Fig. 58Fig.58.
Fig. 57Fig.57.
Changes in Mixed Crystals with the Temperature.—In the case of the different types of systems represented in Fig. 49, a homogeneous liquid solution of the two components will exist at temperatures above the freezing-point curve, a homogeneous mixed crystal at temperatures below the melting-point curve, while at any point between the freezing-point and melting-pointcurves the mixture will separate into a solid phase and a liquid phase. In the case, however, of the two types shown in Fig. 54 the relationships are somewhat more complicated. As before, the area above the freezing-point curve gives the conditions under which homogeneous liquid solutions can exist; but below the melting-point curve two different mixed crystals can coexist. This will be best understood from Figs. 57 and 58. D and E represent, as we have seen, the composition of two mixed crystals which are in equilibrium with the liquid solution at the temperature of the point C. These two mixed crystals represent, in the one case, a saturated solution of B in A (point D), and the other a saturated solution of A in B (point E). Just as we saw that the mutual solubility of two liquids varied with the temperature, so also in the case of two solids; as the temperature alters, the solubility of the two solid components in one another will change. This alteration is indicated diagrammatically in Figs. 57 and 58 by the dotted curve similar to the solubility curves for two mutually soluble liquids (p.101).
Suppose, now, that a mixed crystal of the compositionxis cooled down, it will remain unchanged until, when the temperature has fallen tot′, the homogeneous mixed crystal breaks up into a conglomerate of two mixed crystals the composition ofwhich is represented byx′andx″respectively. From this, then, it can be seen that in the case of substances which form two solid solutions, the mixed crystals which are desposited from the liquid fused mass need not remain unchanged in the solid state, but may at some lower temperature lose their homogeneity. This fact is of considerable importance for the formation of alloys.[277]
A good example of this will soon be met with in the case of the iron and carbon alloys. The alloys of copper and tin also furnish examples of the great changes which may take place in the alloy between the temperature at which it separates out from the fused mass and the ordinary temperature. Thus, for example, one of the alloys of copper and tin which separates out from the liquid as a solid solution breaks up, on cooling, into the compound Cu3Sn and liquid:[278]a striking example of a solid substance partially liquefying on being cooled.
EQUILIBRIUM BETWEEN DYNAMIC ISOMERIDES
It has long been known that certain substances,e.g.acetoacetic ester, are capable when in solution or in the fused state, of reacting as if they possessed two different constitutions; and in order to explain this behaviour the view was advanced (by Laar) that in such cases a hydrogen atom oscillated between two positions in the molecule, being at one time attached to oxygen, at another time to carbon, as represented by the formula—
acetoacetic ester
When the hydrogen is in one position, the substance will act as an hydroxy-compound; with hydrogen in the other position, as a ketone. Substances possessing this double function are calledtautomeric.
Doubt, however, arose as to the validity of the above explanation, and this doubt was confirmed by the isolation of the two isomerides in the solid state, and also by the fact that the velocity of change of the one isomeride into the other could in some cases be quantitatively measured. These and other observations then led to the view, in harmony with the laws of chemical dynamics, that tautomeric substances in the dissolved or fused state represent amixtureof two isomeric forms, and that equilibrium is established not byintra- but byinter-molecular change, as expressed by the equation—
CH3.CO.CH2.CO2C2H5reversible arrowCH3.C(OH):CH.CO2C2H5
In the solid state, the one or other of the isomerides represents the stable form; but in the liquid state (solution or fusion) the stable condition is an equilibrium between the two forms.
A similar behaviour is also found in the case of other isomeric substances where the isomerism is due to difference of structure,i.e.structure isomerism (e.g.in the case of the oximesbeta formandalpha form, or to difference in configuration,i.e.stereoisomerism (e.g.optically active substances), or to polymerism (e.g.acetaldehyde and paraldehyde). In all such cases, although the different solid forms correspond to a single definite constitution, in the liquid state a condition of equilibrium between the two modifications is established. As a general name for these different classes of substances, the term "dynamic isomerides" has been introduced; and the different kinds of isomerism are classed together under the title "dynamic isomerism."[279]
By reason of the importance of these phenomena in the study more especially of Organic Chemistry, a brief account of the equilibrium relations exhibited by systems composed of dynamic isomerides may be given here.[280]
In studying the fusion and solidification of those substances which exhibit the relationships of dynamic isomerism, the phenomena observed will vary somewhat according as the reversible transformation of the one form into the other takes place with measurable velocity at temperatures in the neighbourhood of the melting points, or only at some higher temperature. If the transformation is very rapid, the system will behave like a one-component system, but if the isomeric change is comparatively slow, the behaviour will be that of a two-component system.
