values for x 1 are within f8 h. Consequently, for an unknown oil spill sampled within 4 days of spillage, the SDvalue can, alone, establish the identity of the spilI. However, the calculation of x1 will additionally assign an approximate time of spillage. This information can be very useful as corroborative evidence where a “passing ship in the night” is identified as the spill source and yet is 4 days’ steaming time distant from the actual spill site. Additionally, as was pointed out above, the artificial weathering scheme used to produce “weathered” oils for this set of in-house tests has been known to cause greatly accelerated weathering of oils compared to actual real world weathering situations. Consequently, the 96-h time limit imposed on the weathering framework in order that SDvalues remain less than 0.22 is a significant underestimate for real world weathering situations. Environmental parameters in real world situations are unique; as such, the weathering of spilled oils will proceed a t various rates for different locations and for different times of the year. This problem will be addressed in future work.
LITERATURE CITED C. B. Koons, P. H.Moneghan, and G. S. Baylis, “Pitfalls in Oil Splll Characterization: Needs for Multiple Parameter Approach and Direct Comparison of Spill Material With Specific Parent Oils”, Esso Production Research Co., Houston, Texas. “Oil Spill Identification System”, Department of Transportation, United States Coast Guard, Office of Research and Development, Washington. D.C., Report No. CGD-41-75. (Available to the public through the National Technical Information Service, Springfield, Va. 22161.) D. L. Carmin and A. J. Raymond, J. Chromatogr. Sci., 11, 625 (1973). Waters Associates, Milford, Mass., Bulletin AN154, February 1975. Waters Associates, Milford, Mass., Bulletin ANlO8, December 1973. R. L. Stevenson, J. Chromatogr. Sci., 9, 251 (1971). D. M. Jewell, R. G. Ruberto, and 6.E. Davis, Anal. Chem., 44, 2318 (1972).
(8)K. H. Altgelt and E. Hirsch, Sep. Sci., 5(b), 855 (1970). (9) T. E. Cogswell, J. F. McKay, and D. R. Latham, Anal. Chem., 43, 645 (1971). (10) “Modern Practice of Liquid Chromatography”, J. J. Kirkland, Ed., WileyInterscience, New York, 1971, p 188. (11) J. 6.F. Lloyd, Analyst (London), 100, 529 (1975). (12) Waters Associates, Milford, Mass., Bulletin AN131, September 1973. (13) J. A. Schmttt, R. A. Henry, R. C. Williams, and J. F. Dickman, J. Chromatogr. Sci., 9, 648 (1971). (14) J. W. Frankenfeld and W. Schulz, “Identification of Weathered Oil Films Found in the Marine Environment”, DOT, USCG Office of Research and Development, Washington, D.C., Report NO. DOTCG23,035-A. (Available to the public through the National Technical Information Service, Springfield, Va. 22161.) (15) W. Schulz, Abstracts, 26th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1975, No. 459. (16) J. R. Jadamec, W. Saner, and T. Porro, Abstracts, 26th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1975, No. 456. (17) H. V. Drushel and A. L. Sommers, Anal. Chem., 38, 19 (1966). (18) J. F. McKayand D. R. Latham, Anal. Chem., 44, 2132 (1972). (19) J. W. Frankenfeld, “Weathering of Oil at Sea”, DOT, USCG Office of Research and Development, Washington, D.C., Report No. CGD-7-75. (Available to the public through the National Technical Information Service, Springfield, !la. 22161.) (20) S. P. Wasik, J. Chromatogr. Sci., 12, 845 (1974). (21) Waters Associates, Milford, Mass., Bulletin DS042, February 1974. (22) R. E. Leitch and J. J. DeStefano, J. Chromatogr. Sci., 11, 105 (1973). (23) Waters Associates, Milford, Mass., Bulletin AN1 14, September 1973. (24) L. R. Snyder and J. J. Kirkland, “Modern Liquid Chromatography”, John Wiley and Sons, Inc., New York, 1974, p 435. (25) A. Janik, J. Chromatogr. Sci., 13, 93 (1975). (26) M. Ahmadjian, C. Baer, P. Lynch and C. Brown, Environ. Sci. Techno/.,in press. (27) C. Brown, P. Lynch, M. Ahmadjian, and C. Baer, Am. Lab., December, 59 (1975). (28) C. Brown, University of Rhode Island, Kingston, R.I., personal communication, February 1976. (29) E. L. Crow, F. A. Davis, and M. W. Maxfield, “Statistical Manual”, Dover Publication, Inc., New York, 1960, p 158.
