-4 S E W METHOD FOR T H E DETERXIISATIOS OF SORPTIOS I S O T H E R X I L S O F TAPOURS O S CHARCOAL
RII
L. J. BCRRAGE
Introduction Some little time ago a new and rapid method for the evaluation of the rapour-sorbing properties of solid sorbents, such as charcoal, was described.' The experimental procedure used in carrying through this retenticity test is very simple. Briefly, it consists in saturating a known weight of charcoal with the vapour concerned at a definite temperature and pressure, reweighing, passing dry air through the charged charcoal column at a known rate, and interrupting the process at noted intervals in order to reweigh the charcoal container. The weights of sorbed vapour per unit weight of charcoal thus found (q) are plotted against the total volume of air passed (V). Tangents to this retentiz'ity cuwe are drawn for different q values. The value of any such tangent, -
(3
T, represents the rate of loss of sorbed vapour at the q
figure in question, and, assuming equilibrium to persist between sorbed phase and gas phase during the desorption, is proportional to the equilibrium pressure of the vapour above the exit end of the charcoal column. If these tangents be plotted against their corresponding q values, a sorption isothermal curve or something closely resembling it will be obtained. And if the initial slope of the retentivity curve be carefully determined and compared with the original pressure at which the charcoal was saturated, this isothermal will give the actual relation betn-een pressure of vapour and quantity sorbed. This procedure was primarily worked out in order to obtain B comparative charcoal test for use at relatively low vapour pressures. It was realised that, whilst furnishing data which are of sufficient accuracy a t such pressures for this purpose, it is liable to give misleading results at intermediate pressures, the reason being that the quantity figures obtained are average values for the whole length of the column, whilst the pressures must necessarily correspond to the charge on the charcoal at the end of the column, where the air stream leaves it. Consequently, the method was examined more closely. in order to see whether, suitably modified, it was capable of furnishing true isotherms over the whole pressure range. ~
Part I, A. J. Allmand and J. E. Manning: .Jour. Soc. Chr2m. Ind., 47, 369 T - 3 7 T (1928); Part 11, .4. J. Allmand and L. J. Burrage: loc. cit. 372 T-3j6 T.
S O R P T I O S ISOTHERMALS O F VAPOURS ON CHARCOALS
2203
Nature of Concentration Gradient inside a Charged Charcoal Column during Desorption in an Air Stream
It is clearly important to realise, in some detail, how, during the course of a retentivity test, the charge of sorbed vapour alters in the different parts of the charcoal column, and the following work was accordingly undertaken. T w o experiments were first carried out with Charcoal B,' using three containers in series, these being of wide cross-section in the one, and of narrow cross section in the other case (2.61 and 1.4 c.m.2 respectively). The containers were of the design already described.2 The procedure was as follows. The three containers were dried and placed in the bath at I O O O C . , the taps being open. When temperature equilibrium had been fully established, the taps were closed and the containers taken out, cooled and weighed. (In every case, after being cleaned externally with alcohol and then ether, the container was hung on the balance for ten minutes before weighing.) The charcoal under test, graded by IO- and 12-mesh sieves, was heated in an air oven at 150' for one hour as a preliminary drying. Into each container, a known volume of this half-dried charcoal was introduced, with continuous vertical and horizontal tapping until settling was complete and the upper surface was level. The charcoal in each container was then dried separately by passing dry air through for I $ hours at I~o', and the containers were weighed. Each container was then charged to saturation with CCli at 33 m.m. pressure at rooo, and again weighed. The three containers, now ready for the commencement of the experiment, were joined up in series by the minimum quantity of rubber tubing, placed in a water-bath at IOO', and successive definite amounts of air passed through them, much as in the simple retentivity test, the containers being weighed separately after the passage of each measured volume of air. From the resulting data, a very fair idea of what occurs in a charcoal column of length equal to the sum of the lengths of the columns in the three containers can be obtained, the arrangement clearly being equivalent to one long column cut into three sections. The total quantity of CC1, retained in each container a t any time during the run gives the average charge in m.g. CC1,:gram of charcoal for that particular section. As each column is comparatively short, one may assume that this concentration is very close to that at the middle point of the section. A curve drawn through the points obtained by plotting the concentration or charge of CCla against the position of the corresponding points an the complete three sectzon column will give a close approximation to he distribution of the CCI, in this composite column after the passage of a certain volume of air. A series of such curves for ever-increasing total quantities of air will furnish a quantitative picture of the change in concentration gradient in the column during the whole test. A chemically activated soft-mood charcoal. See J. Phys. Chem., 32, 441 (1928). Reference should also be made to this paper
* Allmand and Burrage: loc. cit., Figure 4.