Temperature-Concentration Diagram.—The relationships which are met with here will be most readily understood withthe help of Fig. 59. Suppose, in the first instance, that isomeric transformation does not take place at the temperature of the melting point, then the freezing point curve will have the simple form ACB; the formation of compounds being for the present excluded. This is the simplest type of curve, and gives the composition of the solutions in equilibrium with the one modification (αmodification) at different temperatures (curve AC); and of the solutions in equilibrium with the other modification (βmodification) at different temperatures (curve BC). C is the eutectic point at which the two solid isomerides can exist side by side in contact with the solution.
Fig. 59Fig. 59.
Now, suppose that isomeric transformation takes place with measurable velocity. If the pureα-modification is heated to a temperaturet′above its melting point, and the liquid maintained at that temperature until equilibrium has been established, a certain amount of theβ-form will be present in the liquid, the composition of which will be represented by the pointx′. The same condition of equilibrium will also be reached by starting with pureβ. Similarly, if the temperature of the liquid is maintained at the temperaturet″, equilibrium will be reached, we shall suppose, when the solution has the compositionx″. The curve DE, therefore, which passes through all the different values ofxcorresponding to different values oft, will represent the change of equilibrium with the temperature. It will slope to the right (as in the figure) if the transformation ofαintoβis accompanied by absorption of heat; to the left if the transformation is accompanied by evolution of heat, in accordance with van't Hoff's Law of movable equilibrium. If transformation occurs without heat effect, the equilibrium will be independent of thetemperature, and the equilibrium curve DE will therefore be perpendicular and parallel to the temperature axis.
We must now find the meaning of the point D. Suppose the pureα- or pureβ-form heated to the temperaturet′, and the temperature maintained constant until the liquid has the compositionx′corresponding to the equilibrium at that temperature. If the temperature is now allowed to fall sufficiently slowly so that the condition of equilibrium is continually readjusted as the temperature changes, the composition of the solution will gradually alter as represented by the curvex′D. Since D is on the freezing point curve of pureα, this form will be deposited on cooling; and since D is also on the equilibrium curve of the liquid, D is the only point at which solid can exist in stable equilibrium with the liquid phase. (The vapour phase may be omitted from consideration, as we shall suppose the experiments carried out in open vessels.) All systems consisting of the two hylotropic[281]isomeric substancesαandβwill, therefore, ultimately freeze at the point D, which is called the "natural" freezing point[282]of the system; provided, of course, that sufficient time is allowed for equilibrium to be established. From this it is apparent thatthe stable modification at temperatures in the neighbourhood of the melting point is that which is in equilibrium with the liquid phase at the natural freezing point.
From what has been said, it will be easy to predict what will be the behaviour of the system under different conditions. If pureαis heated, a temperature will be reached at which it will melt, but this melting point will be sharp only if the velocity of isomeric transformation is comparatively slow;i.e.slow in comparison with the determination of the melting point. If the substance be maintained in the fused condition for some time, a certain amount of theβmodification will be formed, and on lowering the temperature the pureαform will be deposited, not at the temperature of the melting point, but at some lower temperature depending on the concentration of theβmodification in the liquid phase. If isomeric transformationtakes place slowly in comparison with the rate at which deposition of the solid occurs, the liquid will become increasingly rich in theβmodification, and the freezing point will, therefore, sink continuously. At the eutectic point, however, theβmodification will also be deposited, and the temperature will remain constant until all has become solid. If, on the other hand, the velocity of transformation is sufficiently rapid, then as quickly as theαmodification is deposited, the equilibrium between the two isomeric forms in the liquid phase will continuously readjust itself, and the end-point of solidification will be the natural freezing point.
Similarly, starting with the pureβmodification, the freezing point after fusion will gradually fall owing to the formation of theαmodification; and the composition of the liquid phase will pass along the curve BC. If, now, the rate of cooling is not too great, or if the velocity of isomeric transformation is sufficiently rapid, complete solidification will not occur at the eutectic point; for at this temperature solid and liquid are not in stable equilibrium with one another. On the contrary, a further quantity of theβmodification will undergo isomeric change, the liquid phase will become richer in theαform, and the freezing point willrise; the solid phase in contact with the liquid being now theαmodification. The freezing point will continue to rise until the point D is reached, at which complete solidification will take place without further change of temperature.
