T H E K A T E R CONTEST OF BEKZENE BY ALAN W. C. MESZIES
Studies of the degree to which intensive desiccation affects the physical properties of substances have led to results which are notoriously discordant.' Vapor pressure of a desiccated substance has not infrequently been selected for study in preference to other properties as yielding results that are perhaps least equivocal. The earliest publication on this phase of the subject was that of Smith and Menziesz on dried calomel; while the latest is a t present the interesting work of Greel3 who studied especially benzene below 50.6'. I t was shown by Greer that the small quantity of water present in a sample of benzene that had been distilled from phosphorus pentoxide within a closed glass apparatus was sufficient to raise the t'otal vapor pressure by several millimeters of mercury higher than that of another portion of the same benzene dried by silica gel. So many precautions were taken to avoid the presence of water that the impression remains that the benzene sample of higher total pressure must itself have been typical of very well dried benzene. K e believe, however, that very much drier benzene can readily be obtained, and studied in a glass apparatus, with far fewer precautions. If this is so, the other workers in this field may after all have operated on benzene samples which were initially so well dried that Greer's explanation of their anomalous findings would be inapplicable. I n 1910, Menzies4 had shown that the boiling point of benzene dried I. over sodium is found by static and dynamic methods respectively with values identical within o.001~. It might, however, be argued that any abnormal behavior, due to water if present, would be identical for the two methods. 2. More recently, Wright and Menzied studied near Soo, by their method of isot'hermal distillation, samples of benzene dried by sodium and transferred in a dry pipette to a glass apparatus dried without extreme precaution. The change in vapor pressure on distilling off as much as half of the sample did not exceed 0.2 mm. of mercury. I t would appear from the experimental work described under (3) below that lowering of vapor pressure due to removal by distillation of dissolved water from benzene must manifest itself before even one-fourth of the liquid has been fractionated off. The amount of water in our sample, therefore, was so small as not to affect the normal boiling point of benzene by over O.OI', which is the temperature change corresponding to 0 . 2 mm. Much of the literature of this subject may be found cited in the work of J. W,Smith: "The Effects of Moisture on Chemical and Physical Changes" (1929). ? Z . physik. Chem., 76,713 (1911). J. Am. Chem. Soc., 5 2 , 4191 (1930). 4 J. Am. Chem. Soc., 32, 1616 (1910). 5 J. Am. Chem. SOC.,52, 4699 (1930).
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3 . In these circumstances, and because other workers in the field of intensive desiccation had studied benzene a t 80’ and above, it was of interest t o find if the vapor pressure of benzene, as customarily handled in the organic laboratory, and near its normal boiling point, was ordinarily affected by the presence of water. For one might anticipate that, on simple distillation and even without the use of a desiccant, any water present in the benzene would pass away with the earliest fractions by reason of its relative richness in the vapor phase. Accordingly, the experiments described below are designed to test the behavior of undried benzene from which a portion has been dist’illed off. Method A . The apparatus employed was that devised by Menzies for determination of molecular weights of dissolved substances by direct measurement of vapor pressure lowering of a solvent. A description of this may be found in the original article or elsewhere,‘ but Fig. I , although drawn to illustrate Method B, will serve to recall the form of the apparatus. About 130 cc. of a good grade of commercial “C.P. crystallizable benzene conforming to the standards of Murray” was charged into the outer bulb A and boiled for ten minutes under reflux. About 1 5 cc. were then distilled off through the condenser in a non-reflux position, and discarded. The first runnings were cloudy, due to water. The connection D to the conclmser was closed, and a further quantity of like volume was distilled off through the gauge tube C and inner tube B, with the object of st ieai-t partidly removing water adsorbed on the glass walls of these portioas of the apparatus. The inner tube was then charged with a portion of the remaining benzrne. To remove dissolved gases, the connection to the rondenser being closed, benzene vapor was “blown through” the gauge tuhe and inner tube, which was then closed by stopper E, while the connection to the condenser was simultaneously opened. After thermal quilibrium had been reached, the zero reading of pressure was taken, in terms of mm. of difference of levei of the benzene surfaces in the inner tube and the gauge tube. The blowing through process was repeated until the results were constant. (i) SeTwai grams of anhydrous magnesium prrchiorate werc then added to the inner tube. Difficulty was anticipated in removing adsorbed gases from the large surface offered by this solid, hut no such difficulty was experienced. The zero reading obtained, in the manner explained above, was identical, within the experimental error, wjt,h that obtained in the absence of the desiccunt, and remained constant until the experiment mas discontinued afrer two hours. I t may be recalled that readings with this forin of apparatus are not affected by slow change of atmospheric pressure. This result pointed t o the essential absence from the benzene of such an antount of water as would affect the vapor pressure of benzene at 80’ by so much as half a millimeter of mercury. I t may be objected that, the efficiency of magnesium perchlorate as a desiccant a t 80’ is uncertain. (ii) Potassium hydroxide a.s a desiccant has been studied by H a t e r and Starbeather’ 1
Cf. Ytaehler: “Arheitsmethoden in der morg. Chemie,” 3, 169 (1913) J. Am. Chem. Soc., 38, 2036 (1916).