for any experimental details omitted here.
L. J. BURRAGE
2204
The following are the figures for the experiment using wide containers. The volumes of the three sections were respectively 2.340, 2.240 and 2.393 C.C. The results of the experiment with narrow containers are given in Table I B. The figures obtained with larger volumes of air are omitted. They were irregular and clearly subject to some gross form of experimental error. Th volumes of the respective sections were r . 4 j j J 1.400and 1.400 C.C. 1
TABLE IA Charcoal B. CCla. Litres of air passed 0.0 0.1 0 . 2
0.4 0 .j I
.o
I
.8
3.4 6.6 '3 . o 25.8
Section
I
355.2 337.6 309.5 262. j 228.8 178.6 126.0 75.9 40.0
17.6 6.7
IOOOC.
Average q value in m.g./grams. Section 2
Section 3
355.5 355.5 350.0 329.3 300.9 254.9 192.8 127.3 84.9
355.4 355.4 355.4 343.6 338.1 302. I 251.5 174.0
.o
69.3 42.5
52
30.2
111.1
TABLE IB Charcoal B. CCI,. IOO'C. Litres of air passed. 0.0 0.1 0 . 2
0.4 0.6 I
.o
I
.8
Section
358.4 318.8 279.7 231 . o
I
Average q value in m.g./gram. Section 2
352.1 352.6 343.9 301.9
.o
271.2
154.1 I 0 2 .o
225.3
201
I 57.8
Section 3
356.4 359.8 351 . o 327.6 308.4 260.5 195.3
The two sets of results are shown graphically in Fig. I. Along the ordinate are plotted the average charges (q values in m.g. CCla/gram charcoal) in the containers, these corresponding closely to the actual concentrations a t the middle points of the separate sectional columns. Along the abscissa is plotted the total volume of the three-section column in c.c., starting from the point of entry of air. OD, DE, and E F are the volumes of the three sections in the first experiment (Table I A), O F therefore being the volume of the complete column (6.973 c.c.), whilst OD', OE', and OF' are the total volumes, measured from the point of entry of air, corresponding to the middle points of the three separate columns, and consequently to the points in the complete
SORPTION ISOTHERhlALS O F VAPOURS O S CHARCOALS
2205
composite column to which the average measured q values closely correspond. Similarly, OA, AB, BC and OC are the volumes of the constituent columns and of the complete column (4.255 c.c.) in the second experiment (Table I B), and A’, B’ and C’ the points to which the measured q values correspond. It is clear from the curves that the concentration gradient at any point in the complete column, after the passage of a given volume of air, is independent of the cross-section of the column within the limits worked over and merely depcnds on its total volume a t that point, measured from the point of entry of air.
FIG.I
A further experiment was then undertaken, again at IOOOC.,but using four containers in series. These were packed with a characoal which is better at low pressures than Charcoal B, but not so good a t high pressures, viz., Charcoal G’. I t was expected that the length-concentration curves would show corresponding changes. The data are contained in Table I1 and plotted in Fig. 2 . The volumes of the four separate sections in this case were 0.73 j , 2 . 3 4 2 1 3.42 j and 2 , 8 7 5 C.C. respectively, and the positions of their centre points in relation to the total column are shown by A’, B’, C’ and D’ on the abscissa of Fig. 2 . A comparison of the results given by the two charcoals is of interest. Reference t o Fig. I shows that when, say, 0.1litre of air has been passed, the first layers of Charcoal B have already lost a large amount of CCI,, since this charcoal adsorbs badly at low pressures; whilst a t but a short distance from 1
A steam-activated nut charcoal. See Allmand and Burrage: loc. cit.