The diagram also allows us to predict what will be the result of rapidly cooling a fused mixture of the two isomerides. Suppose that either theαor theβmodification has been maintained in the fused state at the temperaturet′sufficiently long for equilibrium to be established. The composition of the liquid phase will be represented byx′. If the liquid is nowrapidlycooled, the composition will remain unchanged as represented by the dotted linex′G. At the temperature of the point G solidαmodification will be deposited. If the cooling is not carried below the point G, so as to cause complete solidification, the freezing point will be found to rise with time, owing to the conversion of some of theβform into theαformin the liquid phase; and this will continue until the composition of the liquid has reached the point D. From what has just been said, it can also be seen that if the freezing point curves can be obtained by actual determination of the freezing points of different synthetic mixtures of the two isomerides, it will be possible to determine the condition of equilibrium in the fused state at any given temperature without having recourse to analysis. All that is necessary is to rapidly cool the fused mass, after equilibrium has been established, and find the freezing point at which solid is deposited; that is, find the point at which the line of constant temperature cuts the freezing point curve. The composition corresponding to this temperature gives the composition of the equilibrium mixture at the given temperature.
It will be evident, from what has gone before, that the degree of completeness with which the different curves can be realised will depend on the velocity with which isomeric change takes place, and on the rapidity with which the determinations of the freezing point can be carried out. As the two extremes we have, on the one hand, practically instantaneous transformation, and on the other, practically infinite slowness of transformation. In the former case, only one melting and freezing point will be found, viz. the natural freezing point; in the latter case, the two isomerides will behave as two perfectly independent components, and the equilibrium curve DE will not be realised.
The diagram which is obtained when isomeric transformation does not occur within measurable time at the temperature of the melting point is somewhat different from that already given in Fig. 59. In this case, the two freezing point curves AC and BC (Fig. 60) can be readily realized, as no isomeric change occurs in the liquid phase. Suppose, however, that at a higher temperature,t′, reversible isomeric transformation can take place, the composition of the liquid phase will alter until at the pointx′a condition of equilibrium is reached; and the composition of the liquid at higher temperatures will be represented by the curvex′F. Below the temperaturet′the position of the equilibrium curve is hypothetical; but as the temperaturefalls the velocity of transformation diminishes, and at last becomespracticallyzero. The equilibrium curve can therefore be regarded as dividing into two branchesx′G andx′H. At temperatures between G andt′theαmodification can undergo isomeric change leading to a point on the curve Gx′; and theβmodification can undergo change leading to a point on the curve Hx′. The same condition of equilibrium is therefore not reached from each side, and we are therefore dealing not with true but with false equilibrium (p.5). Below the temperatures G and H, isomeric transformation does not occur in measurable time. We shall not, however, enter into a detailed discussion of the equilibria in such systems, more especially as they are not systems in true equilibrium, and as the temperature at which true equilibrium can be established with appreciable velocity alters under the influence of catalytic agents.[283]Examples of such systems will no doubt be found in the case of optically active substances, where both isomerides are apparently quite stable at the melting point. In the case of such substances, also, the action of catalytic agents in producing isomeric transformation (racemisation) is well known.
Fig. 60Fig.60.
Transformation of the Unstable into the Stable Form.—As has already been stated, the stable modification in the neighbourhood of the melting point is that one which is in equilibrium with the liquid phase at the natural freezing point. In the case of polymorphic substances, we have seen (p.39) that that form which is stable in the neighbourhood of the melting point melts at the higher temperature. That was aconsequence of the fact that the two polymorphic forms on melting gave identical liquid phases. In the present case, however, the above rule does not apply, for the simple reason that the liquid phase obtained by the fusion of the one modification is not identical with that obtained by the fusion of the other. In the case of isomeric substances, therefore, the form of lower melting pointmaybe the more stable; and where this behaviour is found it is a sign that the two forms are isomeric (or polymeric) and not polymorphic.[284]An example of this is found in the case of the isomeric benzaldoximes (p.203).
