NOTE ON T H E POSSIBLE MAGNITUDE O F THE SORPTION ERROR I N MEASUREMENTS INVOLVING EASILY SORBABLE GASES AT LOW PRESSURES MARCUS FRANCIS
Institute du Radium, Pavillon Curie, Paris, France Received April 3, 1933 INTRODUCTION
Gases which are easily compressible condense readily as a rule on surfaces exposed to them. At sufficiently high pressures changes in the amount of gas condensed on the walls of an apparatus, resulting from alterations in the pressure, are small compared with changes in the quantity of gas contained in the free space of the apparatus. They may therefore be neglected, except in cases where a high degree of accuracy is required; for example, in measurements of gaseous densities for atomic weight determinations. Not so, however, a t low pressures. Here, not only the amount of gas contained in the free space of the apparatus, but also the amount sorbed on the enclosing walls may vary directly with pressure-the socalled “Henry range.” If the initial slope of the sorption isotherm is greater than that of the isochor for the apparatus concerned the two curves will intersect, for the latter’continuesto rise, whilst the former tails off more or less rapidly as saturation of the surface is approached. I n such cases calculations based on the pressure changes observed in an apparatus of known volume may be subject to serious error, and this may escape unnoticed unless there is some possibility of controlling the results by an independent met hod. The diagram of figure 1 represents a case in which the free gas content a t 20°C. of an apparatus of two-liter capacity is compared with the early portion of a typical sorption’ isotherm a t the same temperature. I n general there is no necessity for the two curves to intersect, as in the hypo1 I n the absence of suitable data for glass the start of the 20°C. isotherm for sulfur dioxide on 0.05 g. of wood charcoal (writer’s unpubl’ished results) has been plotted. The smoothed curve is the commencement of a “Freundlich” equation, A = 50.0 pO.’87, where A is given in micromoles and p in millimeters of mercury. An equation 124 p of the Langmuir type, A = ___ mould fit the upper points of the figure reason1+5.52p’ ably well, but falls however below the first three points. 1019
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thetical case depicted here. There will, however, usually be found a range of low pressure over which the slopes will not be widely different, and over this range the same remark applies. APPARATUS
The apparatus in which the experiments about to be described were performed, was constructed for the purpose of examining the suitability
FIQ.1. Curve I : Isochor for apparatus of two-liter capacity a t 20°C. May be considered a straight line for all gases over the pressure range here in question. Curve 11: Typical isotherm for easily condensible gas. Approaches a limiting saturation value a t higher pressures unless capillary condensation intervenes.
af the McLeod gauge as an instrument for measuring low pressures of sulfur dioxide. I n that section of the apparatus with which we are here concerned (figure 2), the following combinations were independently possible :Volume bulb
TL
McLeod gauge No. 1
+--+
Gas train
I(
1 McLeod gauge No. 2
Evacuating system
t.-l->
I
L
+
Gas supply
The estimated surfaces and volumes of the different parts of the system bounded by taps or seals were as follows: gas train (TI to T,), 750 sq. cm., 164 cc.; volume of bulb, R,560 sq. cm., 997 cc.; McLeod gauge No. 1,GI, 660 sq. cm., 319 cc. ;McLeod gauge No. 2, Gz, 290 sq. cm., 269 cc. Volume estimates are probably more accurate than those of surface. The exact significance of estimates of the geometrical surface is uncertain, since
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calculations of sorption data based on such estimates in the case of glass indicate the existence of a n unduly large number2 of superposed layers of closely packed molecules under certain conditions. The two McLeod gauges were similar in size and shape, each having a bulb of ea. 180 cc. volume and a compression chamber consisting of a lengt,h of quill tubing of cross section equivalent.to 7.29 cu. mm. per millimeter, surmounted by a capillary of 1.88 cu. mm. per millimeter. At the highest mark a column of 1 em. of mercury corresponded to a gas pressure of 1.5
FIG. 2. APPARATUSUSED I N THE EXPERIMENTS R, volume bulb; G1 andGz,McLeod gauges; C, condensation bulb containing phosphorus pentoxide; PI and Pz, phosphorus pentoxide drying bulbs; MI and M,, manometers for controlling pressure of gas to be admitted t o micropipette; Ts, single-bore, three-way tap with appendix forming micropipette; TI, tap t o evacuating system; t a p t o reserve of sulfur dioxide. Right-hand side of system:-McLeod Gz, bounded by mercury seals SI and Sz anP by t a p Tg. Left-hand side of system:-McLeod GI,bounded by SI,Ta and T4, plus volume bulb R, plus gas train between Tl and Tr. Tg,
microns of mercury in the system. The upper limit of the reading range of McLeod gauges was a pressure of 4 mm. of mercury. The two gauges were connected directly by way of a mercury seal, SI, but for the remaining connections carefully chosen glass taps lubricated with rubber grease (later replaced by Apiezon) were employed. Separating the system from the gas supply there was a capillary tap, Ts,with three outlets a t 120" to one another but only one bore. One of the outlets had been sealed off to form a short appendix, and by connecting this with a For example, circa fifty layers of water, five layers of carbon dioxide (Langmuir: J . Am. Chem. SOC.38,2283 (1916)).
