E. MATTHEWS 43

SOLUTIONS, PART 11. THE FREEZING-POINT

TION OF BINARY MIXTURES OF VOLATILE LIQUIDS.

DIAGRAMS AND LATENT HEATS OF EVAPORA-

BY WILFRED FISHER WYATT.

Received I 2 fh November, I 9 2 8.

It has been pointed out in a previous communication that when the molal latent heats of evaporation of' binary liquid mixtures are plotted against the molecular concentration of one of the components, irregularities are manifested in certain cases. Examples of such irregularities are afforded by the systems acetone - chloroform, ethyl alcohol - benzene, and ethyl alcohol - carbon tetrachloride. In these cases it was shown that at the freezing-points of the mixtures the solids separating did not consist merely of the pure components. In the case of the system benzene - chloroform which exhibits a regular molal latent heat curve, however, the freezing-point diagram is of the simple eutectic type.

The investigation has now been extended to the systems methyl alcohol - chloroform, ethyl ether - carbon tetrachloride, ethyl acetate - carbon tetrachloride and benzene - ethyl bromide, the latent heats of which were investigated by Tyrer.2 The curves showing the internal molal latent heats of the mixtures (calculated from the latent heats per gram by the method given in Part I.) plotted against the molecular concentration of the second-named component for the above systems are given, together with the freezing-point diagrams, in Figs. I to 4 respectively. The full lines in

Wyatt, Trans. Faraday SOL, 24, 429 (192s). Tyrer, J . Chem. SOC., 99, 1633, 1911 ; 101, SI, and 1104, 1912.

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44 SOLUTIONS, PART LI

the latent heat curves show the variation of this property when referred to the concentration in the liquid phase, and the broken lines to the vapour phase.

Experimental. Materials.-The liquids

used in the freezing-point determinations were purified by standard methods, phos- phorus pentoxide being used as final drying agent whenever possible ; the methyl alcohol was dried by means of metallic cal- cium.

Method-The appara- tus used for the determina- tion of the freezing-points of the mixtures was essenti- ally the same as previously described. For the present experiments, however, the cooling bath was not em- doved because of the risk

, . . . . . . / - I00

O C .

FIG. 1.-Methyl alcohol-chloroform.

liquid from it, when seed- ing mixtures by means of the chilled capillary tube. I t was further observed that the persistent seeding of a mixture as the freez- ing-point was approached contributed much more to the accuracy of the determination than the fine temperature control afforded by the cooling bath.

Temperatures be- tween oo and - 80" were recorded by means of a standard alcohol ther- mometer graduated in tenths of a degree, and below - 80" by a pentane thermometer graduated in fifths of a degree be- tween - 60" and - 190". The correction for the emergent column of each thermometer was found for the actual conditions under which it was used

L d

of contamination by the

U

5500

- 30

- 50

- 70

- 90

- I10

"C.

Freezing-point diagram.

FIG. 2.-Ethyl ether-carbon tetrachloride.

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45 W. F. WYATT

The following thermometric points (with the exception of the transition point of carbon tetra~hloride)~ taken from International Critical Tables, Vol. I., p. 54, were em- ployed :-transition point of carbon tetrachloride, - 48.5' ; the freezing-

points of :-carbon tetra- chloride, - 23-0" ; chloro- form, - 63'5" ; ethyl ace- tate, - 83.6" ; acetone, - 94.3" ; methyl alcohol, - 97%" ; carbon disul- phide, - I I I -6" ; ethyl ether, - I 16-3" ; and ethyl bromide, - I 19'.

Results. In the following re-

sults all concentrations, except where otherwise stated, refer to the mole- cular percentage of the second-named component in the liquid phase.

Freezing.point diagram.

FIG. 3.-Ethyl acetate-carbon tetrachloride,

I . Methyj Alcohol - ChZovafoorm Mixtures.-An examination of Fig. I shows that two minima and two maxima occur in the latent heat curve, the

Latent heat curve.

; 6000 - U

~ ~ _ _ _

20 40 60 80-1 Molecular % ethyI bromide.

FIG. 4.-Benzene-ethyl bromide.

former at 6-5 and about 61 per cent., and the latter at 21.5 and 85 per cent. of chloroform. For con- centrations up to 62 per cent. both positive and negative deviations from the additive law occur, but for concentrations greater than this only positive deviations are exhibited. The curves referring to liquid and vapour concen- trations respectively touch one another, but do not intersect, at the minimum boiling mixture, contain- ing 62 per cent. of chloro- form.

