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I X D U S T R I A L A N D ENGINEERIXG C H E M I S T R Y
the neighborhood of its equilibrium length, it is found that the equilibrium length is independent of the direction from which equilibrium is approached. The rubber could be initially either in the stretched or retracted state. Such a stressstrain curve, therefore, exhibits reversibility and also shows the rubber to be stiffer a t higher temperature in accord with the Joule heat effect and the second law of thermodynamics. The essential difference between the customary and the equilibrium stress-strain curve is that time or frictional effects enter when the rubber is stretched a t a given velocity. In the equilibrium stress-strain curve these effects are eliminated. The absorption of shocks and vibrations by rubber in a practical sense depends upon the fact that the stress-strain curve shows hysteresis under these conditions. Thus, oscillations in the rubber are damped. Consequently the second law of thermodynamics cannot be applied to such irreversible actions. The potential energy of stretched rubber is the tendency to do work and increases with the stretch. On the other hand, the energy content, as defined by the first law of thermodynamics, decreases with the stretch, for when rubber retracts without doing work the temperature decreases. This is all the more interesting since up t o 600 per cent elongation the equilibrium stress-strain curves are identical for different cures, in contrast with the usual stress-strain curves which show the rubber to be stiffer with higher cures. This is additional evidence that vulcanization does not greatly affect
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the elastic properties. The important change in rubber caused by vulcanization is a greater resistance to the plastic flow or permanent set. Acknowledgment
The writer wishes to take this opportunity to express his appreciation to W. A. Gibbons and M. Mooney for suggestions and criticisms. Literature Cited Bouasse, Ann. fac. sci. Toulousc, 6 , 177 (1904). Gibbons, Unpublished discussion of papers in Rubber Division at Swampscott and Columbus meetings of American Chemical Society. Gough, Mem. Proc. Manchester Lit. Phil. Soc., 1, 288 (1805) ; Nicholson’s J., 13, 305 (1806). Hock, India Rubber J . , 14, 419 (1927). Hyde, Trans. Inst. Rubber Ind., 3, 35 (1927). Joule, Joule’s Scientific Papers, Vol. I, p. 413. Le Blanc and Kroger, Z . Elektrochem., S4, 241 (1928). Lewis and Randall, ‘Thermodynamics and Free Energy of Chemical Substances,” p. 119, McGraw-Hill, 1923. Lundal, Ann. Physik, 66, 741 (1898). Partenheirner, IND.ENG.CHRY.,20, 1245 (1928). Schmulewitsch, Ann. Physik, 144, 280 (1872). Somerville and Cope, Rubber Chem. Tech., 9 , 1 (1929). Stevens, J . Soc. Chem. Ind., S1, 280T (1918). Van Rossem and van der Meyden, Rubber Age, 13, 438 (1928). Whitby. “Plantation Rubber and the Testing of Rubber,” p. 453, Longmans, 1920. Williams, IND.END. CHRM.,21, 872 (1929). Wormeley, I n d i a Rubber J . , 43, 218 (1914).
Vapor Pressures of Saturated Equilibrated Solutions of Lactose, Sucrose, Glucose, and Galactose’ E. 0. Whittier and S. P. Gould BUREAUOF DAIRY
INDUSTRY,
WASHINGTON, D.