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the source of supply and the system successively, the pressure in the latter could be increased by amounts as small as desired, the pressure of the supply having been suitably adjusted beforehand. This tap, the only one bounding the system to have on the other side sulfur dioxide a t appreciable pressures, could be isolated by a second mercury seal, Sz, introduced between it and the McLeod gauge GP. TREATMENT OF APPARATUS
Following the usual practice, the constituent parts of the apparatus had been cleaned with chromic-sulfuric acid3 mixture before assembly. After completion of the air calibration, for which one of the McLeod gauges served as reference volume, the apparatus was exhausted thoroughly and baked out a t 250-300°C.with the mercury vapor pump running. Heating elements in the form of nichrome wire spirals wound on Pyrex quill and mounted parallel t o the axes of sheet asbestos cylinders, reinforced where necessary by a backing of sheet “tin,” were found very convenient for heating the gauges, which for this reason had been mounted on iron stands instead of the usual form of wooden support. In, order to permit of their easy removal when not in use the heating ovens were hinged down one side. During the baking-out of the McLeod gauges the gas train and volume bulb were heated a t intervals by the aid of the spirit flame. A cowl of asbestos sheet suitably arranged prevented the formation of a cold cap on the upper hemisphere of the volume bulb. The outgassing treatment was repeated each time the gas in the apparatus was changed and whenever the accumulation of surface moisture was suspected, owing to the attainment of an apparently negative pressure on exhausting4 or to a drop in the calculated pressures on passing from mark to mark on the McLeod gauge. EXPERIMENTAL PROCEDURE
The experiment consisted in distributing a certain quantity of sulfur dioxide at low pressure between the different sections of the apparatus This treatment, though customary in gas work, possesses t h e disadvantage of covering the surface of the glass with a layer of silica gel, thereby increasing the sorption characteristics of the apparatus (cf. H. S. Frank: J. Phys. Chem. 33, 970 (1929)). From this point of view swabbing out with cotton-wool soaked in benzene might be preferable, other things being equal, t h e solvent removing the grease and mechanical action the insoluble matter. 4 Differences in the dryness of the surface of the glass in the compression chamber and its comparison capillary cause the depression of t h e mercury meniscus in the two to differ, although t h e diameters are t h e same i n both (cf. L. Dunoyer: La Technique du Vide, p. 74). The effect was frequently observed after unusually high temperatures had prevailed in the laboratory for some hours a t ? time. Under such conditions there is a disengagement of water from lower sorption levels, which the pentoxide bulbs in the train are not able to remove immediately.
.