The freezing-point dia- gram for this system ex- hibits an eutectic arrest at - I I I *So, composition 12-4 per cent., and a transition point at - 77-s0,

composition 44.8 per cent. I t is seen that the depression- of the freezing- 3 McCullough and Phipps, 'jf. Arner. Ghem. SOC., 50, 2213, (1928).

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46 SOLUTIONS, PART I1

point of chloroform by the addition of large quantities of methyl alcohol is small, and analyses of the solids separating from mixtures containing more than 44.8 per cent. of chloroform indicated that they consist of d i d solutions which are granular in appearance. The solids separating from mixtures containing less than 44.8 per cent., however, are fine white needles. Evidence that these needles consist of a compound formed between one molecule of methyl alcohol and one molecule of chloroform was obtained by cooling the 40 and 50 per cent. mixtures to temperatures well below their freezing-points; the 40 per cent. mixture when cooled to 6" below its freezing-point was still not completely solid, but the 50 per cent. mixture became quite solid at the transition point, and the granular crystals which first separated slowly changed to the needlelike crystals of the compound.

During the experiments with mixtures of these liquids it was observed that although pure methyl alcohol showed no noticeable increase of viscosity at its freezing-point, yet mixtures containing up to 30 per cent. of chloro- form showed a marked increase of viscosity as the freezing-points were approached.

2. Ether - Carbon TefrachZoride Mixfures.-The latent heat curve for this system shows no irregularities and exhibits negative deviations from the additive law over the whole range of concentrations when referred to CCI, in the liquid phase, but positive deviations when referred to CCl, in the vapour phase.

The important features of the freezing-point diagram are as follows :- ( I ) Eutectic point between ether and the compound z(C2H&0,CC14 at - 118*5", composition 5.4 per cent. (2) Transition point due to a change from the compound (C,H5)20,CC1, to the compound 2 (C2H5)20,CC14 at - 1 0 7 O , composition 18 per cent. (3) Melting-point of the compound (C2H5)20,CC14 at - 86.5". (4) Transition point at - 48-2", composition 89 per cent. (5) Metastable eutectic point between ether and the com- pound (C2H5),0,CC1, at - I 2 2 -5", composition I 2 -5 per cent.

As in the case of the system methyl alcohol - chloroform, the composi- tions of the compounds were determined by finding the mixtures which con- tained the least proportion of carbon tetrachloride and completely solidified at the transition points. It was observed that as the 25 *I per cent. mixture was cooled below its freezing-point much solid, consisting of flaky crystals, separated; at the transition point this flaky solid changed into a denser crystalline form, the "thinning " of the mixture due to the change being very marked. The mixture was then cooled to 6" below this temperature and liquid was still present. The 33.4 per cent. mixture, on the other hand, completely solidified at the transition arrest, and the flaky solid which first separated changed into the denser form. The difference between these t w o crystalline forms was well shown during an experiment with a 20.7 per cent. mixture, by cooling it well below its stable freezing-point without seeding. At - 106.5" a fine white solid separated, and on allowing the temperature to rise to - 105%" all of this solid melted leaving no trace of turbidity. Then from the liquid there grew white flaky balls which left the mixture between them perfectly clear, and the temperature rose to about - 102". I t was concluded that the fine white solid and the flaky balls consisted of the compounds 2 (C2H5)?0,CC14 and (C2H5),0,CC14 respec- tively. The 50 per cent. mixture solidified at constant temperature and the solid grew as a hard, transparent mass from the sides of the tube. In order to determine whether or not an eutectic point occurs between the compound (C2H5)20,CC14 and pure CCl, a few drops of the latter liquid were added

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W. F. WYATT 47

to the above solution and the freezing-point again found. It was observed that the addition of 0-3 gram of CC1, to 2 0 grams of the mixture raised the freezing-point, indicating that no eutectic arrest occurs.

(3) EthyZ Acetate - Carbon TetrachZoride Mixtures.-The latent heat curve for this system is almost a straight line coinciding with that required by the additive law.