RECEIVEDfor review May 13,1976. Accepted July 1,1976.
Determination of the Specific Surface of Adsorbents by the Dynamic Adsorption Method Henryk Golka and Boguslawa Jezowska-Trzebiatowska. Institute of Chemistry, University of Wroclaw, Wroclaw, Poland
Logarithmic equations were derived for the adsorbate distribution among the sequential adsorbent samples in a column during the dynamic adsorption from the flowing mixture of carrier gas and benzene. To check the equations, a series of benzene adsorption measurements on several Al2O3samples were carried out. It has been found that the derlved equations properly describe the adscwption course and that the specific surfaces of examined samples determined by them are Identical to those determined by other methods.
Studies on the adsorption of volatile compounds of rhenium, molybdenum, and selenium (1, 2) require a special method for analysis of the adsorbate distribution along the adsorbent packed in a column, because of the specific conditions such as high temperature, adsorption a t dynamic conditions, and unusually long retention times. Theoretically derived equations for the distribution of an adsorbate on an adsorbent allowed: a) the determination of the specific surface of solids and b) calculation of the dimensions of adsorption columns a t a fixed efficiency. In order to check the accuracy of the derived equations, 1754
adsorption measurements of benzene on adsorbents of known surface were carried out a t dynamic conditions.
EXPERIMENTAL The measurements were carried out on the apparatus shown in Figure 1. Its most important element consists of a multisectional, dismantable adsorption column (8, Figure 1).At first, samples of studied adsorbents(0.02-0.1 g) were placed in each section. Samples of A1203 of different specific surfaces, made from ignited Al(OH)3 ( 3 ) , called A1203(I),&@&I), A1203(III) and adsorbent B ( I , 2 ) (MgO A1203) were used in our studies as adsorbents. The number of sections in a column was usually 5-6. A neutral gas current (hydrogen, 20 ml/min) was passed through the column. The carrier gas flow was maintained constant by use of a three-stage reducing valve (2a-c, Figure 1). Hydrogen was purified by passing through a column (3, Figure 1)packed with A-4 molecular sieve and a liquid nitrogen trap (4,Figure 1).Before starting the measurements, the adsorbent samples, placed in the column (8a-e, Figure l), were heated for 1h a t 200 ‘C. After the impurities ( 4 ) were removed from the adsorbent surface and equilibrium in the system was reached (decay of the recorder baseline drift), the column was cooled down to 25 O C . That temperature was kept constant using an ultrathermostat. Then the thermostated column (8, Figure 1)was cut from the path I of carrier gas (valve 6, Figure 1) and put into the circuit of gas (path I) saturated with benzene in a bubbler (7, Figure 1).The volume of the gas which was passed through the column was measured with a gas burette (12a,
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
+
Table I. Results of the Dynamic Adsorption Measurement of Benzene on A1,0, (I) Section
Sample weight, g
Peak area, mm2
la
Adsorbed amount of benzene, N cm3
2a
a b
0.0704 3307 5140 1102 0.0692 2016 C 0.0641 837 438 d 0.0407 187 81 e 0.0870 4 43 sum 0.3314 a Corresponds to individual measurement series.
3a
la
2a
3a
5315 1484 868 460 316
1.1582 0.7053 0.2933 0.0654 0.0014 2.2246
1.7966 0.3852 0.1504 0.0283 0.0151 2.3756
1.8564 0.5183 0.3031 0.1607 0.1105 2.9490
Table 11. Results of the Dynamic Adsorption Measurement of Benzene on A1,0, (11) Section a b C
d e f sum
Sample weight, g 0.0289 0.0248 0.0256 0.0532 0.0503 0.0576 0.2404
Peak area, mmz
Adsorbed amount of benzene, N om3
la
2a
3a
la
2a
2060 511 449 407 132 48
2263 593 414 398 128 28
2323 752 605 858 596 326
1.689 0.418 0.368 0.334 0.108 0.040 2.957
2.105 0.554 0.386 0.371 0.119 0.026 3.561
3a
.
