THE SOLUBILITY RELATIOSS O F S d P H T H h L E K E *
B Y H. LEE TVARD
I n a very imrortant series of papers Hildebrandl and his collaborators have produced much evidence to show that the deviations from the ideal solubility l a w may be ascribed to differences in the internal pressures of the solvent and solute and t o polarity. Hildebrand and Jenks hare developed a method for the evaluation of solubility data and applied it to the solubilities of naphthalene, iodine and rhombic sulphur. The method consists in plotting the logarithm of the mol fraction of the substance separating as the solid phase against the reciprocal of the absolute temperature. I n case an ideal solution is formed in a given solvent, the resulting curve is a straight line, whose slope (8)is given by the expression S = L, 4.j 8 where L, is the molal heat of fusion of the solid. If the molal heat of fusion varies with the temperature, there is a corresponding deviation from the straight-line relationship, but as a first approximation, it can be neglected. For non-ideal solutions of substances of not very cliff erent pressures or no very pronounced polarities, the curves approach straight lines. Rlortimer2 has applied the above method to the quantitative determination of solubility curves on the assumptions (a) that the logarithm of the mol fraction versus the reciprocal of the absolute temperature curves are straight lines passing through a point on the I T axis corresponding to the absolute melting point of the solid phase and (b) that the deviations of the slopes of such straight lines from the ideal slope, S = L, 4.j 8 are due to internal presSure differences of solvent and solute. From a series of such curves he tletermines the internal pressures of various pure substances relative to naphthalene. From these in turn he calculates the solubility of the various compounds in one another, even applying the method to the case of polar solvents. Since the above conclusions are so important in the theory of solutions and are, in addition, of so much practical value, especially to the organic chemist, it seems highly desirable t o make a systematic investigation of the exact form of the log S versus I T curve for various typical solutes in a number of solvents of differing internal pressures and polarities. I n the present paper an attempt has been made to accomplish this for the solubility of naphthalene. The choice was made because both Hildebrand and Mortimer have used the literature measurements for this substance as an example, and these are rarely complete enough to determine the exact details of the curves. Also, the napththalene cleril-atives form a series of compounds with varying * Contribution from the Department of Chemistry, Kashington 1-niversity. 'Hildebrand: J. .Im. Chem. SOC., 38. 1452 (1916): 39, 2297 (1917): 41, 1067 (1919); 43, joo (1921);Hildebrand and Jenks: 42, 2180 (1920):43, 2 1 7 2 (1921):Taylor and Hildebrand: 45, 682, (192.3); Hildebrand, Hogness and TaSlor: 45, 2828 (1923). See also Hildebrand: Solubility, .Im. Chem. SOC.Monographs (1924). hlortimer: J. Am. Chem. Soc., 44, 1416 (1922); 45, 633 (1923).
SOLVBILITT RELATIOXS O F h-APHTHALEXE
1317
internal pressures and polarities. and of convenient melting points for determination of the complete solubility curves. I t is hoped in later papers t o give the results of such measurements. Solubility measurements for naphthalene in benzene, chlorhenzene and carbon tetrachloride have been macle by Schroderl employing the synthetic method. The procedure n a s t o heat weighed quantities of solvent and solute in a sealed tube, rotated in a n a t e r bath, and observe the temperature a t which the crystals had nearly disappeared. The tube was then sloivly cooled until the crystals were seen to increase both in size and numher. The mean temperature betneen these two values, observed with very s l o rates ~ of heating was taken as the solubility point. The accuracy of the determination was given as 0.5' at the higher naphthalene concentrations and 1.0'a t the lower. Temperatures were not stated closer than 0 . jo. The materials were carefully purified and their melting or boiling points noted. The results obtained agree as closely with the author's observations as could be espected from the accuracy claimed by Schroder. The average deviation in the mol fraction shown hy the two sets of data is 0.0034. When reduced to a percentage basis the average difference is o.;? and the masiniuni less than two percent. Speyers? has measured the qoluhility of naphthalene in methyl, ethyl and propyl alcohols and in chlcroform and toluene. His method is t o saturate the solution at a fixed temperature and to analyze by qpecific gravity measurements. He does not specify the procedure by which this latter determination is carried out, but. in some specific gravity measurements for another purpo.e, states there inight be a n uncertainty of three units in the third decimal place in certain cases though the usual error was in the fourth place. Moreover, no attempt n a s made t o purify the naphthalene and its melting point is not given. The solubilitieq for toluene do not lie on a smooth curve n hen plotted by the method of Hiltlehrand and are very much greater than the corresponding solubilities for benzene as determined by Schroder and the author. Since the internal pressures of toluene and benzene are so nearly alike, approximately the same niclecular solubilities should tie expected. There seems to be little doubt that Speyer's value. for toluene are considerably in error. His determinations for methyl alcohol qhov sonien hat higher qoluhilitieq than those here reported but except for the lon est mol fraction of naphthalene the differenceq are not very great. I?tard3 rei-orts the -olubilities of naphthalene in hexane, carhon disulphide, and carbon tetrachloride He doe. not state the method used or the purlty of his materials. Hiq vxlueq for hexane agree fairly ne11 with those of the author except at the highest and 1on-e.t temyeratureq. The loner point- when plotted hy €€il('ebmnd'~ niethod do not lie on the same curve a. the rest and those at the highest teml-eratures n oult! seem t o show great deviations from the ideal law with large mol fractions of naphthalene, which is L ery doubtful. k C h r m , 11. 449 (1893) (4)14 294 19021.
