A NOTE ON THE SORPTION OF WATER VAPOR BY GLASS E. P. BARRETT1
AND
A. W. GAUGER*
Experiment Station of the School of Mineral Industries, The Pennsylvania State College, State College, Pennsylvania Received October 5, 195.8
A study of the condition of water in coals of various ranks which is now in progress in this laboratory requires a knowledge of the sorption of water vapor by the glass system used in the investigation, over a range of pressures from 10 to 1000 microns at temperatures varying from 25’ t.0 30°C. The data available over this range are very meagre. Frank (1) has made a very precise measurement from 1 to 100 microns a t 20°C. but the writers know of no published data, other than Frank’s, which are reliable below 50 per cent relative humidity. Although the data were required simply to furnish a correction factor, it seemed unwise to extrapolate Frank’s isotherm to 1000 microns, not only because of the effect of varying previous histories of Frank’s glass and that of the authors, but also because the water vapor-glass isotherm must show a point of inflection somewhere between a relative humidity of 0.006, which is the upper limit of Frank’s isotherm, and a relative humidity of 0.5, which is the lower limit attained by McHaffie and Lenher (2). Such a point of inflection must exist, since Frank’s isotherm is concave toward the pressure axis while that of McHaffie and Lenher is convex. It was primarily to receive assurance that this point of inflection did not occur below 3.5 per cent relative humidity that the measurements herein described were made. APPARATUS AND PROCEDURE
The apparatus is shown in figure 1. SI, Sz, S3,S h and S5 are mercury seal stopcocks. T, contains the coal and was shut off a t S3 during these measurements. T4is used in the study of coal and is of no significance in these measurements. F is a 12-liter Pyrex flask and M a mercury manometer. This manometer was made from 17-mm. Pyrex tubing and, after careful cleaning, was sealed empty to the apparatus. It was then “torched out” under high vacuum for half an hour, and after cooling mercury was distilled into it through a side arm (not shown). It was read with a micrometer microscope and calibrated against the McLeod gauge, using 1 2
Research assistant. Director, Mineral Industries Research, 47
48
E . P. BARRETT AND A . W. GAUGER
dry air as the calibrating gas. It was self-consistent to one micron a t pressures below 100 microns and its absolute accuracy was about one per cent at the highest pressure measured (ca. 1700 microns). Tzis a tube of distilled water kept a t a constant temperature by the thermostat B. The volume between Sq and Sb including T3was measured before sealing to the apparatus. By closing S4 and opening S5, T3was saturated with water vapor a t a pressure calculable from the temperature of the thermostat. Then by closing Ss and opening S4 a definite weight of water was introduced into the system. Since the thermostat was operating only slightly below room temperature, the relative humidity in Ta was fairly high, and it was necessary to
FIG. 1. APPARATUS FOR STUDYINQ AQUEOUSTENSION OF WATERI N COAL
add to the weight of water vapor in its free space the weight sorbed on the walls in order to calculate the weight of water vapor added to the system. This correction, which did not exceed 10 per cent of the total weight introduced, was obtained from McHaffie and Lenher’s isotherm for high relative humidities. As it was not convenient to thermostat the entire apparatus, consistent results were not easy to obtain, but by exercising some care to keep the laboratory temperature fairly constant, it was possible to test the rectilinear character of the relationship between the logarithms of relative humidity and the weight sorbed per unit area over the range in question. Before starting a measurement, the system was pumped to a high vacuum (10-6 mm. or better) and pumping was continued for a t least twelve hours. The glass was therefore “dry)’ in the ordinary sense, but it
49
SORPTION O F WATER VAPOR BY GLASS
must be remembered that several days exposure to a vacuum of mm. are necessary to remove the last traces of moisture a t ordinary temperatures. RESULTS
The data obtained are given in tables 1 and 2 and are shown graphically in figure 2. I n the tables W , W,, and W , are, respectively, the weight of TABLE 1
Isotherm ut 302°K. W
P
0.58 1.16 1.74 2.32 2.90 3.48 4.06 4.64 5.22 8.70 11.60
41 86 133 182 731 277 375 374 423
0.476 1.00 1.54 2.11 2.68 3.21 3.77 4.34 4.91 8.32 11.15
717 960
wo
WoIA
0.104 0.16 0.20 0.21 0.22 0.27 0.29 0.30 0.32 0.38 0.45
2.9 4.5 5.5 5.9 6.2 7.5 8.2 8.5 8.9 10.7 12.7
4.5 6.1 7.8 9.3 11.o 12.6 14.2 24.1 32.3
R.H.