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The vapor pressures of saturated equilibrated soluappear to be those of Browne ERTAIN confectioners’ tions of sucrose, glucose, galactose, and lactose have ( 2 ) . Since he used anhydrous products are made only been determined at 25’ C. These values indicate sugars and exposed them to s e a s o n a l l y in many that the hygroscopic tendencies of these sugars differ, 60 per cent and 100 per cent localities on account of the decreasing in the order listed. Calculated values undesirable effects of the high humidity, his results involve have been obtained for the percentage of atmospheric incomplete hydrate formation percentage of atmospheric huhumidity with which each saturated sugar solution midity. Chocolate coatings in a number of cases. Conwould be in vapor pressure equilibrium. It is suggested and paper wrappings are fresequently, they are not dithat lactose might possibly be substituted in part for quently used as a means of rectly comparable with those sucrose or glucose in confectioners’ products in which com b a t i n g t h i s difficulty. of this paper, except in the the hygroscopic tendencies of these sugars are obAnother Dossible method that c a s e of sucrose. He found jectionable. would be more desirable in that sucrose did not take up many cases is the substituwater from an atmosphere of tion of less hygroscopic sugars for the sucrose and glucose 60 per cent humidity, but did from a saturated atmosphere. ordinarily used. Since the tendency of a soluble substance to take up water Lactose is a sugar that could be made available in large from the atmosphere is dependent on the relationship bequantities for such a purpose, if its properties appear suitable. tween the values of the vapor pressure of the saturated soluIts comparatively low degree of sweetness would be a dis- tion of the substance and of the partial pressure of the wate1 advantage in some cases and an advantage in others. Fur- vapor in the atmosphere, vapor-pressure measurements on thermore, a generally desirable therapeutic action is claimed saturated solutions may be used as the basis for comparing for lactose. If the demand for this sugar should become hygroscopic tendencies. It must not be forgotten, however, sufficient to insure continuous large-scale production, its cost that the rate of absorption of water vapor from the atmoscould undoubtedly be decreased considerably by changing or phere involves a number of other factors in addition t o vapor improving the methods of isolation. pressure, as pointed out by Edgar and Swan ( 3 ) . The results of this paper show that lactose is considerably I n spite of the fact that it is not strictly true that the vapor less hygroscopic than either glucose or sucrose. Galactose pressure of a dry soluble solid is that of its saturated solution, is also considered on account of its hydrolytic relationship to it is generally admitted that hygroscopic tendencies of solids lactose. are, for practical purposes, those of their saturated solutions. The only data in the literature on hygroscopicity of sugars The fact that a surface solution approaching saturation is 1 Received October 11, 1929. produced practically as soon as a thin film of water is deposited
C
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
on the surface of the solid justifies the assumption that the vapor pressure of this solution controls the hygroscopic tendency of the substance. Furthermore, water is taken up, even by the most hygroscopic of sugars, a t such a moderate rate that alpha-beta equilibration takes place a t a speed closely approaching that of solution of the sugar. Measurements of vapor pressure on saturated solutions of soluble salts a t various temperatures have been made by Edgar and Swan (5) and by Adams and Merz (1). Hudson ( 4 ) determined the vapor pressure of saturated lactose solutions a t 10-degree intervals from 50' to 100" C. The results herewith reported cannot be compared with those of Hudson on account of the temperature differences. The difficulties in measuring the vapor pressure of viscous cane-sugar solutions have been pointed out by Wood (6). Additional difficulties have appeared in this work on saturated solutions because of the involvement of alpha-beta equilibrium in the case of the aldose sugars and the slowness of attainment of solubility equilibrium in some cases. The attainments of alpha-beta equilibrium and solubility equilibrium are mutually involved in saturated aldose solutions.
Figure 1-Apparatus
for Determining Vapor Pressures of Saturated Solutions
For example, a solution of lactose in equilibrium with solid lactose a t 25' C. contains alpha- and beta-lactose in the equilibrium ratio; the solution is saturated with alpha, but is undersaturated with respect t o beta. If such a solution is partly evaporated, alpha-lactose will commence t o crystallize and this will continue until the solution is no longer supersaturated with respect to alpha-lactose. In the meantime, however, the separation of alpha-lactose upsets the alpha-beta equilibrium and change of beta to alpha begins and continues until the normal ratio is again attained. The formation of alpha-lactose in this process increases the amount of alpha that must crystallize before solubility equilibrium is reached. The interplay of these factors continues until both types of equilibria are simultaneously attained. In the case of solutions of each of three aldose sugars used, preliminary determinations with a polariscope showed that 24 hours were sufficient for attainment of alpha-beta equilibrium a t 26" C. when solubility equilibrium was not involved. The ultimate criterion employed for simultaneous solubility and alpha-beta equilibria in the solutions used in this work was the attainment of vapor-pressure values stable over several days' time.
