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IODINE AND CHLORINE.

I. _Invariant systems._

-------------------------------------------------------------------------

Phases present.

Temper-

Pressure.+--------------------+-----------------+-------------- ature.

Solid.

Liquid.

Vapour.

--------+----------+--------------------+-----------------+-------------- 7.9

11 mm.

I_{2},[alpha]-ICl

I[wavy]Cl_{0.66}

I + Cl_{0.92} 0.9

--

I_{2},[beta]-ICl

I[wavy]Cl_{0.72}

-- 22.7

42 mm.

[alpha]-ICl,ICl_{3}

I[wavy]Cl_{1.19}

I + Cl_{1.75} [-102

<1 atm.="" icl_{3},cl_{2}="" i[wavy]cl_{m}="" i="" +="">

II. _Melting points._

A. Iodine,[246] 114.15 (pressure 89.8 mm.).

C. [alpha]-Iodine monochloride, 27.2 (pressure 37 mm.).

E. Iodine trichloride, 101 (pressure 16 atm.).

G. [beta]-Iodine monochloride, 13.9.

Since the vapour pressure at the melting point of iodine trichloride amounts to 16 atm., the experiments must of course be carried out in closed vessels. At 63.7 the vapour pressure of the system trichloride--solution--vapour is equal to 1 atm.

Pressure-Temperature Diagram.--In this diagram there are represented the values of the vapour pressure of the saturated solutions of chlorine and iodine. To give a complete picture of the relations between pressure, temperature, and concentration, a solid model would be required, with three axes at right angles to one another along which could be measured the values of pressure, temperature, and concentration of the components in the solution. Instead of this, however, there may be employed the accompanying projection figure[247] (Fig. 43), the lower portion of which shows the projection of the equilibrium curve on the surface containing the concentration and temperature axes, while the upper portion is the projection on the plane containing the pressure and temperature axes. The lower portion is therefore a concentration-temperature diagram; {166} the upper portion, a pressure-temperature diagram. The corresponding points of the two diagrams are joined by dotted lines.

[Ill.u.s.tration: FIG. 43.]

Corresponding to the point C, the melting point of pure iodine, there is the point C_{1}, which represents the vapour pressure of iodine at its melting point. At this point three curves cut: 1, the sublimation curve of iodine; 2, the vaporization curve of fused iodine; 3, C_{1}B_{1}, the vapour-pressure curve of the saturated solutions in equilibrium with solid iodine. Starting, therefore, with the system solid iodine--liquid iodine, addition of chlorine will cause the temperature of equilibrium to fall continuously, while the vapour pressure will first increase, pa.s.s through a maximum and then fall continuously {167} until the eutectic point, B (B_{1}), is reached.[248] At this point the system is invariant, and the pressure will therefore remain constant until all the iodine has disappeared. As the concentration of the chlorine increases in the manner represented by the curve B_f_H, the pressure of the vapour also increases as represented by the curve B_{1}_f__{1}H_{1}. At H_{1}, the eutectic point for iodine monochloride and iodine trichloride, the pressure again remains constant until all the monochloride has disappeared. As the concentration of the solution pa.s.ses along the curve HF, the pressure of the vapour increases as represented by the curve H_{1}F_{1}; F_{1} represents the pressure of the vapour at the melting point of iodine trichloride. If the concentration of the chlorine in the solution is continuously increased from this point, the vapour pressure first increases and then decreases, until the eutectic point for iodine trichloride and solid chlorine is reached (D_{1}). Curves Cl_{2} solid and Cl_{2} liquid represent the sublimation and vaporization curves of chlorine, the melting point of chlorine being -102.

Although complete measurements of the vapour pressure of the different systems of pure iodine to pure chlorine have not been made, the experimental data are nevertheless sufficient to allow of the general form of the curves being indicated with certainty.

Bivariant Systems.--To these, only a brief reference need be made. Since there are two components, two phases will form a bivariant system. The fields in which these systems can exist are shown in Fig. 43 and Fig. 44, which is a more diagrammatic representation of a portion of Fig. 43.

I. Iodine--vapour.

II. Solution--vapour.

III. Iodine trichloride--vapour.

IV. Iodine monochloride--vapour.

[Ill.u.s.tration: FIG. 44.]