Since in Fig. 59 theαmodification has been represented as the stable form, the transformation of theβinto theαform will be possible at all temperatures down to the transition point. At temperatures below the eutectic point, transformation will occur without formation of a liquid phase; but at temperatures above the eutectic point liquefaction can take place. This will be more readily understood by drawing a line of constant temperature, HK, at some point between C and B. Then if theβmodification is maintained for a sufficiently long time at that temperature, a certain amount of theαmodification will be formed; and when the composition of the mixture has reached the point H, fusion will occur. If the temperature is maintained constant, isomeric transformation will continue to take place in the liquid phase until the equilibrium point for that temperature is reached. If this temperature is higher than the natural melting point, the mixture will remain liquid all the time; but if it is below the natural melting point, then theαmodification will be deposited when the system reaches the condition represented by the point on the curve AC corresponding to the particular temperature. As isomeric transformation continues, the freezing point of the system will rise until it reaches the natural freezing point D. Similarly, if theαmodification is maintained at a temperature above that of the point D, liquefaction will ultimately occur, and the system will again reach the final state represented by D.[285]
Examples.—Benzaldoximes.The relationships which have just been discussed from the theoretical point of view will be rendered clearer by a brief description of cases which have been experimentally investigated. The first we shall consider is that of the two isomeric benzaldoximes:[286]—
Fig. 61 gives a graphic representation of the results obtained.
The melting point of theαmodification is 34-35°; the melting point of the unstableβ-modification being 130°. The freezing curves AC and BC were obtained by determining the freezing points of different mixtures of known composition, and the numbers so obtained are given in the following table.
Fig. 61Fig.61.
The eutectic point C was found to lie at 25-26°, and the natural freezing point D was found to be 27.7°. The equilibrium curve DE was determined by heating the liquid mixtures at different temperatures until equilibrium was attained, and then rapidly cooling the liquid. In all cases the freezing point was practically that of the point D. From this it is seen that the equilibrium curve must be a straight line parallel to the temperature axis; and, therefore, isomeric transformation in the case of the two benzaldoximes is not accompanied by any heat effect (p.197). This behaviour has also been found in the case of acetaldoxime.[287]
The isomeric benzaldoximes are also of interest from the fact that the stable modification has thelowermelting point (v.p. 202).
Acetaldehyde and Paraldehyde.—As a second example of the equilibria between two isomerides, we shall take the two isomeric (polymeric) forms of acetaldehyde, which have recently been exhaustively studied.[288]
In the case of these two substances the reaction
3CH3.CHOreversible arrow(CH3.CHO)3
takes place at the ordinary temperature with very great slowness. For this reason it is possible to determine the freezing point curves of acetaldehyde and paraldehyde. The three chief points on these curves, represented graphically in Fig. 62, are:—
Fig. 62Fig.62.
In order to determine the position of the natural melting point, it was necessary, on account of the slowness of transformation, to employ a catalytic agent in order to increase the velocity with which the equilibrium was established. A drop of concentrated sulphuric acid served the purpose. In presence of a trace of this substance, isomeric transformation very speedily occurs, and leads to the condition of equilibrium. Starting in the one case with fused paraldehyde, and in the other case with acetaldehyde, the same freezing point, viz. 6.75°, was obtained, the solid phase being paraldehyde. This temperature, 6.75°, is therefore the natural freezing point, and paraldehyde, the solid in equilibrium with the liquid phase at this point, is the stable form.
With regard to the change of equilibrium with the temperature, it was found that whereas the liquid phase contained 11.7 molecules per cent. of acetaldehyde at the natural freezing point, the liquid at the temperature of 41.6° contains 46.6 molecules per cent. of acetaldehyde. As the temperaturerises, therefore, there is increased formation of acetaldehyde, or a decreasing amount of polymerisation. This is in harmony with the fact that the polymerisation of acetaldehyde is accompanied by evolution of heat.
While speaking of these isomerides, it may be mentioned that at the temperature 41.6° the equilibrium mixture has a vapour pressure equal to the atmospheric pressure. At this temperature, therefore, the equilibrium mixture (obtained quickly with the help of a trace of sulphuric acid) boils.[289]
SUMMARY.—APPLICATION OF THE PHASE RULE TO THE STUDY OF SYSTEMS OF TWO COMPONENTS
In this concluding chapter on two-component systems, it is proposed to indicate briefly how the Phase Rule has been applied to the elucidation of a number of problems connected with the equilibria between two components, and how it has been employed for the interpretation of the data obtained by experiment. It is hoped that the practical value of the Phase Rule may thereby become more apparent, and its application to other cases be rendered easier.