2.528 0.819 0.658 0.936 0.649 0.357 5.947
a Corresponds to individual measurement series.
Table 111. Results of the Dynamic Adsorption Measurement of Benzene on A1,0, (111)
Section
Sample weight, g
-
Peak area, mm2
la
2a
Adsorbed amount of benzene, N cm3 la
a b
0.0468 9096 6512 7.959 0.0750 2800 2994 2.450 c 0.0983 1843 2211 1.612 d 0.1063 766 678 0.671 e 0.1140 110 126 0.096 f 0.0887 16 17 0.014 sum 0.5291 12.802 a Corresponds t o individual measurement series.
2a
5.696 2.619 1.934 0.757 0.141 0.019 11.167
Figure 1. Apparatus used for studies of the dynamic adsorption of
benzene (1) Carrier gas container, (2) (a, b, c, d) reducing valves, (3) molecular sieve, (4) liquid nitrogen trap, (5) comparative catharometer cell, ( 6 ) valve, (7) bubier containing benzene, (8) (a, b, c, d, e) multisection adsorption column, (9) catharometricmeasurement cell, (IO)flow meter, (11) gas burette, (12) power
supply and recorder
Table IV. Results of the Dynamic Adsorption Measurements of Benzene on Sorbent B
Section
Figure 1).After a given amount of benzene-H2 mixture was passed through, the adsorption column was connected again to path I. To avoid any perturbation of measurements, which might be caused by the adsorbed benzene migration, the column was cooled simultaneously to -72 "C (dry ice-EtOH). Next, the column sections were subsequently heated up to 100 "C in the following order-section 8e, 8d, 8c, 8b, and 8a, causing fast and complete desorption of benzene from a given adsorbent sample. By passing various volumes of benzene-saturated H2 through the column, various amounts of the adsorbate were injected on the column and therefore (at a given amount of the adsorbent) different amounts of benzene were adsorbed on subsequent sections of the column. The amounts of adsorbed benzene in individual measurement series are given in Tables I-IV (also see Figures 3a and 4a). The benzene desorbed was determined quantitatively using a Sorptiograph S-272 (made by I C s 0 Blachownia Slaska-Poland), operating on a chromatograph principle. A catharometer of 1000 mVlmglm1 sensitivity was used in this unit as detector. The amounts of desorbed benzene were determined from peak areas. The calibration indicated that the relation, P = f ( n ) , where P = peak area in mm2 and n = amount of benzene, is linear in the applied concentration range. To find the total amount of adsorbed benzene, the determination of two calibration points for each measurement series was sufficient. The results of benzene adsorption on
a b C
d e
sum a b C
d e sum a
Sample weight, g
Peak area, mmz
Adsorbed amount of benzene, N cm3
la
2a
la
2a
0.0204 0.0291 0.0254 0.0255 0.0340 0.1324
1761 363 138 43 30
1952 504 315 190 74
1.5156 0.3120 0.1184 0.0374 0.0258 2.0092
1.6798 0.4336 0.2711 0.1638 0.0637 2.6120
0.0578 0.0525 0.0590 0.0549 0.1082 0.3324
2858 1098 1062 636 123
0.9962 0.3833 0.3701 0.2219 0.0429 2.0151
Corresponds t o individual measurement series.
the adsorbents A1203(I), A1203(II),A1203(III), and B are given in Tables I-IV. Figure 2 presents one of the chromatograms of benzene distribution among the adsorbent B samples, placed in subsequent column sections.