L . J. BURRAGE
2206
the beginning of the column, the charcoal is still saturated to 33 m.m. pressure, the point where the pressure begins to fall below this figure being a t 2 , ; C.C. If we consider Charcoal G on the other hand, again after 0 . 1 litre of air has been passed, the fall in q in the first layers is not nearly so great as with
Charcoal Litres of air passed
Section
.o
0.3 0.7
5 469 4 414 9 354 4
0
100°C.
Average q value in m.g. 'gram. Section z Section 3 Section 4
I
502
0.1
TABLE I1 G. CC1+
460.4
471.3
472 4
454.1
471.3 471.3
474 4
1.5
2;o
3.1
200 0
435.6 103.8 359.5 298.j
6.3
0
460.0 436.I 38i . 4
138 5
228.0
32j.1
12.7
91 4
252.6
25.5
55.3
164.8 108.5
Si4 4 465 4 428 4 3ii 8 323 4 262 j 191.4
183 .;
the Charcoal B; tliePe first layers lose their CCll less readily because G is much the better charcoal of the two a t low pressures. But, as a result of this, the layers further along the column must necessarily lose more CCII than is the case with B; and thus the point in the column where the pressure begins to fall below 33 n1.m. is a t j.4 c.c., a very different figure from that given by Charcoal B. These curves then afford a ready means of differentiation be5001 1x1
300
I
z
J
5
4
7
6
0 *I
8'
C'
-
8
c
7
9
I O C L
7
FIG.2
tween charcoals which are good at low and poor a t high pressures, and those which are good a t high and poor at lorn pressures. For the rest, the diagrams show that the general picture is the same in the two cases. A point of interest is the amount of CCl, left in the first Iayers of the charcoal when 2j.j litres of air have been passed, I n the case of Charcoal G , the first layers still hold about 2 0 - 3 0 m.g.,igrsm, whereas the first 0.8 C.C. of Charcoal B have, to all intents and purposes, no CCl, left in
SORPTIOS ISOTHERMALS O F T.4POURS O S CHARCOALS
2207
a t all. h further consideration of the diagrams will make it plain why the errors in the simple retentivity test ( I ) are of far greater importance at medium than at low and at high pressures and ( 2 ) are of greater importance with chemically activated charcoals, bad at low pressures (such as Charcoal B, activated by ZnCIP) than with steam activated charcoals, good at loiv pressures (such as G.). The az'erage q:l gradient, the magnitude of which is a measure of the degree of inexactitude of the simple retentivity test, is clearly greater at intermediate pressures (i.e. at intermediate q values at the exit end of the column) than at low and high pressures; and, further, is greater with charcoals which are poor than with charcoals which are good at low pressures. The author has the data for similar diagrams for CCl, and for HsO at 2 s C , but their reproduction is considered unnecessary as they are of the sanir general type as those for CCli at 100'. Determination of True Isotherms Consider a charged column of charcoal and let a certain amount of air be passed through so that the concentration gradient is as shown. The shaded portion will represent the remaining adsorbed substance. Dividing this tiy the \wight of charcoal, we get the weight adsorbed in m.g.,'gram. S o w this quantity should, if the retentivity test were quantitatively correct, correspond to the partial pressurc of the vapour in the issuing air, i.e. to the pressure
FIG.(a)
FIG.(b)
over the end of the column; but clearly it does not do so, the pressurc it1 question corresponding to the heavily charged charcoal layer :it the end of the column. Thus the average quantity figure is too small for the corresponding pressure furnished by the test. Suppose now we consider :i shorter column. Then the error due t o averaging the quantity will not be SO great. If this shortening of the column be carried to the limit, we shall reach a stage where the pressure over the charcoal does actually correspond to the quantity, in other words, what the author terms the pressure-length effect will have disappeared. The curves in Figs. I and 2 are all of the type shown in Fig. (b), and clearly, in accordance with what has just been said, whilst the q values taken from such a set of curves (obtained by the passage of increasing volumes of air over a charged column of definite volume) will more nearly correspond to the p values read off from the slope of the corresponding retentivity curves, the
