used with catalyst 3, a large pore alumina, n a s close to equilibrium data. Faster rates shoned steadily increasing deviations from equilibrium values. Calculated Pore Volume Distribution Values. T h e primary objectil e of t h e nitrogen adsorption and desorption d a t a was t o allow characterizntion of t h e catalyst sample by pore size distribution calculation, folloiring the method of Barrett, Joyner, and Halenda ( 2 ) . It wts, therefore, of interest to note the effect of deviations from equilibrium pressure and volume values, on the calculitted distribution. The most effective comparison can be made from the plot of cumulative pore volume 1's. pore radius. This type of plot avoids overemphasis on small data fluctuations, and also avoids arbitrary divisions of the pore volume. The cumulative pore volume plots are given in Figures 4, 5, and 6 for the calculations from the desorption isotherms of the three samples. The volume distribution for catalyst 1 \vas insensitive t o the rate, as would be expected from the coincidence of the isotherm points. The cumulative pore
0 5
0.6
07
08
09
PIP0
Figure 3. Experimental nitrogen isotherm points on large pore alumina catalyst no. 3
Figure 2. Experimental nitrogen isotherm points on large pore alumina catalyst no. 2.
obseivations can be made from the isotherms:
04
Table 11.
Pore Volume Distribution of Catalyst Samples Calculated from Adsorption and Desorption Branches o f the Nitrogen Isotherm
Radius A. 25Cb300 200-250 150-200 100-150 90-100 80-90 70-80 60-70 50-60 40-50 30-40 20-30