CALCULATION METHODS In order to find mathematical relations in the examined
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
1755
1
Defining x = n,/n,,
Secii qns
1 - x = ng/no and since n,
+ ng = no n,dx --rdl
a,bno(l - x ) V bn,(l - x )
+
Integration of Equation 6 a t limiting conditions: p = p o , x =Oat1 = O p = pi, x = x i a t 1 = li
results in bnoxi = ambrli - In (1 -xi) + -
I' I
V At low partial pressures of the adsorbate, bn,xi/V prove this, Equation 7 was rewritten
_-
Figure 2. Chromatogram of benzene distribution among the sample weights of the sorbent B, placed in subsequent column sections, during the thermal benzene desorption. The arrow indicates the direction of the gas mixture flow
process, and to find quantitative relations between the amount of substance adsorbed in a given volume (du) of an adsorbent and its distance from the start of filling of the column, equilibrium conditions of the adsorption and desorption processes were assumed. Since the experiment was carried out a) a t low carrier gas flow rates and b) using short, lightly packed columns, the following assumptions were made: 1) Carrier gas pressure decay along the column is very small. 2) The adsorption process is running under the equilibrium conditions. 3) The amount of substance adsorbed on the adsorbent surface is determined only by the adsorption isotherm. For further calculations the Langmuir isotherm was applied:
The amount of substance adsorbed in each volume element dv of the adsorbent packed in the column is given by
a =d n2 du
Balancing the adsorbed substance flowing through a given volume element (du = dx-dy-dz) of the filling, it should be considered that, because of the gas phase flow forced in the macroscopically favored direction, e.g., along the z axis, an additional transfer of the sorbed substance along the two remaining directions x and y occurs. That transfer is caused by the diffusion, convection, and turbulence of the main gas stream. At limited cross sections of a column, these phenomena lead to the decay of the adsorbate concentration gradient in those two directions ( x , y). That makes possible a balancing of the adsorbate in one favored direction-the gas phase flow direction. Therefore,
(3) The negative value was used to denote the decrease of the amount of substance adsorbed in a given volume element a t an increasing distance from the beginning of the column. Following Dalton's law, the partial pressure of adsorbate was expressed by concentration. Since the total carrier gas pressure in the column during the experiment was equal, p c = const = 1atm, --= dn,
rdl 1756
(7)
V
a,bn,lV 1 bn,/V
+
(4)
* 0. T o
Y=AX+B
(8)
where Y = [In (1 - xi)]/~i,X = li/Xi, B bn,/V. Differences between the results obtained from Equation 7 at bn,xlV = 0 and Equation 8 were negligible. Therefore, for further calculations Equation 7 was used in its simplest form:
- In (1- x )
=A 1
(9)
where A = b.C, b = adsorption-desorption equilibrium constant, and C = constant value at given experimental conditions. If gas pressures are differing from 1 atm, Equation 7 becomes
- N In (1- xi) + bnopcxi = a,bp,rli
(74
Equation 7a should be linearly dependent on the absolute value of the total gas pressure. However, if the pressure gradient along the column becomes Ap,/Al # 0, Equation 7a should behave nonlinear. The influence of pressure gradients was not examined in detail. The polylayer adsorption could be thought of as a series of parallel systems (5,6). Each of them could be defined by the reactions: sorbate
ki + free surface + single adsorption complex k-1
sorbate
kz + single adsorption complex + k-2
double adsorption complex, etc. where kllk-l= bl, kzlk-2 = b2, k3lk-3 = bS, etc. Usually, the equilibrium constant b l for the first layer is remarkably higher than bp, b3, . . . for the second and next layers, because the sorbent-sorbate interaction is decreasing rapidly with increasing distance. I t is usually assumed that bl >> bz 3 b3 . . . (10) In the case of multilayer adsorption, the assumption of the applicability of the Langmuir isotherm for each layer independently, as in the BET theory, leads to the conclusion that differences among the subsequent equilibrium constant values (Equation 10) should influence the slope (A-b) of Equation 9 (if experimental data are presented as log (1 - x ) vs. 1). In the ideal case the plot of log (1- x ) vs. 1 should be composed of linked straight-line sectors of various slopes. Their joint points called L , (Figure 3.) are situated a t distance E , in the coordinate system, and should correspond to the adsorbent coverage completion/beginning by adsorbate molecules within a given monoenergetic surface section of adsorbent. The layer
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
1
Ncm: 9
i
20
30
20
10
10
-*
2.1
. I:
e
Figure 4. Dynamic adsorption of benzene on Al203 (I). (a) Distribution of benzene among the sequential sorbent samples in the column for injected benzene doses: (in Ncm3) 0 = 6.715, 0 = 7.169, 0 = 8.899. (b) Application of Equation 9 to describe the adsorption
Figure 3. Dynamic adsorption of benzene on sorbent B (a) Distributionof benzene among the sequential sorbent samples in the column for injected benzene doses: (in Ncm3) 0 = 13.217, 0 = 17.180, 0 = 6.063. (b) Application of Equation 9 to describe the adsorption.