1.4 2.9
TABLE 2 Isotherm at 298.5"K. W
0.542 1.08 1.65 2.17 2.71 3.26
P
37 78 123 164 207 250
I
wz
t
Wol A
R.H.
0.436 0.92 1.45 1.94 2.44 2.95
0 .IO9 0.16 0.20 0.23 0.27 0.31
3.1 4.5 5.6 6.5 7.6 8.7
1.5 3.2 5.1 6.8 8.6 10.4
-
water vapor introduced, the weight in the free space (as calculated from the observed pressure), and the weight sorbed, all in grams X lo3. W , / A is the weight sorbed per unit area. P is the pressure in microns and R. H . is the relative humidity X lo3. Figure 2 is a logarithmic plot of the results and clearly shows the rectilinear character of the isotherm over the range in question. I n figure 2 appears a line lying to the left of the experimental curve and with a slightly steeper slope. This is Frank's isotherm, shown as a solid line over the range of his observations and extrapolated as a broken line to cover the range of the authors' measurements.
50
E. P. BARRETT AND A. W. GAUGER
2
4
6
810
20
FIG.2. SORPTION OF WATERVAPORBY GLASS The logarithms of the relative humidities X 108 are plotted as ordinates against the logarithms of weight sorbed in grams x 10s per sq. cm. as abscissae. Centers of circles are points obtained a t 302°K. Vertices of triangles are points obtained a t 298.5”K. ’DISCUSSION
The results obtained are somewhat higher than Frank’s. There are three factors which may account for the difference. First, a difference in the composition and history of the glass; second, a difference in the extent of drying before beginning a measurement; third, an error in McHaffie and Lenher’s results used to correct the computation of the weight of water introduced. It seems unlikely that the use of McHaffie and Lenher’s data could introduce serious error, through being itself a t fault, not only because it was used for making a fairly small correction, but also because of the good agreement between the two corrections made from quite different points on their isotherm. At 258.5”K. the correction is about 4 parts in 53 and a t 302°K. it is about 2 parts in 56, and yet the writers’ measurements at these two temperatures as corrected by McHaffiie and Lenher’s data are in good agreement. It is impossible to consider the second source of discrepancy, because Frank says simply that his adsorption chamber was “dried under vacuum.” If Frank pumped his glass for considerably shorter periods of time than
SORPTION OF WATER VAPOR BY GLASS
51
was used in these measurements, his results would, of course, be lower than those obtained in this laboratory. It seems most probable that the chief source of difference lies in a different treatment of the glass. Frank allowed his glass to stand in contact with chromic acid for one week, during which time the acid was boiled several times. He thereafter cleaned the glass by soaking in distilled water and rinsing with conductivity water. The writers’ sorption chamber was cleaned with boiling soap solution and rinsed with hot distilled water, and was then subjected to steam a t atmospheric pressure for four hours. It was thereafter rinsed with hot distilled water several times before being attached to the system. SUMMARY
A test of the rectilinear character of the logarithmic form of the sorption isotherm for the system water vapor-glass has been made over a range of relative humidities from to 3 X The results agree closely with the very precise measurements of Frank and indicate that his isotherm may be extrapolated to include pressures of the next higher order of magnitude. The authors wish to express their thanks to the National Research Council for a Grant-in-Aid which made the study on the condition of water in coal possible. REFERENCES (1) FRANK,H. S.: J. Phys. Chem. 33, 970 (1929). (2) MCHAFFIE, I. R., AND LENRER,S.: J. Chem. SOC.137, 1561 (1925).