The important features of the freezing-point diagram are as follows : (I) Eutectic arrest between ethyl acetate and a compound of the two com- ponents at -goo, composition 16.5 per cent. (2) A flat portion of the diagram between 25 and 42 per cent., due to the formation of a compound. (3) Either a transition point or a eutectic point at about - 8 7 O , composition 42 per cent., depending upon whether the compound has the composition CH,COOC2H,,CC1, or 2 (CH3COOC2H,),CC1,. (4) Transition point at - 47-83 composition 86.1 per cent. Difficulty was experienced in attempt- ing to decide whether or not a maximum occurs in the flat portion of the curve at 33.3 per cent. A mixture of this composition appeared to solidify at constant temperature, but since, for a small temperature lowering very much solid separated from all mixtures in this region, and rendered adequate stirring very difficult, the evidence was not regarded as conclusive. The above portion of the curve was therefore repeated on three different occa- sions, using freshly prepared samples of the pure liquids, and on each occasion a maximum was indicated. Additions of ethyl acetate and of carbon tetrachloride to a 33-3 per cent. mixture also indicated a maximum, but owing to the very flat nature of the curve the evidence, although point- ing strongly to the existence of the compound 2 (CH,COOC2H,),CCl,, cannot be regarded as absolutely conclusive.

(4) Benzene - EthyZ Bromide Mixfures.--The latent heat curve for this system shows no irregularities, but it is seen that the curve relating to liquid concentrations shows negative deviations from the additive law almost throughout the entire range, and the curve relating to vapour concentra- tions shows mostly positive deviations.

The freezing-point diagram is of the simple type, exhibiting an eutectic point at - 1 2 0 ' 5 O , composition 95-5 per cent. of ethyl bromide.

Discussion.

In agreement with the results obtained in Part I. it is found that the system showing irregularities in the molal latent heat curve has a complex freezing-point diagram (Fig. I). On the other hand a regular molal latent heat curve does not necessarily indicate that compounds between the two components are impossible (Figs. 2 and 3). In the systems ether - carbon tetrachloride and ethyl acetate - carbon tetrachloride compounds are formed between the two components at low temperatures, but the fact that the latent heat curve at the boiling-point shows negative deviations in the first case, and follows the additive law in the second, suggests that the compounds are not stable at elevated temperatures.

The positive deviations of the latent heat curve for the system methyl alcohol - chloroform indicate that relatively large forces of attraction exist between the two species of molecules at the boiling-points of the mixtures, and especially is this the case in mixtures containing excess of chloroform. The decrease in the value of the latent heat at a rate greater than that re- quired by the additive law for mixtures of low chloroform concentration may be due to the dissociation of methyl alcohol complexes on the addition

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48 SOLUTIONS, PART I11

of chloroform, but further discussion as to the meaning of the maxima and minima is not justified on the evidence at present available.

The freezing-point diagram of the system methyl alcohol - chloroform bears a close resemblance to that of the system ethyl alcohol - carbon tetra- chloride (Part I.), for in both cases there is only a small depression of the freezing-point of the halogen compound on the addition of large quantities of the alcohol. In both systems also, for mixtures rich in chloroform or carbon tetrachloride, alcohol separates along with the halogen compound in a series of solid solutions.

The increase of viscosity on mixing two liquids, as observed in the case of methyl alcohol and chloroform, has been interpreted by previous workers as indicating compound formation between the components, and this view is supported by the results obtained in this work. A marked increase of viscosity at low temperatures was also noted for the system acetone - chloro- form during the determination of the freezing-points of these mixtures, but was not recorded in Part I.

The transition point at about - 48" in the carbon tetrachloride systems will be discussed in Part 111. of this series.

Summary. The investigation of the freezing-point diagrams and latent heats of

binary mixtures of volatile liquids has been extended to the systems CH,OH-CHCl,, (C2H5)20-CC14, CH,COOC2H5-CC1, and C6H6- C2H5Br. The latent heat curve for CH,OH-CHCl, mixtures shows two minimum and two maximum values, but for the other systems regular latent heat curves are observed. The large positive deviations from the additive law for the system CH,OH-CHCl, indicate that relatively strong forces of attraction exist between the two species of molecules at elevated temperatures.

Cryoscopic evidence for the existence of the following compounds has been obtained : CH,OH,CHCl, ; (C2H5)20,CC14 ; 2(C2H5)20,CC14, and

The form of the latent heat curves for mixtures containing CCl,, indi- cates that the above compounds of this substance are not stable at elevated temperatures.

Bramley, 7. Chem. SOL., 10g, 10, 434 (1916); Kremann, Ahren's Vortrdlge, 27 94 (1924) ; Kremann, Die Eigenschaften der bindlren Flussigkeitsgemische, p. 222.

Z(CH,COOC~H~) ,CC~~

Th Universii'y, ShefieZd.

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