T H E DETERMINATION O F SORPTION ISOTHERMALS ON CHARCOAL BY T H E RETENTIVITY TECHKIQUE-EXPERIMENTS WITH CARBON TETRACHLORIDE AND R A T E R BY A . J. ALLMAND AND L. J. BCRRAGE
A new technique for the determination of the sorption isothermals of vapours on charcoals-the retentivity method-has recently been described.' It was stated that the isothermals obtained by this method showed discontinuities, being made up, in fact, of a series of loops cutting one another at definite pressures. The present paper contains details of the results obtained, and deals, in particular, with the evidence for the discontinuous structure mentioned. The majority of the charcoals have already been described, viz. A,? B,? C,* G,3H,3 K,' L.' The new charcoals Mz, M3, and M4 areallsteamactivated, and made from the same raw material (palm nut kernel). They form a series of decreasing bulk density (determined on particles of 10-12 mesh, respectively 0.584, 0.478 and 0 . 4 7 7 ) ~to which corresponds an increasingly severe treatment during activation. M I (bulk density 0.62) is an unactivated charcoal prepared from the same raw material by simple carbonisation, without subsequent steam treatment. Table I contains a synopsis of certain relevant facts concerning the experimental work and the results. Column I gives the designation of the charcoal, column 2 the number of separate charcoal containers making up the total composite charcoal column, column 3 the initial pressure (in mm.) of vapour at which the charcoal was s a t w +,ed,column 4 the number of points on the experiment?' retentivity curve, i . e . the number of times during the experiment that the air-stream was stopped and the charcoal containers weighed, column j the number of tangents drawn to the derived retentivity curve obtained by logarithmic extrapolation, and column 6 the number of breaks found in the derived isothermal. Figs. 1-4contain in detail examples of the actual isothermals obtained. I n Table I1 are collected the pressures in mm. a t which the three lowest hreaks have been found with different carbon tetrachloride isothermals. To these data are added the corresponding figures for the three most satisfactory experiments carried out with water vapour, ciz. those with Charcoals G, K (after thorough previous extraction with water in order to remove hygroscopic mineral matter?) and hl4. It will be noticed that Charcoal G gave an additional break on its water vapour isothermal at a lower pressure, 0.07 mm., 1 2
3 4
Burrage: J. Phys. Chem., 34, 2 2 0 2 (1930). J. Phys. Chem., 32, 452 (1928j. J. SOC.Chem. Ind., 47, 372 T (1928). See Allmand and King: Proc. Roy. Soc., 130A,
2 1 0 (1930).
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than in the other cases. Several of the water isothermals showed breaks a t about this pressure and some, in addition, a t about 0.02 mm. We are somewhat doubtful as to their real existence. A remarkable feature of the values of the pressures in Table I1 is the manner in which they appear to approximate to one or other of three fairly closely defined figures (see average values), which in turn correspond to the first three breaks. This coincidence between the pressure values is rendered still more striking by the facts (i) that the two sorbates worked with, carbon
ISOTHERMALS ON CHARCOAL BY RETEXTIVITY TECHNIQL-E
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FIG.2
tetrachloride and water, are of very different nature and furnish isothermals of quite different types and (ii) that measurements with the former substance a t two different temperatures are included. The concordance between the different pressure values very largely disappears a t higher pressures. Fig. 5 contains a graphic summary of the pressures, between the limits of j-I;mm., a t which breaks have been observed during our deterniinations of isothermals; experiments with water vapour in which the initial charging pressure was below the latter figure are not included. The breaks are seen to vary in number from case to case, and to
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A . J . ALLMAND AND L. J . BURRAGE
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