L. J. BURRAGE
2208
smaller the volume of, or the shorter, the column used in the test, complete concordance between q and p values will only be obtained when the column is of vanishingly small linear dimensions, corresponding to the extrapolated value of g o in the figure. Equally clearly (and still more so from the actual experimental diagrams I and 2 1 , the extrapolation of such curves would be very uncertain. Inspection of the curves suggested, however, the existence of some kind of logarithmic connection between v (the volume of the column) and q. This was followed up, and it was found that, if the logarithm of the total c'olztme of t h e colunin were plotted against the logarithm of the average p ialue for the total colunrn, the result was in every case a straight line graph (for columns of the lengths used), corresponding to the equation (I = k. vn or log q = log k n. log v
+
This fact allows of an accurate extrapolation of the data and of as close an xpproach to q. values (i.e. to q values which when plotted on a retentivity diagram will furnish exactly corresponding pressures) 3s is found necessary in practice. Table I11 contains some of the dataof Tables I A and I B expressed in this way. The first line gives the logarithm of the total volume (in c.c.) of the column and indicates the nature of its components. Thus 1-2-3-5 represents a column corresponding to all the three narrow containers considered in Table I B; 1 - 2 - W Po P I 2? 2 3 2 5 26 represents a column consisting of the FIG.3 first two sections of wide containers dealt Kith in Table I h. Subsequent lines contain the logarithms of the average q values (in m.g./gram) of these composite columns after passage of the number of litres of air stated in the first column of the Table. The data are plotted in Fig. 3. Table I V and Fig. 4 contain the corresponding data for Charcoal G, drawn from Table 11. The third, fourth, and sixth columns of results con19
84
LOO f
4
Litres of air passed 0.1 0 . 2
0.4
0.6 I
.o
I
.8
TABLE I11 Charcoal B. CCl+ IOO'C. log v = o . 163 (1-W
log q =
0.369 (I-W)
0.456
(I-2-N)
2.504
2.528
2.525
2.447 2.364 2.303 2.188 2.009
2.491 2.419 2.359
2.493 2.425 2.355
2.252
2.277
2.100
1.112
0.629 (I-2-3N) 2.536 2.511 2.456 2.414 2.327 2.179
0.661
(I-2-W) 2.540 2.517
0.843~ (I-2-3-W) 2.544
2.421 2.334
2.529 2.494 2.462 2.390
2.200
2.280
2.470
SORPTIOS ISOTHERMALS OF VAPOURS ON CHARCOALS
2209
tain data from another experiment, of which the author has not given the full details. I n the case marked with a n asterisk, the charcoal was allowed to stand at 100’ for 20 hours before the desorption was carried out. The fact that the points lie on the same straight lines as do the other points for the same volume of air passed is evidence that no appreciable “sinking-in” of the CCh has taken place during this period, Le. that the charging process leads rapidly to a real equilibrium state.
Litres of air paased
TABLE IV Charcoal G. CCl,. log v=1.866
(1) 0 . 1 l o g q = 2.672 0.3 2.618
0.7
2.550
1.5
2.431 2.301
3.1 6.3 12.7 25.5
2.142
1.961 1.744
0.488 (1-2)
2.660 2.635 2.596 2.534 2.446 2.324 2.178 1.991
0.512
*
100°C.
0.646
0.813
0.894
2,584 2.514 2.420 2.290
2.676 2.662 2.641 2.602 2.534 2.443 2,324
0.972 (1-2-3-4) 2.665 2.658 2,639 2.603 2.545 2.461 2,349
2.125
2.160
2.205
(1-2-3)
2.657 2.635 2.597 2.541
2.668 2.649
2.667 2,656
2.611
2.625
2.560
2.450
2.327
-
2.487 2.385 2.235
-
-
Having found out this convenient method for extrapolation of the experimental data, it was next necessary to discover how far the same had to be carried in order, in practice, to reach true q, values. The extrapolated quantity figures were read off on Fig. 3 for a volume of one cubic millimeter (Le. log v = - 3 ) , a retentivity curve constructed from these values combined with theappropriate volumesof air, and tangents to the curve takenat intervals.