The interest and importance of investigations into the conditions of equilibrium between two substances, lie in the determination not only of the conditions for the stable existence of the participating substances, but also of whether or not chemical action takes place between these two components; and if combination occurs, in the determination of the nature of the compounds formed and the range of their existence. In all such investigations, the Phase Rule becomes of conspicuous value on account of the fact that its principles afford, as it were, a touchstone by which the character of the system can be determined, and that from the form of the equilibrium curves obtained, conclusions can be drawn as to the nature of the interaction between the two substances. In order to exemplify the application of the principles of the Phase Rule more fully than has already been done, illustrations will be drawn from investigations on the interaction of organic compounds; on the equilibria between optically active compounds; and on alloys.
Summary of the Different Systems of Two Components.—Before passing to the consideration of the application of the Phase Rule to the investigation of particular problems, it will be well to collect together the different types of equilibrium curves with which we are already acquainted; to compare them with one another, in order that we may then employ these characteristic curves for the interpretation of the curves obtained as the result of experiment.
In investigating the equilibria between two components, three chief classes of curves will be obtained according as—
I. No combination takes place between the two components.
II. The components can form definite compounds.
III. The components separate out in the form of mixed crystals.
The different types of curves which are obtained in these three cases are represented in Figs. 63, 64, 65. These different diagrams represent the whole series of equilibria, from the melting point of the one component (A) to that of the other component (B). The curves represent, in all cases, the composition of the solution, or phase of variable composition; the temperature being measured along one axis, and the composition along the other.
We shall now recapitulate very briefly the characteristics of the different curves.
Fig. 63Fig.63.
If no compound is formed between the two components,the general form of the equilibrium curve will be that of curve I. or II., Fig. 63. Type I. is the simplest form of curve found, and consists, as the diagram shows, of only two branches, AC and BC, meeting at the point C,which lies below the melting point of either component. The solid phase which is in equilibrium with the solutions AC is pure A; that in equilibrium with BC, pure B. C is the eutectic point. Although at the eutectic point the solution solidifies entirely without change of temperature, the solid which is deposited is not a homogeneous solid phase, but a mixture, or conglomerate of the two components.The eutectic point, therefore, represents the melting or freezing point, not of a compound, but of a mixture(p.119).
Curve II., Fig. 63, is obtained when two liquid phases are formed. C is an eutectic point, D and F are transition points at which there can co-exist the four phases—solid, two liquid phases, vapour. DEF represents the change in the composition of the two liquid phases with rise of temperature; the curve might also have the reversed form with the critical solution point below the transition points D and F.
Fig. 64Fig.64.
In the second class of systems (Fig. 64), that in which combination between the components occurs, there are again two types according as the compound formed has a definite melting point (i.e.can exist in equilibrium with a solution of the same composition), or undergoes only partial fusion; that is, exhibits a transition point.
If a compound possessing a definite melting point is formed, the equilibrium curve will have the general form shown by curve I., Fig. 64. A, B, and D are the melting points of pure A, pure B, and of the compound AxByrespectively. ACis the freezing point curve of A in presence of B; BE that of B in presence of A; and DC and DE the freezing point curves of the compound in presence of a solution containing excess of one of the components. C and E are eutectic points at which mixtures of A and AxBy, or B and AxBycan co-exist in contact with solution. The curve CDE may be large or small, and the melting point of the compound, D, may lie above or below that of each of the components, or may have an intermediate position. If more than one compound can be formed, a series of curves similar to CDE will be obtained (cf.p.152).
On the other hand, if the compound undergoes transition to another solid phase at a temperature below its melting point, a curve of the form II., Fig. 64, will be found. This corresponds to the case where a compound can exist only in contact with solutions containing excess of one of the components. The metastable continuation of the equilibrium curve for the compound is indicated by the dotted line, the summit of which would be the melting point of the compound. Before this temperature is reached, however, the solid compound ceases to be able to exist in contact with solution, and transition to a different solid phase occurs at the point E (cf.p.134). This point, therefore, represents the limit of the existence of the compound AB. If a series of compounds can be formed none of which possess a definite melting point, then a series of curves will be obtained which do not exhibit a temperature-maximum, and there will be only one eutectic point. The limits of existence of each compound will be marked by a break in the curve (cf.p.143).
Fig. 65Fig.65.
Turning, lastly, to the third class of systems, in which formation of mixed crystals can occur, five different types of curves can be obtained, as shown in Fig. 65. With regard to the first three types, curves I., II., and III.,these differ entirely from those of the previous classes, in that they are continuous; they exhibit no eutectic point, and no transition point. Curve II. bears some resemblance to the melting-point curve of a compound (e.g.CDE, Fig. 64, I.), but differs markedly from it in not ending in eutectic points.