capacity at that point may be determined from the plot of a vs. 1. That dependence determines the adsorbate quantity (calculated for the adsorbent mass unit) adsorbed in a given element hV on its distance from the beginning of a column. The capacities of layers a t distances 1, will be given by the corresponding a, values. Because of the simultaneous existence of several adsorption-desorption equilibria (Equation 10.) the plot of log (1x ) vs. 1 for experimental data would be a curve composed of linear sectors. The distance 1, could in that case be evaluated by the extrapolation of the straight-line sectors of that curve. Differences in the slopes of those sectors should occur for those layers only which differ in the equilibrium constants (Equation 10). That method can be much simplified. Differentiation of Equation 9, given in its exponential form and substituting differentials by increments leads to Equations 12 and 13, which relate directly the amount of adsorbed molecules in a given layer element to its distance from the beginning of adsorbent filling in the column: 1 - x =exp(A.b.l)
-
(11)
d(1 - x ) = A b eAbLdl (12) Ax In - = A . b 1 const (13) A1 The slope of Equation 13 also contains the equilibrium constant b and, hence, the plot of In (AxlAl) vs. 1 should exhibit a bending in the formation area of the completed adsorbate layer on the adsorbent.
- +
RESULTS The results of the benzene adsorption measurements on sorbents B, Al203(I), A1203(II), A1203(III)are listed in Tables I-IV. For a convenient direct comparison, these results were recalculated introducing the adsorption degree ( x ) and weight fraction of the adsorbent (gi). The adsorption degree of the examined compound a t a given distance ( l i ) of the column packing is defined as the sum of ratios of adsorbate amounts in subsequent layers to the total amount injected to the column: xi = AX,
+ Ax2 + . . . + AX^
Axi = ni/n,
(14)
The weight fraction of the ith layer of the adsorbent was defined as the ratio of its mass to the total mass of an adsorbent in a column: gi = milma
(15)
One may prove that the weight fraction gi of the ith layer of adsorbent is equal to the volume and length fractions of the given layer, respectively: gi = A u ~= Ali
and therefor e
C gi = v i = li = A l l + A12 + . . . + Ali
(16)
1
The equivalence of the weight, volume, and length fractions allowed the easy illustration of changes in the amounts of adsorbed molecules along the column packing. It allows also the use of the very illustrative term of distance of ith adsorbent layer from the column start.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
1757
Table V. Determined Values of the Specific Surface of Examined Samples
Sorbent
A1203 (1) Alto, (11) A1,0, (111) Sorbent B
Specific surface determined, mZ/g
Injected amount of benzene, Ncm3/g
From Eq. 9
6.715 7.169 8.899 12.290 14.813 2 4.7 9 8 C 24.196 21.106 6.063
a'm 5.50 5.10 5.20 9.76 9.76 10.4 13.5 13.6 5.20 5.17
64.6 60.0 61.0 114.9 114.9 122.0 159.0 160.0 61.0 60.7
10.4
122.0
SO
...
13.217C 17.180C
a'm 5.07
...
By other
From Eq. 13
methods SO
SO
59.5
...
...
5.15 9.88
60.5 117.0
60.2a
21.4
... ...
252.0
... ...