The conditions for the existence of these systems will probably be best understood from Fig. 44. Since the curve B'A' {168} represents the pressures under which the system iodine--solution--vapour can exist, increase of volume (diminution of pressure) will cause the volatilization of the solution, and the system iodine--vapour will remain. If, therefore, we start with a system represented by _a_, diminution of pressure at constant temperature will lead to the condition represented by _x_. On the other hand, increase of pressure at _a_ will lead to the condensation of a portion of the vapour phase. Since, now, the concentration of chlorine in the vapour is greater than in the solution, condensation of vapour would increase the concentration of chlorine in the solution; a certain amount of iodine must therefore pa.s.s into solution in order that the composition of the latter shall remain unchanged.[249] If, therefore, the volume of vapour be sufficiently great, continued diminution of volume will ultimately lead to the disappearance of all the iodine, and there will remain only solution and vapour (field II.). As the diminution of volume is continued, the vapour pressure and the concentration of the chlorine in the solution will increase, until when the pressure has reached the value _b_, iodine monochloride can separate out. The system, therefore, again becomes univariant, and at constant temperature the pressure and composition of the phases must remain unchanged. Diminution of volume will therefore not effect an increase of pressure, but a condensation of the vapour; and since this is richer in chlorine than the {169} solution, solid iodine monochloride must separate out in order that the concentration of the solution remain unchanged.[250] As the result, therefore, we obtain the bivariant system iodine monochloride--vapour.

A detailed discussion of the effect of a continued increase of pressure will not be necessary. From what has already been said and with the help of Fig. 44, it will readily be understood that this will lead successively to the univariant system (_c_), iodine monochloride--solution--vapour; the bivariant system solution--vapour (field II.); the univariant system (_d_), iodine trichloride--solution--vapour; and the bivariant system _x'_, iodine trichloride--vapour. If the temperature of the experiment is above the melting point of the monochloride, then the systems in which this compound occurs will not be formed.

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 realized, 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.

The phases which occur are--Solid: ice, sulphur dioxide hydrate, SO_{2},7H_{2}O. Liquid: two solutions, the one containing excess of sulphur dioxide, the other excess of water, and represented by the symbols SO_{2} [wavy] _x_H_{2}O (solution I.), and H_{2}O [wavy] _y_SO_{2} (solution II.).

Vapour: a mixture of sulphur dioxide and water vapour in varying proportions. Since there are two components, sulphur dioxide and water, the number of {170} possible systems is considerable. Only the following, however, have been studied:--

I. _Invariant Systems: Four co-existing phases._ (_a_) Ice, hydrate, solution, vapour.

(_b_) Hydrate, solution I., solution II., vapour.

II. _Univariant Systems: Three co-existing phases._ (_a_) Hydrate, solution I., vapour.

(_b_) Hydrate, solution II., vapour.

(_c_) Solution I., solution II., vapour.

(_d_) Hydrate, solution I., solution II.

(_e_) Hydrate, ice, vapour.

(_f_) Ice, solution II., vapour.

(_g_) Ice, hydrate, solution II.

III. _Bivariant Systems: Two co-existing phases._ (_a_) Hydrate, solution I.

(_b_) Hydrate, solution II.

(_c_) Hydrate, vapour.

(_d_) Hydrate, ice.

(_e_) Solution I., solution II.

(_f_) Solution I., vapour.

(_g_) Solution I., ice.

(_h_) Solution II., vapour.

(_i_) Solution II., ice.

(_j_) Ice, vapour.

[Ill.u.s.tration: FIG. 45.]

Pressure-Temperature Diagram.[251]--If sulphur dioxide is pa.s.sed into water at 0, a solution will be formed and the temperature at which ice can exist in equilibrium with this solution will fall more and more as the concentration of the sulphur dioxide increases. At -2.6, however, a cryohydric point is reached at which solid hydrate separates out, and the system becomes invariant. The curve AB (Fig. 45) therefore represents the pressure of the system ice--solution II.--vapour, and B represents the temperature and pressure at which the invariant system ice--hydrate--solution II.--vapour can exist. At this point the temperature is -2.6, and the pressure 21.1 cm. If heat is withdrawn from this system, the solution will ultimately {171} solidify to a mixture of ice and hydrate, and there will be obtained the univariant system ice--hydrate--vapour. The vapour pressure of this system has been determined down to a temperature of -9.5, at which temperature the pressure amounts to 15 cm. The pressures for this system are represented by the curve BC. If at the point B the volume is diminished, the pressure must remain constant, but the relative amounts of the different phases will undergo change. If suitable quant.i.ties of these are present, diminution of volume will ultimately lead to the total condensation of the vapour phase, and there will remain the univariant system ice--hydrate--solution. The temperature of equilibrium of this system will alter with the pressure, but, as in the case of the melting point of a simple substance, great differences of pressure will cause only comparatively small changes in the temperature of equilibrium. The change of the cryohydric point with the pressure is represented by the line BE; the actual values have not been determined, but the curve must slope towards the pressure axis because fusion is accompanied by diminution of volume, as in the case of pure ice.