el,
a
I?taid Bull, 13 9, 82 (1893
The solubility of naphthalene in nitrobenzene has been measured by Kremann,l employing the synthetic method. The recults are in fair agreement with those here reported except for two points where the variation amounts to about two and a half percent. Eaperirrzental. The procedure chosen for the solubility determinations was the well known synthetic method of Alesejew,? which consist5 in rotating a sealed tube containing definite quantities of solvent and solute in a bath and noting the temperature at which all of the solid phase disappears. The apparatus employed was very similar to that described by Schroder, except that an electric motor with a variable speed control was used for stirring in the place of a water motor, and the beaker employed for a bath had a capacity of four liters to secure better tenirerature regulation. The synthetic method offers the advantage that the analysis of the saturated solution is not necesm y ; a procediire which is rather undertain in the case of most organic substances. I n addition. determinations may be readily carried out at temreratures above the boiling yoint of the solvent. The principal difficulty in the method is to be certain that equilibrium i q reached at the tenirerature at Tyhich the last of the solid phase disappears, which temperature will hereafter bc designated the solubility point. I n order t o minimize the uncertainty-. the temperature was always raised until all of the solid phaze nent into colution and the tube I n s then cooled as rapidly a. roesible while rotating or shaking. This procedure produced crystals which TX ere of ?me11 si7e p n d very uniform. The bath was then heated slowly and a few degrees below the Folutiility point, the rate was adjusted so that the temperature did not rise more than a tenth of a tenth of a degree per minute. The temyerature was taken when but a yery few $mall crystals remained. Frequently the attainment of equilibrium was checked by still slower rates of heating and the points obtained were not more than a tenth of a degree lon-er. In order to ascertain the error possible in the failure t o attain equilibrium, a t u b e was selected at random which had previously shonm a solubility point of 28.7'. The solvent was benzene. This sample was heated until all the naphthalene went into solution and then rapidly cooled to secure small crystals of uniform size. It was then heated to 28.8' a t the usual rate, at which point no perceptible naphthalene was left undissolved. On very s l o ~ cooling crystals suddenly appeared a t 2 ; .3 '. The amount coming out at that temperature TTas considerable and the individual crystals quite large. The same procedure was repeated by heating t o 28.7' and this time the crystals appeared at 27.6' the phenomena being essentially as before. On heating to 28.6', on the contrary, some crystals still remained though the amount m s very small. On s!owly cooling to 28.;' a perceptible increase in the number of crystals was noticed and at 28.4' this was absolutely unmistakable. Since in the first two cases the supercooling phenomenon was so evident, it is believed that there was not enough naphthalene remaining undisolved at 28.7' t o act Iiremann. Monntsheft. 2 5 . 1246 r i g o l ) . A%lexejeiv.\Tied. .Inn. 2 8 , 3( 5 (18b6).