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1023
and measuring the pressure in each side independently. After each redistribution the apparent quantity of gas present in the apparatus was calculated from the known volumes of the different sections and the observed temperatures and pressures. The total quantity was then compared with an arbitrarily chosen initial reading for which the whole of the free gas had been collected in McLeod gauge No. 2, hereafter referred to as the right-hand side of the apparatus. When equality of pressure had been attained in both sides of the system, a further transfer of gas could be effected by applying a wad of cotton-wool soaked in liquid air t o an appropriate section of the train for a short time and manipulating the taps accordingly. By proceeding in this manner it was possible to distribute the gas in either direction a t will from a state of uniform pressure, or to collect practically the whole6 of the free gas in either side of the system. RESULTS
The results of a typical series6of distributions in both directions, repeated three times, are given in figure 3, in which the percentage variation from the initial reading has been plotted against the time in days from the start. I n order t o avoid undue extension of the time scale the intervals between the last readings on one day and the first on the next have been omitted. Lines joining the experimental points merely indicate the sequence of the readings; they do not, in general, represent an attempt at intrapolation. In the course of the first distribution from right t o left an apparent disappearance of gas was observed, sorption on the new surface exposed more than counterbalancing desorption from the old consequent upon the pressure drop in the right-hand side. Completion of the transfer was accompanied by a partial recovery of the loss. After returning the gas t o the right-hand side of the system a gain of some 16 per cent (fifth day of figure 3) ensued, increasing to 20 per cent (eleventh day of figure 3) on repeating the cycle of operations. Sorbed gas was evidently being slowly disengaged from the enclosing walls of the system. Since early experiments of Herbert’ had already shown the impractica5 There was always a small after-release of gas on isolating a section of the apparatus which had been exposed t o liquid air for a short period of time; hence the transfer of gas never appeared t o be quite complete on attempting to collect t h e whole of t h e free gas in one side of t h e apparatus. 6 I n all, three series of distributions were performed with different quantities of gas in t h e system in each case. For reasons of economy i n space the intermediate series only is discussed here; the preceding and following series, with twice and half as much free gas, respectively, in the system yielded entirely analogous results. 7 J . M. B. Herbert (unpublished work performed a t Manchester University). F. P. Burt (Trans. Faraday SOC.130, 183 (1932)) reports that in the case of ammonia sorbed on glass several days exposure to t h e vacuum produced by charcoal cooled in liquid air is insufficient for t h e recovery of even half the quantity of gas sorbed during a week’s run a t atmospheric pressure.
THE JOURNAL OF PHYSICAL CHEMIETRP, VOL. XXXVII, NO.
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bility of completely recovering sulfur dioxide sorbed on glass, even on raising the temperature, there did not appear to be any likelihood of success attending an effort a t recovery from a system a t room temperature. Accordingly no attempt was made to push the matter to a limit; instead, a condensation bulb in the system was immersed in liquid air for three periods of ten minutes or so and the whole of the condensed gas was then allowed to expand into (a) the whole, (b) the right-hand side, and (c) the left-hand side, respectively, of the system. The peaks on the thirteenth, fourteenth and fifteenth days of figure 3 represent the earliest points taken in each case. It may be observed that the peak corresponding to the readings of the fourteenth day is double. This is due to the removal of a small quantity of permanent gas, which was found to have collected in the course of the experiments. It owed its origin in all probability to traces of air dissolved in the t a p grease8 during a check air calibration performed shortly before and now released under the combined influence of the low pressure in the apparatus and the high laboratory temperatures obtaining about this time on occasion. A correction based on the assumption of a uniform disengagement with time yields the dotted curve of figure 3; the general form of the curve is in no wise altered thereby and the effect is small compared with the other effects observed, so that a correction on this basis would appear to be not unjustified. Examination of the curve of figure 3, particularly in the corrected form, indicates that, for all practical purposes, a state of quasi-equilibrium had been reacheds at the beginning of the series of distributions, i.e., the purely time-conditioned effect was negligible. The increasingly large develop ment of free gas on collecting in the right-hand side of the system would appear to be due to the hysteresis effect associated with the sorption of easily condensible gases on glass surfaces, Exposure of the system to the low sulfur dioxide pressure obtaining a t liquid air temperatures effects the release from the glass of quantities of gas, which are not resorbed immediately on raising the pressure again. The effect may be considered to be the counterpart of the phenomenon observed by Burt and Jones.10 These observers found that in a sorbing system where sorption was proceeding with extreme slowness it was possible to cheat time, as it were, by temporarily raising the working pressure. On returning the system to the original pressure the sorption was found to 8 It is not practicable t o heat the tapa of an apparatus during the outgassing and thus accelerate the liberation of dissolved gases from the lubricant, consequently the last traces are released with extreme slowness. 0 The apparatus had been standing a t low pressure for a fortnight before t h e series was commenced. 10 F. P. Burt, loc. cit.
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APPARENTFREEGASI N SYSTEM PLOTTED AGAINSTTIMEFROM ARBITRARILY CHOSENSTARTING POINT The lower, dotted line indicates that the truly time-conditioned desorption is practically n i l ; the main disengagement of gas is provoked by the low pressure resulting from the liquid air treatment. The pressures obtaining at the distributions corresponding t o the salient points of the curve were as follows: FIG.
3.
VARIATION OF
AN
Start$. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1st peak4 (5th d a y ) , . , . . , , . , . . , . , . . 2nd peak$ (11th day). . . . . . . . . . . . . . . 3rd peak* (13th d a y ) , . . . . . . . . . . . . . . 4th peak1 (14th day). . . . . . . . . . . . . . 5th peaklt (14th d a y ) , . . . . . . . . . . . . . Gth peaks (15th d a y ) , . . . . . . . . . . . . . .