LOG $3
FIG.4
The tangent value a t the initial saturation quantity must necessarily correspond to 3 3 m.m., and hence a factor was obtained, read off from the retentivity curve, by which other tangent values could be converted to true pressures. Having obtained such a set of p and q values, the isotherm was then drawn. As the extrapolation of the logarithmic q/v plots was considerible, the whole process was repeated, taking as the extrapolated volume I
L. J. BURRAGE
2210
(log v = o), and, from the retentivity curve, a second isotherm constructed in the same way. This was found to agree very closely indeed with the isotherm obtained by extrapolation to v = I cubic millimetre. We are therefore justified in concluding that, working with a column of volume one c.c., the pressure-length effect is overcome in practice, i.e. that the average q value in such a coluinn at a n y instant during the retentivity test corresponds so closely to the partial pressure of mpour in the air leaving the colzcmn as to allous of true isotherms being deduced Jrom the retentizity curve. That' this is a valid conclusion has been amply demonstrated by the agreement shown between the data obtained in this way at 2 j" and those obtained by a static technique.' The only corresponding evidence for 100' is more or less of an indirect nature, since it depends, as far as the static technique is concerned, on isotherms calculated from isosteres, themselves somewhat extrapolated from measurements carried out at lower temperatures. Provided that the pressure never exceeds about 0 . 1 m.m., the method is perfectly sound, experimental error apart. If hoivever the pressure rises above this value, this extrapolation cannot be resorted to unless there is a sufficient number of points in the particular region of the isostere concerned, the reason being the existence of a break in the isosteres in the region of this pressure. The following comparison shows the type of agreement obtained between the two different methods. The difference between the figure is of the same order as the error when using the static technique. C.C.
Charcoal
Value of q in mg./gram at IOO' Static method Retentivity method
Pressure
B
0.009
B -12
0.033 0.264
nun.
5.7 10.0
40.3
5.0 .o 39.3 I2
Isotherms for CC14 at 25' C As already mentioned, this method of obtaining true i-otherms is equally applicable at 2 jo. Exactly the same general procedure is followed as described above. Table 5' contains a specimen set of data obtained from retentivity Litres of air passed 0 , 2
0.6 I .2
2
0
5 .o 8. o
TABLE 1 ('harcoal B. ("?la. log v = o
log q
=
140
3.030 2.970 2.895
li4
0
z j oC. 0
4.12
o 623
3.028 2.968
3.032
3.040
2.891
2,945
3 ,010 2.975
2.888
2.922
__
2.828 2.674
2
2.574
2.598
2
,492
2.514
2.590
2.642
2,484
2.557
I2
.o
2
20
.o
2.376
~
,683
~
2.762
,666
2.801 2
,713
Experiments hg R . Chaplin, submitted to the Royal Society for publication. An air-activated soft-wood charcoal. See J. Phys. Chem., 32, 4.11 (1928).
SORPTIOS ISOTHERMALS O F VAPOURS ON CHARCOALS
experiments at zjo with Charcoal B charged with CCla. Fig. the linear extrapolation is valid in this case also.
2211
j
shows that
"'"1
FIG.5
The following figures show the type of agreement found at 25' C . between data obtained by the new method and data obtained by (i) a static technique in absence of air' and (ii) direct charging at a definite pressure in an air stream.* Charcoal
B B B Charcoal
Static Technique P q 23
. 8 m.m.
20.8
I O ~ mg.,'g. I
Retentivity Technique P q 2 3 , 4 m.m. 1048 mg.,'g.
I002
20.8
1006
18.8
952
965
18.8
Direct charging
q at
B
374 o m g. gram.