13.5 5.15 5.19 10.37 10.30
159.0 60.6 61.0 122.0 121.2
116.0b 297b 62.0b
a Specific surface determined by the classic BET method from the benzene adsorption isotherm. b Specific surface determined by the thermal desorption of nitrogen method (7,8). C Benzene injected in excess vs. capacity of sorbent sample used.
sE
On the basis of such calculated data, plots of x vs. I , showing the mean increase x in sequential adsorbent layers for increments Al = 0.1, were drawn. The resulting averaged data were used in the graphic solution of derived Equations 9 and 13. Determination of the Specific Surface from Equation 9. From intersections of the extrapolated straight line sectors of the log (1- x ) vs. 1 plots, only the 1, distance of those points from the start of the column packing were determined directly. The amount am'of adsorbed molecules at this L , point, at the distance I, was determined indirectly from plots ai vs. l i where ai = Axi-n,/Alpmo. Examples of plots log (1 - xi) vs. 1, and ai vs. li for the dynamic benzene adsorption on Al203(I), and on adsorbent B are given in Figures 3 and 4. The x , and a,'values read from the diagrams and calculated on the basis of surfaces occupied by the adsorbed benzene molecules are listed in Table V; these values correspond to the cross-points (L,) of straight-line sectors of the log (1- x ) vs. 1 dependence. The surface occupied by the benzene molecules was calculated from the formula:
so=
t"
0 0 4
18
16
14
12
10
08
u,.N.u 22,414
0.6
Determination of the Specific Surface from Equation 13. Equation 1 3 can be transformed: log ai = a * li
+ C'
Examples of log ai vs. li plots for the dynamic benzene adsorption on A1203(I) and on adsorbent B are given in Figures 5 and 6. These plots are not straight lines, according to assumption. From the L , points, determined by extrapolation of their linear parts, the log a', values were determined. The calculated layer capacity a ', and, calculated from Equation 17, surface C, occupied by the adsorbed benzene molecules are listed in Table V. Extensive simplification of Equation 9, given by Equation 13, might lead to inaccurate results and therefore we would recommend the use of Equation 9 to determine the So values.
DISCUSSION The capacity a', of the layer determined from the L , points on logarithmic plots and, hence, calculated surface occupied by the adsorbed benzene molecules proved to be practically equal to the specific surface values of studied adsorbents determined by another method (thermal desorption of nitrogen (7,8)).That agreement of specific surface values for the examined samples seems to confirm our assumptions and derived equations. 1758
9
ANALYTICAL CHEMISTRY, VOL. 48,
04
(18)
NO. 12,
02
, A , 02
04
06
,
08
, 10 L
Figure 5. Dynamic adsorption of benzene on sorbent B Application of Equation 15 to the determination of the specific surface of the examined sample, for injected benzene doses: (in N cm3) 0 = 13.217. 0 = 17.180,0 = 6.063
The results obtained indicate that the deflection areas of logarithmic plots correspond to the formation of the complete adsorbate layer on the adsorbent surface in the case of benzene. The slope of straight-line sectors of a logarithmic plot is proportional to the equilibrium constant of the sequential adsorption stages. The deflection point on the log (1- x ) vs. 1 plot appears a t the change of equilibrium constant, Le., at transition from monomolecular to polymolecular adsorption. The observed discrepancies of the specific surface values determined for the Al203(III) adsorbent from thermal desorption and our method are most likely due to the molecular sieve effect.
OCTOBER 1976
14
.
12
-
in more remote column sections-at smaller surface coverages. For those samples, the slopes of relations log ai vs. l i and log (1 - x ) vs. 1 differed from those determined for samples of higher coverage. Those peak splittings are not due to benzene impurities. The high purity of benzene used was confirmed by gas chromatography. It seems that the peak splitting is analogous to that described by Kuge and Yoshikawa (9). According to those authors, the peak splitting corresponds to the point B on the Second Type Isotherm, following the Brunauer classification ( 3 ) . It should be noted that unpublished results (IO)on dynamic adsorption of selenium(1V) and rhenium(VI1) oxides indicate that the appearance of a deflection point on log (1 - x ) vs. 1 plots has to be interpreted as the filling of the monoenergetic surface section on the adsorbent surface, and not as the formation of a complete monolayer of the adsorbate molecules.
0
10 "
88.
0.6.
04
NOMENCLATURE
.
