Part 14 (1/2)

{165}

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.