G
j I I
0
9 m m pressure Retentivity Technique
.+
3 j o o m g gram. 500 0
Water Vapour Isotherms at 25'C. This technique has also been applied with success t o obtaining water vapour isotherms at z jo,and it seems very probable that, as true isotherms can be obtained in this way for such widely differing substances as CC11
FIG. 6 hleasurements by A. Puttick, submitted to the Royal Society for publication. 2 A stream of air charged with CC1, a t a h i g h e r r was passed through a vessel cooled by melting ethyl malonate ( - jO"C.), the C la partial pressure being thereby reduced to 0.9 m.m. 1
L. J. BURRAGE
2212
and H20, the method is of general application, both as regards sorbate and also temperatures a t which the experiment is carried out. Table VI and Fig. 6 show that the linear extrapolation holds for water, and Table VI1 and Fig. 7 that this linear relationship is again, within limits, independent of the cross-section of the container used.
TABLE VI Charcoal K’ (water extracted). Water Vapour. Litres of air passed 0.0
log V=0.959
l o g q = 2.214
2 5 O
C.
1.049 2.214
1.114 2.214
0.4
2
,213
2.210
2.212
I ,2
2.205
2.203
2.8 6 .o 12.4 18.8
2.189
2 .
2.151
2.157 I ,091
2.206 2 . I94 2.167 2.114
2 ,018
2.055
2.074 1.984 I ,876 1,743 1.573 1.114
25.2
31.6 38.0 50 .o
I I
l O g q = 2.193
0.2
2.189 2.180 2.161 2.126 2.041 1,834
0.6 1.4 3.0 6.2 12.6 19.0
I . jOI
I
1.711
1.377
log v = o . 7 8 6 I I10 Wide containers
0.0
1.987 ,911 I ,823 1 ’ 593
,932 ,831
TABLE VI1 Charcoal A. Water S’apour. Litres of air passed
I88
2.193 I88 2,185 2.176 2,157 2.119 2.034 1.936 2 .
C.
2 jo
o 870
1 049 Narrow containers
2 . I93 2,190 2 ’ I79 2.163 2.128 2.049
I
,874
I
,636
’ I93 2.192 2.186 2.176 2.153 2.104 2.009 1.873
2
The points in Fig. 7 do not lie on such good linear plots as in other cases. This is due to the fact that Charcoal X exhibits to a rather marked degree the phenomenon known as “drift” or “sinking-in,” and hence it is difficult to get points beyond a certain degree of accuracy, more particularly with water vapour, which is known from experience in static experiments to behave thus with this charcoaLz S o comparison is possible with data obtained from static experiments3 as the aqueous vapour pressures to which the charcoals were of
-4 peat charcoal of French origin, activated with phosphoric acid. Packing density
mesh granules 0.318. J. Phys. Chem., 33, 1694 (1929). LOC.cit., and ibid., p. 1682.
10-12
SORPTIOS ISOTHERMALS O F VAPOURS ON CHARCOALS
2213
exposed were there greater than in the present case (15-18 mm.) and hence, owing to “sinking-in,” the quantities of water taken up also greater. One point should be noted in connection with the water vapour experiments. h certain amount of water is found, on desorption in dry air, to be 1.2,
FIG.7 so firmly held by the charcoal that it can be regarded, for all practical purposes, as permanently bound.’ This amount is subtracted from each q value before the logarithm is taken for the extrapolation diagram, and is later added to the extrapolated retentivity figures. Some Experimental Details An important point in connection with these experiments is the (I) method of dealing with the vapour absorbed in the grease ring at the top of the container; this applies to all measurements of substances which attack the grease. Since the conversion factor of the retentivity curve tangent values to true pressures depends entirely on the measurement of the tangent at the saturation quantity, it is obvious that the form of the retentivity curve in this region must be known with the greatest possible accuracy, and hence the amount of sorbate retained by the grease ring must be known and allowed for after each passage of air. With this end in view, when working with CClr, a weighed empty container of the same dimensions, with a grease ring fully charged to 3 3 m.m. pressure, was put in series at the exit end of the experimental container proper. Since the containers are of the same size, the amount absorbed by the grease is approximately the same in the two cases, and hence, by weighing the empty container at the end of each run, a sufficiently accurate correction may be made for the sorbate held in the grease in the container proper. I n order to measure tangents with equal accuracy at all points in the (2) different parts of the retentivity curve, it may be necessary to expand or contract the horizontal or vertical scale up to 2 0 0 times or more, subsequently of course reducing all values to the normal scale of I c.m. = I O mg./gram or 5 litres of air. One rather unexpected feature about the technique of the method regarded as a whole is the high degree of reproducibility with which J. Phys. Chem., 33,
1 1 5 1 , 1161
(1929).