SOLUBILITY RELATIOY-S O F N - U H T H A L E S E
1319
as a nucleus for crystallization. The amount necessary for this must, of course, be rather small. The solubility point. therefore, seems t o be lower than 28.;". As the growth of crystals was very marked a t 28.4' and perceptible a t 28.;' the true so1iibilit)- point is probably 25.6' or I I O degree lower than that previously determined. In order t o fix the mahimum error nhich might be made with very rapid heating rates, the tube containing a large amount of crystals n-as introduced into the bath at 2 j' and the heating carried out with the full flame of a burner. All crystals had disappeared at 29.1' in less than tn-o minutes after introducing the tube. I t would seem t o he very difficult to make an error of half a degree when the solid phase is produced by sudden cooling. On the hasis of the above and other determinations it i, believed that the error due to equilihrium not being estahlished at the time the temperature is read, should never exceed 0.2' except 1:ossiblp in the cace of very slightly ~olublesubstance>. This error coultl lie very materially reduced hy very slow methods of heating employing thermostatic control. The ultimate accuracj of the method should be limited only hy the amount of the c.olid phase visihle t o the eye. or rerhaps, hy the increaced soluhility of fine crystals over the coarCe ones usually prewnt in the analytical procedure. Schroder ebtimates thtit half of r, milligram of a n organic substance should be visihle and thif v e m s a resonable figure. The increase of d u h i l i t y for wiall crystals of organic substances does not seem t o have been investigated but since their numher is coniparativel!- small in this procedure, it TT oultl prohzlily be inconsic'erable. I t is the ruthor's orinion that the synthetic method can be ma& more accurate than the analytical for all organic suhqtnnces except for the few cases in TI hich an accurate analytical method is known. The thermometers employed n ere compared with primary standards n hich had been certified Ijj- the Bureau of Standards and which were met1 for standardization purpoqes only. The error in checking should not exceed a tenth of a degree and the inaccuracy of the solubility point should, therefore, not ke greater than three tenths of a degree. The tubes uced in the determination? nere of very thin walls. They mere first filled n i t h the Lolid phase, then d r a n n down t o ahout five nim. a t the center of the constriction and the required quantity of liquid solvent introduced. dfter sepling, the tubes nere again weighed in order t o ascertain the qmnll loss of solvent n hich usually took place in the sealing process. Prepnrntzori o f M a f e r i n l s . Saphthalene n-a< prepared by repeated rec r j stallization from methyl alcohol of ('. P. naphthalene. The final material shonecl a freezing point of 80.o~'-So.1' n-hen measured on a large sample in a test tube. Some residues from the above process gave a freezing point but 0.05' Ion-er. but nere not uced. The freezing point of the purest material
agreed very ne11 n i t h the point of 8c.1' obtained a t the Bureau of Standards.
H. LEE WA4RD
1320
The benzene used in the preliminary curve was prepared by shaking technical material repeatedly with concentrated sulphuric acid to remove thiophene, and subsequent fractional distillation. The corrected boiling point was 8 0 . 0 ° - 8 0 . ~ 0and the freezing point j.4'. The material employed for the final curve mas prepared by the method of Richards and Shipley' and the recrystallization as carried on until the freezing point showed no further rise as measured with a Beckmann thermometer. The material was kept over sodium wire and was distilled just before use. The melting point was 5.5". Toluene of the "Analyzed" grade rf-asbite redistilled through a fractionating column and showed a corrected boiling point of I 10.8' to I 10.9'. Commercial, water-white, chlorbenzene was three times fractionated through a Hempel column. In the last distillation nearly all the material distilled over a range of 0.03'. The middle portion showing a corrected boiling point of 132.0' k 0 . r was used in the determinations.
FIG.I .
Solubility Relationships of Sapthalene.
Absolute, commercial methyl alcohol, with an odor very similar to that of pure ethyl alcohol, was distilled through a fractionating column. Almost all the material distilled within 0.1'. The main fraction was redistilled over sodium and the middle fraction of the distillate used for the measurements. Boiling point 64.7' corrected. Sormal butyl alcohol from the fermentation process was four times distilled through a Hempel column, employing metallic sodium as a dehydrating agent for the third distillation. Range during the last distillation 117.4' to 1 1 7.95' corrected. The fraction taken boiled from 117.6' t o 117.8'. Carbon tetrachloride was twice distilled and showed a corrected boiling point of 76.70' to 76.jj'. Technical aniline was twice distilled, the product boiled a t 184.0'-184.1' under normal pressure, and showed but a slight color. Richards and Shiplev: J. Am. Ctiein. soc.,36,1825 (1914).
SOLCBILITT RELATIONS O F SAPHTHALESE
1321
The nitrobenzene was once distilled and showed a range of not more than Boiling point z IO.^', corrected. Acetic acid was prepared by the recrystallization of the C . P. glacial acid and showed a melting point of 16.5'. The acetone was purified by means of calcium chloride and sodium amalgam, a s described by Frankforter and C'ohen,' and on distillation showed a corrected boiling point of j6.1'.
0.1'.
FIG.2 .
Soluliilit>-of Sapthalene in -Acetic -1cid.
The hexane first used n-as prepared by the distillation of a pre-war sample, nhich had been obtained from petroleum. The fraction employed had a hollinp point of 68.0'-68.9' corrected. and probably contained small amounts of isohexanes and polymethylenes. h sample of y-nthetic, normal hexane, furnished by the Eastman Kodnk ('cnirnny, was distilled and the hulk of the ninterial came over at 68.8'-69.0', corrected. This fraction showed essentially the same solubility for naphthalene as the former one.
Erperimentcrl Resltlts. The experimentally determined qoluhilities aprear in Tables I t o S I . From these tlatz the mol fraction of naphthalene mas calculated and plotted against the reciprocal of the alxolute solubility temrernture. The resulting curves. except thoFe for toluene, acetic acid antl nitro benzene, appear in Fig. I . The curves for toluene and nitro-benzene are almost identical with that for benzene antl the curve for acetic acid appears in Fig. 2 . The experimental points lie much too nearly on the curves t o be represented on the