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A fourth univariant system can be formed at B. This is the system hydrate--solution II.--vapour. The conditions for the existence of this system are represented by the curve BF, which may therefore be regarded as the vapour-pressure curve of the saturated solution of sulphur dioxide heptahydrate in water. Unlike the curve for iodine trichloride--solution--vapour, this curve cannot be followed to the melting point of the hydrate. Before this point is reached, a second liquid phase appears, and an invariant system consisting of hydrate--solution I.--solution II.--vapour is formed. We have here, therefore, the phenomenon of melting under the solution as in the case of succinic nitrile and water (p. 122). This point is represented in the diagram by F; the temperature at this point is 12.1, and the pressure 177.3 cm. The range of stable existence of the hydrate is therefore from -2.6 to 12.1; nevertheless, the curve FB has been followed down to a temperature of -6, at which point ice formed spontaneously.

So long as the four phases hydrate, two liquid phases, and vapour are present, the condition of the system is perfectly defined. By altering the conditions, however, one of the phases can be made to disappear, and a univariant system will then be obtained. Thus, if the vapour phase is made to disappear, the univariant system solution I.--solution II.--hydrate, will be left, and the temperature at which this system is in equilibrium will vary with the pressure. This is represented by the curve FI; under a pressure of 225 atm. the temperature of equilibrium is 17.1. Increase of pressure, therefore, raises the temperature at which the three phases can coexist.

Again, addition of heat to the invariant system at F will cause the disappearance of the solid phase, and there will be formed the univariant system solution I.--solution II.--vapour. In the case of this system the vapour pressure increases as the temperature rises, as represented by the curve FG. Such a system is a.n.a.logous to the case of ether and water, or other two partially miscible liquids (p. 103). As the temperature changes, the composition of the two liquid phases will undergo change; but this system has not been studied fully.

The fourth curve, which ends at the quadruple point F, is {173} that representing the vapour pressure of the system hydrate--solution I.--vapour (FH). This curve has been followed to a temperature of 0, the pressure at this point being 113 cm. The metastable prolongation of GF has also been determined. Although, theoretically, this curve must lie below FH, it was found that the difference in the pressure for the two curves was within the error of experiment.

Bivariant Systems.--The different bivariant systems, consisting of two phases, which can exist within the range of temperature and pressure included in Fig. 45, were given on p. 170. The conditions under which these systems can exist are represented by the areas in the diagram, and the fields of the different bivariant systems are indicated by letters, corresponding to the letters on p. 170. Just as in the case of one-component systems (p. 29), we found that the field lying between any two curves gave the conditions of existence of that phase which was common to the two curves, so also in the case of two-component systems, a bivariant two-phase system occurs in the field enclosed[252] by the two curves to which the two phases are common. As can be seen, the same bivariant system can occur in more than one field.

As is evident from Fig. 45, three different bivariant systems are capable of existing in the area HFI; which of these will be obtained will depend on the relative ma.s.ses of the different phases in the univariant or invariant system. Thus, starting with a system represented by a point on the curve HF, diminution of volume at constant temperature will cause the condensation of a portion of the vapour, which is rich in sulphur dioxide; since this would increase the concentration of sulphur dioxide in the solution, it must be counteracted by the pa.s.sage of a portion of the hydrate (which is relatively poor in sulphur dioxide) into the solution.

If, therefore, the amount of hydrate present is relatively very small, the final result of the compression will be the production of the system _f_, solution I.--vapour. On the other hand, if the vapour is present in relatively small amount, it will be the first phase to disappear, {174} and the bivariant system _a_, hydrate--solution I., will be obtained. Finally, if we start with the invariant system at F, compression will cause the condensation of vapour, while the composition of the two solutions will remain unchanged. When all the vapour has disappeared, the univariant system hydrate--solution I.--solution II. will be left. If, now, the pressure is still further increased, while the temperature is kept below 12, more and more hydrate must be formed at the expense of the two liquid phases (because 12 is the lower limit for the coexistence of the two liquid phases), and if the amount of the solution I. (containing excess of sulphur dioxide) is relatively small, it will disappear before solution II., and there will be obtained the bivariant system hydrate--solution II.

(bivariant system _b_).

In a similar manner, account can be taken of the formation of the other bivariant systems.

A behaviour similar to that of sulphur dioxide and water is shown by chlorine and water and by bromine and water, although these have not been so fully studied.[253] In the case of hydrogen bromide and water, and of hydrogen chloride and water, a hydrate, viz. HBr,2H_{2}O and HCl,2H_{2}O, is formed which possesses a definite melting point, as in the case of iodine trichloride. In these cases, therefore, a retroflex curve is obtained. Further, just as in the case of the chlorides of iodine the upper branch of the retroflex curve ended in a eutectic point, so also in the case of the hydrate HBr,2H_{2}O the upper branch of the curve ends in a eutectic point at which the system dihydrate--monohydrate--solution--vapour can exist. Before the melting point of the monohydrate is reached, two liquid phases are formed, as in the case of sulphur dioxide and water.

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CHAPTER X

SOLID SOLUTIONS. MIXED CRYSTALS