of the system would not affect the existing pressure measurably. The measuring instrument was a specially constructed McLeod gage with a total bulb and stem capacity of 18.054 CC. The graduated portion of the stem had a capacity of 1.000 cc., was graduated to 0.01 cc., and was read to 0.001 cc. Between the gage and the rest of the system were connected a phosphorus pentoxide tube and a calcium chloride column. As a further precaution against error due to presence of water vapor in the gage, that part of the system was evacuated to a pressure of a few hundredths of a millimeter of mercury each time before use. Sugars of c. P. grade were crystallized according t o accepted procedure, dried in a vacuum oven below 100" C., and made up into saturated solutions as required. The galactose used was tested bacteriologically to prove the absence of lactose and glucose. The isoteniscope bulb was about two-thirds filled with the saturated solution of the sugar, a few crystals of the sugar were added, and the bulb was sealed to the rest of the isoteniscope. The solution was then boiled a t 25' C. until the volume of the liquid was noticeably decreased. The pressure in the system was then equalized, the oil levels being used as index, and the pressure read on the gage. The balance of the oil levels was always checked after reading the gage, while the gas in the gage stem was still under compression. The formula used for computing the vapor pressure was hv/( V - v), in which h is the difference in millimeters in the heights of the mercury columns after compression, V is the volume in cubic centimeters of the gas before compression, and v the volume after compression. The boiling was repeated till checking readings indicated that all air had been removed from the space between the sugar solution and the confining oil. The isoteniscope was then closed off from the rest of the system and allowed to stand overnight in the constant-temperature bath. Readings were then taken a t daily intervals until the measured pressure was constant for several successive days. Finally, the solution was boiled for a few minutes and another reading taken, the purpose of this being to make sure that no air had leaked into the isoteniscope since the previous boiling. A few minutes' boiling disturbs the equilibrium of the solution almost negligibly. The whole procedure was carried out a t least twice for each sugar. Results The values given in Table I are the means of six values for lactose, five each for glucose and sucrose, and three for galactose. I n the third column are given the relative humidities corresponding to each vapor pressure. These were obtained by dividing each vapor pressure by the partial pressure of water vapor in an atmosphere saturated a t 25" C. and multiplying by 100. These figures indicate, for example, that sucrose will absorb water from an atmosphere which is more than 77.4 per cent saturated with water vapor, but that an atmosphere must be more than 93.1 per cent saturated before lactose can absorb water from it. Data on Four Sugars-Temperature 25' HUMIDITY O F AIRIN VAPOR PRESSURE OP EQUILIBRIUM WITH SAID. SOLN. SATD.SOLN. Per cent (of 23.756) Mm. Ng (OD C.) 18.39 19.26 20.47 22.11
Table I-Hygroscopicity SUGAR
Apparatus and Procedure
Determinations were made by the static method of Smith and Menzies (6). A diagram of the apparatus is shown in Figure 1. It was necessary to use bulbs of from 15 to 25 cc. in capacity in order that enough water might be present to displace all the air initially over the solution. An oil prepared for use in a vacuum pump was used as balancing and confining liquid. The isoteniscope was suspended in a water bath maintained a t 25" * 0.02" C. A balloon flask was made a part of the system to insure that slight changes in the volume
VOl. 22, KO.1
Sucrase G1ucose Galactose Lactose
Literature Cited (1) (2) (3) (4) (5) (6)
Adams and Merz, IND. ENQ.CHEM..21, 305 (1929). Browne. Ibid., 16, 712 (1922). Edgar and Swan, J . A m . Chem. Soc., 44, 570 (1922). Hudson, I b i d . , SO, 1767 (1908). Smith and Menzies, Ibid., 32, 1412 (1910). Wood, Trans. Faraday Soc., 11, 29 (1915).
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