Lefl-hand aide
Right-hand side
0.00004 mm. Hg 0.00030 mm. H g 0.00278 mm. Hg 0.01519 mm. Hg 0.00098 mm. H g 0.00049 mm. Hg 0,02306 mm. Hg
0.08318 mm. Hg 0,09640 mm. H g 0.08626 mm. Hg 0.01497 mm. Hg 0.1311 mm. H g 0.1289 mm. Hg 0.00010 mm. Hg
* After distributing throughout whole system. t After removal of trace of permanent gas.
t Bulk of gas collected in right-hand
side of system.
I Bulk of gas collected in left-hand side of system.
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have attained values which would not have been reached in reasonable time had the sorbing pressure been maintained constant throughout. In the present case we secure an increased desorption by temporarily reducing the pressure in the system to very low values. The three peaks mentioned above (figure 3) are of interest for the light they throw on the loosely held surface gas in the system. Owing to the time required for the expanding gas to distribute itself throughout the different parts of the apparatus on evaporation some minutes elapse, after removal of the liquid air, before it is possible to take readings. Nevertheless, the three peaks are, qualitatively a t least, in the relative positions we should expect to find in the case of a given quantity of sorbate admitted to (a) the whole, (b) the smaller part, and (e) the larger part, respectively, of a sorbing surface. Quantitative agreement is hardly to be expected in view of the difficulty of securing identical conditions in each case. If we suppose, however, that the relative positions of the peaks under consideration are, in the main, due to differences in the instantaneous sorption, it follows that the amount of gas actually condensed by the liquid air corresponds to a still greater departure from the initial reading than is indicated by the highest peak recorded in the figure, that of the fourteenth day. CONCLUSIONS
From the foregoing it appears evident that, in an apparatus containing an easily sorbable gas a t low pressures, there is, in addition to the free gas in the system as calculated from the known volumes and observed temperatures and pressures obtaining, an indeterminate amount of sorbed gas potentially available for release on reducing the pressure. This sorbed gas escapes observation under the usual conditions of experiment, though it may equal or even exceed in amount the free gas in the system. In the absence of time effects the gas would be difficult to detFct, but by taking advantage of the time-lag in the attainment of sorption equilibria its presence may be revealed and an estimate, almost certainly too low, made of the amount involved. In a typical apparatus containing sulfur dioxide a variation in the content of free gas amounting to more than 50 per cent referred to an arbitrary starting point, was observed. The ratio of surface to volume was not unduly large and the range of pressure variation in the system was from 0.0001 to 0.10 mm. of mercury, Le., not excessively low. Although the experiments described above were performed with sulfur dioxide in the system, the effect observed is in no way confined to this gas. It depends, as seen from figure 1,only on the relative shapes of the isochor and the sorption isotherm for the particular gas and apparatus concerned. Accordingly it may be expected to be present in any apparatus containing an easily sorbable gas a t a sufficiently low pressure. Hence, in the absence
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of any control in the form of an independent methodll of determining the quantities of substance involved, caution should be exercised in the interpretation of experimental results obtained under conditions similar to those considered above. SUMMARY
1. Comparison of the forms of a typical sorption isotherm and the isochor of an apparatus of two-liter capacity indicate that, over a certain low pressure range, the gas sorbed on the walls of a relatively simple apparatus may approach or even exceed in amount that contained in the free space of the system. 2. I n the absence of some independent form of control this sorbed gas may entirely escape detection, but by taking advantage of the time-lag in the attainment of equilibrium in the case of glass as sorbent we may nevertheless observe a part of the pressure-sensitive sorbed gas. 3. As an example, some measurements with sulfur dioxide are quoted, in the course of which a temporary disengagement of sorbed gas amounting to more than 50 per cent of the free gas at an arbitrary starting point was observed. The total quantity of sorbed gas in the system must have been still greater, since the instantaneous resorption could not be measured. 4. Attention is called to the necessity for exercising caution in the interpretation of experimental results involving measurements of pressure changes in apparatus containing easily sorbable gas a t low pressures, via., pressures below 0.1 mm. of mercury. The experimental work on which this communication is based was performed at Frankfurt-am-Main during the tenure of a Fellowship awarded by the Alexander von Humboldt Stiftung, B.erlin, to the Trustees of which the writer tenders his thanks. Such an independent form of control might be furnished, in sorption work for instance, by the McBain sorption balance or some other suitable form of microbalance.
