Sulphur transition point

G. Tschermak also obtained crystals of potassium hydrosulphate, KHSO4, by the slow decomposition of potassium ethyl sulphate, or by treating alcoholic potash with sulphuric add. J. Kendall and M. L. Landon obtained evidence of the hydrosulphate from the f.p. curve of the Components its m.p. is given as 218 6°, and there are three modifications with transition points at 182° and 202°. 

The remarkable variations in the viscosity of molten sulphur have already been mentioned. Definite measurements have been made at a series of temperatures by the method of rotating cylinders,5 and it, has been found that exposure of the liquid to air, especially below 160° C., has a marked effect on the viscosity from 160° C. onwards. The viscosity of purified (twice distilled but not gas-free) sulphur lias a value at 123° C. of 0 1094 C.G.S. units this falls to a minimum of 0 0709 at 150° C., rises gradually up to about 159° then rapidly increases above this temperature an exact transition point is not observed, however. The maximum for purified unexposed (gas-frcc) sulphur occurs at about 200° C. and has a value of 215 C.G.S. units. For purified (not gas-free) sulphur after prolonged exposure to the air, the maximum occurs at about 190° C, and may have a value as high as 800 C.G.S. units. Such high viscosity appears to be due to impurities, the chief being sulphuric acid, resulting from exposure to the air sulphur dioxide and ammonia in solution also have an appreciable effect. 

The comparison of theoretical conclusion with experimental results in the case of the transformation of sulphur has substantiated Nemst s Theorem regarding the possibility of calculating A from purely thermal data in a very satisfactory way It will be observed that as T increases A decreases, finally becoming zero at the transition point U, on the other hand, increases as T increases The behaviour of this system is thus represented by the curves U and A belongmg to type I in the figure (Fig 52)... 

Since in the neighbourhood of the triple point —Sg—V the curves Si—and Sg—are arranged similarly to the curves S—and L— in the neighbourhood of the triple point S—L—it follows that in the case of two crystalline forms of a substance, the metastable form will, at a given temperature, have a higher vapour pressure than the stable form. The vapour pressure curve, for example, of rhombic sulphur must at temperatures above 95 5° (the transition point) lie above that for the stable form, monoclinic sulphur and at temperatures below the transition point the vapour pressure curve for monoclinic sulphur must He above that for rhombic sulphur (p, 54). 

Law of Successive Reactions.— When sulphur vapour is cooled at the ordinary temperature, it first of all condenses to drops of liquid, which solidify in an amorphous form, and only after some time undergo crystallisation or when phosphorus vapour is condensed, white phosphorus is first formed, and not the more stable form, violet phosphorus. It has also been observed that even at the ordinary temperature (therefore much below the transition point) sulphur may crystallise out from solution in benzene, alcohol, carbon disulphide, and other solvents, in the monoclinic form, the less stable crystals then undergoing transformation into the rhombic form a similar behaviour... 

Under atmospheric pressure, the transition point was found by Reicher to lie at 95 5°. Rhombic sulphur is metastable above, and nionoclinic sulphur Tr metastable below the transition point. Both forms, however, can be obtained in the metastable state, and transformation to the stable form may take place with great slowness (p. 46). The vapour pressure of sulphur between the temperatures of 50° and 210°... 

Curve OC is the transition curve and represents the change of the transition point with pressure. Since the passage of the rhombic into monoclinic sulphur is accompanied by an increase of volume (At> = 0 01393 c.c. per gram), it follows from the theorem of Le Chatelier that the transition point is raised by increase of pressure. By means of this value of Az and the heat of transformation, 3 12 cal. cM... 

B. Sulphur Dioxide and Water.—In the case just studied we have seen that the components can combine to form definite compounds possessing stable melting-points. The curves of equilibrium, therefore, resemble in their general aspect those of calcium chloride and water, or of ferric chloride and water. In the case of sulphur dioxide and water, however, the melting-point of the compound formed cannot be realised, because transition to another system occurs retroflex concentration-temperature curves are therefore not found here, but the curves exhibit breaks or sudden changes in direction at the transition points, as in the case of the systems formed by sodium sulphate and water. The case of sulphur dioxide and water is also of interest from the fact that two liquid phases can be formed. 

A. Water. Other systems of the substance water. B, Sulphur. Metastable systems. Bivariant systems. C. Tin. Transition point. D. Phosphorus, E. Liquid Crystals or Anisotropic Liquids. Nature of liquid cryst s. Equilibrium relations in the case of liquid crystals. 

Figure 4.2 illustrates the case of sulphur, a system that exhibits two crystalline polymorphs. The area above the curve ABEF is the region in which orthorhombic sulphur is the stable solid form. The areas bounded by curves ABCD and FECD indicate the existence of vapour and liquid sulphur, respectively. The triangular area BEC represents the region in which monoclinic sulphur is the stable solid form. Curves AB and BC are the vapour pressure curves for orthorhombic and monoclinic sulphur, respectively, and these curves intersect at the transition point B. 

The broken lines in Figure 4.2 represent metastable conditions. If orthorhombic sulphur is heated rapidly beyond 95.5 °C, the change to the monoclinic form does not occur until a certain time has elapsed curve BB, a continuation of curve AB, is the vapour pressure curve for metastable orthorhombic sulphur above the transition point. Similarly, if monoclinic sulphur is cooled rapidly below 95.5 °C, the change to the orthorhombic form does not take place immediately, and curve BA is the vapour pressure curve for metastable monoclinic sulphur below the transition point. Likewise, curve CB is the vapour pressure curve for metastable liquid sulphur below the 115 °C transition point, and curve B E the melting point curve for metastable orthorhombic sulphur. Point B, therefore, is a fourth triple point (110 °C and 1.7 N m ) of the system. 

Van t Hoff then deals with chemical equilibrium on the basis of the law of mass action, and the change of equilibrium constant with temperature, introducing the case of condensed systems in the absence of vapour and a transition point (point de transition). Physical equilibria are special cases of chemical equilibria. Graphical methods with vapour pressure curves (e.g. for the allotropic forms of sulphur) are introduced. The principle of mobile equilibrium is explained for homogeneous and heterogeneous equilibria, and the Thomsen-Berthelot principle criticised (see pp. 614, 620). The last chapter, on affinity ,gives the definition The work of affinity (A) is equal to the heat produced in the transformation (q), divided by the absolute temperature of the transition point (P) and multiplied by the difference between this and the given temperature (P) ... 

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