0.2-
a2 04 06 08 io L Figure 0. Dynamic adsorption of benzene on sorbent A1203 (I)
Application of Equation 15 to the determination of the specific surface of the examined sample for injected benzene doses: (in Ncm3)0 = 6.715,0 = 7.169, 8 = 8.899
The layer capacity a', determined from the deflection point of the discussed plots is in some cases twice as large as normally observed. That was observed only if relatively large amounts of benzene were injected to a column. It seems likely that, in those cases, the deflection point appeared either as the results of completion of the second layer or because of the layer capacity increase. An increase of the layer capacity might be due to the reorientation of adsorbed molecules. It seems that the possibility of observing the deflection point on a logarithmic plot at higher surface concentrations of adsorbed molecules could be considered as a confirmation of the sensitivity of our method. The influence of the gas carrier flow rate on the slope and L , distance makes some difficulty in application of that method. The optimal setting of the carrier gas flow rate was found a t 20-80 cm3/min for our column (80-280 cm3/min. cmz). For such flow rates, the most reproducible results were obtained because the 1, values were ranging from 0.3 to 0.5. Flow rates outside those limits are unfavorable because they are leading to an increase of the cross-angle of extrapolated straight-line sectors of logarithmic plots. The benzene dose affects the accuracy as well. We have found that the dose of benzene should be sufficient to find as small as possible, but quantitatively detectable, amounts of benzene adsorbed in the last column section (see Figure 2.). At this condition, we found that a, = urn'. The appropriate dose is usually determined within 3 runs at varying amounts of the adsorbate. A t measurements of the dynamic adsorption of benzene, an interesting phenomenon was observed: on the desorption chromatogram of benzene, a t higher carrier gas flow rates of 30 ml/min and for adsorbent samples of higher surface occupancy, a splitting of peaks was observed. That splitting was not observed at the benzene desorption from samples placed
a = amount of a substance adsorbed in N cm3/g. ai = amount of benzene adsorbed on the sorbent sample in a given column section per 1-g sample. a, = monolayer capacity in N cm3/g. a', = amount of adsorbed benzene in N cm3/g of the adsorbent determined from L , points. A = b*C. a = slope, including the expression for the equilibrium constant b. b = adsorption-desorption equilibrium constant. C = constant value a t given experimental conditions. C' = constant, including also the expression for the equilibrium constant b. 1 = column length. li = distance of the sorbent sample from the beginning of the packing in a column. I , = distance of L, points from the packing start of the column. L , = cross point of the extrapolated straight-line sectors of the log (1- x) vs. 1 dependence. m , = total mass of a sorbent in the column. mi = mass of ith analyzed sorbent sample in a given column section. no = total amount of adsorbate injected in the column. ni = amount of adsorbed substance on ith analyzed sorbent sample in a given column section. n, = amount of adsorbed substance. n g = amount of adsorbate in gaseous phase. N = Avogadro number. w = the surface occupied by the benzene molecule. p = pressure of the sorbed component. r = cross section of a column. So = specific surface of the examined sample in N cm3/g. u = sorbent volume. V = carrier gas volume.
LITERATURE CITED '
( 1 ) H. Golka, T. Mikulski, and B. Jezowska-Trzebiatowska,Przem. Chem., 51,
99 (1972). (2)B. Jezowska-Trzebiatowska and H. Golka, Przem. Chem., 53, 101
-
119741 ,. . .,.
(3)S.Brunauer,L. S. Denning, W. E. Denning, and E. Teller, J. Am. Chem. SOC., 62, 1723 . - -119401 ~ (4)N. E. 1Bujanova, G.B. Gudkova,and A. P. Karnaukhov, Kinet. Katal., 6,1085 (196ti). (5)J. OScik, "Adsorpcja", PWN, Warszawa, 1973. (6)S. Brunauer, P. H. Emmett, and E. J. Teller, J. Am. Chem. SOC.,60,309 (1938). (7) 0.G. Grubner, Z.Pbys. Chem., 216,287 (1961). (8)F. M. Nelsen, and F. T. Eggertsen, Anal. Cbem., 30, 1387 (1958). (9)Y. Kuge and Y. Yoshikawa, Bull. Chem. SOC.Jpn., 38, 948 (1965). (IO)H. Golka, unpublished results.
RECEIVEDfor review January 2, 1975. Accepted May 21, 1976.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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