Further, in the case of the formation of a compound, the composition of the solid phase remains unchanged throughout the whole curve between the eutectic points; whereas, when mixed crystals are produced, the composition of the solid phase varies with the composition of the liquid solution. On passing through the maximum, the relative proportions of A and B in the solid and the liquid phase undergo change; on the one side of the maximum, the solid phase contains relatively more A, and on the other side of the maximum, relatively more B than the liquid phase. Lastly, when mixed crystals are formed, the temperature at which complete solidification occurs changes as the composition of the solution changes, whereas in the case of the formation of compounds, the temperature of complete solidification for all solutions is a eutectic point.
The third type of curve, Fig. 65, can be distinguished in a similar manner from the ordinary eutectic curve, Fig. 63, I., to which it bears a certain resemblance. Whereas in the case of the latter, the eutectic point is the temperature of complete solidification of all solutions, the point of minimum temperature in the case of the formation of mixed crystals, is the solidification point only of solutions having one particular composition; that, namely, of the minimum point. For all other solutions, the temperature of complete solidification is different. Whereas, also, in the case of the simple eutectic curve, the solid which separates out from the solutions represented by either curve remains the same throughout the whole extent of that curve, the composition of the mixed crystal varies with variation of the composition of the liquid phase, and the relative proportions of the two components in the solid and the liquid phase are reversed on passing through the minimum.[290]
In a similar manner, type IV., Fig. 65, can be distinguished from type II., Fig. 64, by the fact that it does not exhibit aeutectic point, and that the composition of the solid phase undergoes continuous variation with variation of the liquid phase on either side of the transition point. Lastly, type V., which does exhibit a eutectic point, differs from the eutectic curve of Fig. 63, in that the eutectic point does not constitute the point of complete solidification for all solutions, and that the composition of the solid phase varies with the composition of the liquid phase.
Such, then, are the chief general types of equilibrium curves for two-components; they are the pattern curves with which other curves, experimentally determined, can be compared; and from the comparison it will be possible to draw conclusions as to the nature of the equilibria between the two components under investigation.
1.Organic Compounds.
Fig. 66Fig.66.
The principles of the Phase Rule have been applied to the investigation of the equilibria between organic compounds, and Figs. 66-69 reproduce some of the results which have been obtained.[291]
Fig. 66, the freezing point curve (curve of equilibrium) foro-nitrophenol andp-toluidine, shows a curve of the simplest type[292](type I., Fig. 63), in which two branches meet at an eutectic point. The solid phase in equilibrium with solutions represented by the left-hand branch of the curve waso-nitrophenol (m.p. 44.1°); that in equilibrium with the solutions represented by the right-hand branch, wasp-toluidine (m.p. 43.3°). At the eutectic point (15.6°), these two solid phases could co-exist with the liquid phase. This equilibrium curve, therefore, shows thato-nitrophenol andp-toluidine do not combine with one another.
In connection with this curve, attention may be called to the interesting fact that although the solid produced by cooling the liquid phase at the eutectic point has a composition approximating to that of a compound of equimolecular proportions of the phenol and toluidine, and a constant melting point, it is nevertheless amixture. Although, as a rule, the constituents of the eutectic mixture are not present in simple molecular proportions, there is no reason why they should not be so; and it is therefore necessary to beware of assuming the formation of compounds in such cases.[293]
Fig. 67, on the other hand, indicates with perfect certainty the formation of a compound between phenol andα-naphthylamine.[294](Cf.curve I., Fig. 64.)
Phenol freezes at 40.4°, but the addition ofα-naphthylamine lowers the freezing point as represented by the curve AC. At C (16.0°) the compound C6H5OH,C10H7NH2is formed, and the system becomes invariant. On increasing the amount of the amine, the temperature of equilibrium rises, the solid phase now being the compound. At D, the curve passes through a maximum (28.8°), at which the solid and liquid phases have the same composition. This is the melting point of the compound. Further addition of the amine lowers the temperature of equilibrium, until at E solidα-naphthylamine separates out, and a second eutectic point (24.0°) is obtained. BE is thefreezing-point curve ofα-naphthylamine in presence of phenol, the freezing point of the pure amine being 48.3°.
On account of the great sluggishness with which the compound of phenol andα-naphthylamine crystallizes, it was found possible to follow the freezing point curves of phenol and the amine to temperatures considerably below the eutectic points, as shown by the curves CF and EG.