L. J. BURRAGE
2214
the retentivity curves can be drawn through the experimental points and the great accuracy with which the tangents can be read off. This can best be illustrated by some actual examples taken from experiments in which, for one reason or another, the retentivity curves were plotted on more than one scale. In Table YIII, column z gives the ratio of the abscissa scale actually employed in drawing the curve to the normal scale of I c.m. = j litres of air; column 4 gives the angle which a tangent to the curve draivn at a point
TABLEY111 Isot h e m
I.
CClt-IO0"
Ratio of actual scale q value to normal in scale' mg /gm 2.
3.
1 4
80
2
C,Tl,- roo'
C'Cl,-25"
H20-25'
do.
Actual angle
4. 68 8" I T .90
Tangent of angle
578
o 322
190
59.3'
I
684
do.
40 . o o
0
839
IO
460
64,9O
2
2;
do.
40.5O
0
135 854 jo
5
50
26.j"
0
I
do.
68
2O
? j
3;
. 2 O
71.90
0
759
3 06
Pressure in m.m. corresponding to q
6.
7.
o 644
0.121
644
0.121
5. 2
IO0
50
Corrected tangent
0
84.2 83.9
5.89 j.88
21.35
2.03
11.35
2.03
2 . j
9.90 9.90
2 . j
I ,90 1.91
j .oo 5.02
corresponding to the q yalue in 3 makes with the abscissa, and column j contains the tangent of that angle; whilst column 6 contains this tangent reduced to its normal value by multiplying by the figure in column 2 , and column ;gives the pressure deduced from the corrected tangent. Discussion The method described seems to present the following advantagm.(I) It is specdy. I t can be used ovpr a wide temperature range. (2) -&s the retentivity curre gives the continuous rate of change of (3) pressure with sorbed quantity, the obtaining of an isotherm with any required number of points is merely a matter of drawing tangents at sufficiently close intervals. This peculiar advantage of the method has led to the discovery that sorption isotherms of vapours on charcoal are composed of a series of loops, passing from one to another with discontinuous changes in curvatures. (4) I t i s capable of accuracy rivalling that of the static technique.
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( 5 ) It can be applied over a very wide pressure range and will give results of the same order of accuracy a t 2om.m. pressure or over as it does a t 0.001m.m. The author will, in conclusion, deal with some criticisms of the method which have been made to him from time to time.
(a) T h a t i t is very unlikely that equilibrium will be reached betuven the sorbate o n the charcoal and the t u p o u r in the air-stream under the retenticity test conditions. Air streams of velocities of I O O C.C. to 400 C.C. per minute have been investigated, using Charcoal B with CC14 at, IOO’, and Charcoal L’ with water vapour a t 2 5 ’ . There was no corresponding change in the retentivity curve when plotted in the form of quantity retained against litres of air passed. It would seem then that equilibrium must be set up sufficiently rapidly to cope with the conditions of the retentivity experiment. (b) T h a t , granted that apparent eyz~ilibriumconditions are attained i n the retenticity experiments, these conditions cannot be those of true eqitilibriitm, which will o n l y be set u p ojter a “sinking-in” process, demanding time, has been completed. If the test is interrupted o w n i g h t , t h e n t h i s “sinking-in” process will huce had time, partly at all erents, to take place. Hence, when the desorption i s resumed ajter such a n interruption, the ietentii,ity curre will not be continuous with the part piei’iously determined. The author finds, in the vast majority of cases, that, if a charcoal, charged t o 3 3 mm. pressure with CCl,, is allowed to stand at 100’ for 2 0 hours, there s no preceptible change in the pressure. This shows that there is no fear of any “sinking-in” taking place over-night a t room temperature if the experiment on the charcoal is interrupted. Of all the charcoals examined for this drift effect, only three showed it-Charcoal A and two other soft-wood charcoals. (c) It will be seen in a subsequent paper that each isotherm consists of a series of loops, and further it will appear that the pressures at the junction of t h e different loops are, to a very large measure, independent of the nature of the charcoal, of the temperature, and of the substance adsorbed. This is such a remarkable and unforeseen conclusion that the first point that naturally presents itself is whether or not the existence of these “breaks” is real or merely inherent in the experimental method. T h u s , it appeared possihle that the “breaks” might, in some w a y , be connected with the interruptions i n the retentii,ity experiment due to the necessity of weighing the container, particularly as t h e z’ariozis time intervals betueen successii,e iveiyhi n g s are m u c h the same, whatever the charcoul. A special experiment was carried out in which the time intervals were purposely made irregular, long and short periods of varying lengths following A steam-activated beech-wood charcoal of Dutch origin. Packing density of mesh granules 0.441,