THE W A T E R CONTENT OF BENZENE
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a t joo, a t which temperature they found that it is in equilibrium with an aqueous pressure not greater than o . o q mm. of mercury, although possibly less. Several grams of potassium hydroxide, freshly fused a t a red heat and powdered on cooling in a pre-dried hot mortar, were introduced with a fresh sample of benzene treated as formerly, in the same manner as with magnesium perchlorate. After blowing through as before, the same constant zero reading was obtained as in the absence of the desiccant. This points to the same conclusion as in (i). Melhod B. I t may be objected that these desiccants are, in the presence of benzene, for some reason unable to function, or, a t least, do not function in the time allowed. The same disability has indeed been imputed to phosphorus pentoxide in like circumstances. To meet this objection, the diesociation pressure of a salt hydrate pair was measured within the inner tube of the apparahs. If no vapor is present in the benzene vapor, then the full aqueous pressure furnished by the salts should be found as a quantity additive to the pressure of benzene vapor, but if the benzene vapor already contains some water vapor, the pressure found and measured should be the difference between the true aqueous dissociation pressure of the salts and the partial pressure of water already existant in the benzene vapor before the salts are added. As a suitable salt hydrate equilibrium to utilize for this purpose was chosen that represented the equation ZnS04, (XH,)&O,, 6 H 2 0 ZnSO1, (KH1)*S04,zH?O 4H20. The pressures of this system have been measured in eleven observations by Caven and Ferguson' from 30.7' to 79.4'. The double salt is one of those which is readily prepared in a pure condition. At So0, a pressure over 90 mm. of mercury is to be expected. In order to be able to measure such a high pressure in our apparatus, about j cc. of mercury was placed in the inner tube B, as shown in Fig. I , in addition to the benzene. The manipulat,ion was similar to what has already been described. ilbout 3 grams of salt, were employed, obtained by freshly efflorescing the hexahydrate in an air oven at 90' until the water content corresponded to 5 . 5 H20, and subsequently guarding against adsorption of water. Corrections to the increase of pressure observed were necessary (a) for the pressure due to liquid benzene and solid salt resting upon the mercury, and (b) for the density reduction of mercury and benzene at 8oo (factors of 13.60 and 0.81 j respectively) to mercury at The value of correction (a) was determined by separate experiments made outside the apparatus with tubes of similar dimensions to those forming part of the apparatus. Equilibrium was reached within an hour, and repetition of the blowing through process yielded concordant results. The dissociation pressure found was 95.7 mm. a t 80.2'. The temperature was known t o +o.I' by use of a certified mercurial thermometer of recently determined zero-point. Assuming as correct the three-constant equation
+
GO.
I
.J. Chern. Soc., 125, 1307 (1924).
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given by Caven and Ferguson to represent their results, i 0 . 1 ' corresponds a t 80.2' to h0.5 mm. pressure. Looking also to possible errors in pressure measurement, our value may well be in error by I mm. The pressure a t 80.2' according to Caven and Ferguson's equation is 96.6 mm., which is 0.0 mm. higher than our value, but within our error of experiment. A possible source of error not taken into consideration above is the alteration in the vapor pressure of benzene itself by reason of the water now dissolved in it. The amount (expressed as mol fraction) of such water will bear nearly the same ratio to the amount a t saturation as its partial pressure does to that of pure water. For benzene saturated with water is in equilibrium, as to partial pressures of the components, with water saturated with benzene, whose aqueous pressure is obviously near that of pure water, The Duhem-Margules relation is not very helpful in evaluating this correction to the partial pressure of benzene, partly owing to lack of data. It may be pointed out that this relation obviously fails to hold in cases such as that of heptane and ethyl alcohol' where one of the partial pressure-composition isotherms passes through a maximum. Study E of the published partial pressure-composition curves for pairs of non-ideal liquids leads one to anticipate a lowering by water of the partial vapor pressure of benzene much less than 2 . 8 mm., which is approximately that demanded C in this case by Raoult's law. Liquid pairs are known where the partial pressure of the solvent (as ethyl alcohol) is actually raised by the presence of the solute (as heptane). For these reasons, no attempt was made to apply the small and uncertain correction for possible error from this cause. However, it seems unlikely that water present before the introduction of the salts influenced the pressure of FIG.I the benzene used for the experiment by as much as two mm. of mercury. The utility of the method here employed for measuring dissociation pressures of salt hydrate pairs will be referred to in an article2 which follows. Conclusions From these experiments it would appear that, if ordinary undried benzene be distilled a t about 80" and the first one-quarter fraction be rejected, the Smyth and Engel: J. Am. Chem. SOC., 51, 2660 (1929). By Menzies and Hitchcock.
THE WATER CONTENT OF BENZENE
1659
remaining benzene exhibits a vapor pressure that has suffered alteration from the possible presence of water in an amount not exceeding about 0.2 percent. This corresponds to a change in the normal boiling point of less than 0.IO. Beyond these limits, apparently abnormal vapor pressure results obtained by workers with intensively desiccated benzene cannot be attributed to the water content of their “ordinary” benzene, provided the latter has received even the very simple treatment of partial distillation outlined above.
Summary It is pointed out that benzene dried by sodium and transferred to a glass apparatus has been found free from such an amount of water as would alter the normal boiling point by 0.01’. Two methods have been described for examining the water content of the vapor of benzene at its boiling point. It is shown that, if one-fourth of a sample of ordinary benzene, containing water, is distilled off a t atmospheric pressure, the remainder contains less water than would affect the normal boiling point by 0.1’. The bearing of this upon work with intensively desiccated benzene is pointed out. A simple form of indirect method of measuring dissociation pressures of salt hydrates has been described. Pnnceton Universzty, January, 1951.