12-12
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L. J. BURRAGE
one another, instead of the usual regular increase as given in Tables 1-1'11. This procedure furnished an isotherm with the breakh at the same pressures as before. (d) Another objection made w a s that, i f the r u n i s interrupted oi'er-night or over the week-end, as sometimes happens, the concentration gradient inside the charcoal column i s bound to change and to become less steep-which will cause a break in the retentivity curve on the resumption of the r u n . Examination of the curves given by experiments which had been interrupted overnight or over the weekend showed no signs of such breaks. During a certain experiment at IOO', the run was deliberately interrupted, and the container allowed to stand at 100' for one hour. If such a change in concentration gradient takes place overnight at room temperature, it should happen far more rapidly at IOO', and hence a break should appear subsequently in the retentivity curve. This however was not found to be the case. Finally, a container was filled with charcoal and charged to saturation with CC1, at 33 m.m. -4second container was filled with dry uncharged charcoal, and directly connected with the other through the container taps, the two containers being allowed to stand at room temperature for 1 7 0 hours. Here we have an extreme case, charcoal charged at 33 m.m. pressure and charcoal containing no CCl,, in direct communication with one another. I t is true that the two taps, although of wide bore, will slow down somewhat the setting up of equilibrium between the charcoal columns in the two containers; but nevertheless, if the levelling-out of the concentration gradient referred to above were actually to occur, one would expect a fairly rapid increase in the weight of the second container in the present case. This was actually found to be only 18 m.g. during the 1 7 0 hours (4 m.g.:/gram of charcoal). I t follows that the alteration of the concentration gradient in the charcoal column under the conditions of the test must be negligible for practical purposes. When it is remembered how the diffusion of the molecules of vapour through the gas phase in the container is bound to be slowed down by the large excess pressure of air present, this is not surprising. In any case, it agrees with many observations made in the course of static experiments on the large retarding effect of small amounts of indifferent gas on the rate of diffusion of a vapour present at low pressures. (e) I t appeared possible that the breaks might be connected in some w a y with mechanical imperjections in the fiexible metal strip used for drawing the retentivity curves. The fact that alterations in the scale on which the retentivity curves were drawn had no effect on the final isothermals of course is a strong argument against this objection. Actual experiments with other flexible metal strips were however carried out. They did not result in any essential changes in the nature of the derived isothermals.
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SllmmnrP An investigation has been made of the manner in which the concentration gradient in the charged charcoal column changes with the quantity of air passed, under the conditions of the simple charcoal retentivity test of Allmand, Manning and Burrage. (I)
As a result of these experiments, the above test has been so modified (2) as to yield true sorption (or desorption) isothermals. (3) This new method of determining sorption isothermals appears to be capable of general application t o cases of the sorption of vapours by solids. It possesses to a marked degree the advantages of speed, simplicity, accuracy and flexibility. (4) The resulting sorption isothermals are discontinuous in nature, being made up of a series of loops cutting one another at definite pressures.
Cniuersity of London, King’s College. M a y 86,1930.
