724 DUNNINGHAM : THE SYS'lXM ETHYL ETHER-WAFER-

LXXIIL- The System Ethyl Ether- Water-Potassium Iodide-iMercui i c Iodide. Part 11. Solutions Saturatcd with Respect to Solid Phases in the Four-component System.

By ALFRED CHARLES DLJNNINGHAM. AFTER the consideration of the three-component systems (this vol., p. 368), the simplest manner in which t o approach the somewhat complicated four-component system is by an examination of a series of isotherms, beginning with the simplest type, and developing this into the Complicated form representing the system under con- sideration.

Fig. 8 * shows such a series in a four-component system in which two of the components are liquids. The surfaces showing saturation with respect to the solid phases are projected on a plane parallel to two sides of the tetrahedron representing the system, which do not cut each other. The tetrahedra have been omitted for the sake of clearness.

Fig. 8a shows the typical isotherm in which the two solid com- ponents A and B do not form a compound. Fig. 8b shows a similar isobherm in which a compound of A afid B only, called G, is formed.

* For Fig. 7, see this vol., p. 377.

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POTASSIUM IODIDE-MERCURIC IODlDE. PART 11. 7 25

I n both these cases the curves of saturation in the two three- component systems in which the solids appear are similar, and are joined by surf aces extending right through the f our-component system. I n Fig. 8c a second compound, D, appears, containing the two solid components and one of the liquid ones. The surface of saturation with respect t o this compound naturally disappears before the right-hand three-component system is reached. This gives rise to a point z, representing a solution in equilibrium with C, L), and B.

Fig. 8d shows an isotherm which closely resembles Fig. 8c, except

FIG. 8tr. FIG. 8b.

A C B

FIG. 8c. FIG. 8d.

for the fact that the two liquids are not now miscible in all pro- portions. The saturation field of A is thus divided into two parts separated by the two-liquid area, A L , + A L2. Similarly, the field of C is divided into two parts by the two-liquid area, which extends into the D field, where the two liquids become identical in the critical point K,. A two-liquid area is also found in the saturation surface of B, the two liquids becoming identical in the critical point X2. This figure, of course, only shows one of the very many forms which are theoretically possible in a system of this type, but the division of the saturation surfaces of the solid phases corresponds with that actually found in the system under con- sideration.

If, now, in Fig. 8d the right-hand three-component system is made 3 ~ 2

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726 D U N N I N G H A M : THE SYSTEM ETHYL ETHER-WATEB-

similar to that in the system potassium iodide-mercuric iodide- ether, the most likely types for the four-component system are shown in Figs. 9a and 9b , which are readily intelligible when it is remembered that the binodal curve of the three-component system becomes a cone-shaped solid in the four-component system, and that the intersection of the surfaces of saturation of the solid phases with the binodal cone renders certain portions of i t metastable.

FIG. 9n.

FIG. 9b.

The type of equilibrium actually found is shown by Fig. 9b. It is as well, therefore, t o consider the intersection of the surfaces of saturation with the binodal cone a little more closely. I n Fig. 10, Krl-IFE and HGPE represent the saturation surfaces of B and C respectively. These surfaces intersect along the line F E , which therefore stands for solutions in equilibrium with both solid phases. The iines K E and H E show the saturation curves of B and C respectively in the three-component system, so that a, b , c, a i d d

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POTASSIUM IODIDE-MERCURIC IODIDE. PART TI. 7 21

are the points of intersection of the binodal curve with these lines. The binodal curve itself is represented by abdc, the metastable portions being dotted. a’b’d/d and a/’bl/ are therefore sections of the binodal cone in the four-component system, the stable por- tions of this con0 being the triangular strips acalf and bdb“. It will be noticed that there are no stable two-liquid layers after alf and b / / , which represent two liquid layers in equilibrium with one another and with B and C. Any attempt t o advance from these points in the direction of P results in a diminution of the quantity of the liquid aft, because the mixture simply moves a little further into the tetrahedral complex BCb//a/’, a// disappearing when the composition of the mixture reaches the plane BCbfj.

FIG. 10.

Beyond this there is only the ordinary equilibrium of B, C, and solutions on bNF.

We can now consider the diagram, which shows the actual equilibrium. Fig. 11 is a diagrammatic projection of the tetra- hedron on a plane parallel to two sides which do not intersect, such that the position of any point on the projection is found from the relationships

0 being taken as the origin, BC as the x axis, and 140 as the y axis.

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728 DUNNINGHAM : THE SYSTEM ETHYL ETHER-WATER-

FGHJZ is the isotherm in the system potassium iodide-mercuric iodide-water, abcfmgde that in the system potassium iodide- mercuric iodide-ether, P Y X and a M P that in the system potassium iodide-water-ether, and Z Q X and PPe that in the system water- ether-mercuric iodide. From what has already been said, the saturation surfaces will easily be distinguished. They are numbered 1, -2, 3, and 4, t o correspond with potassium iodide, potassium mercuri-iodide, potassium mercuri-iodide hydrate, and mercuric iodide respectively. The saturation qurface of potassium iodide is divided into the

FIG. 11.

two portions FNRG arid MUba, representing aqueous and ethereal solutions respectively. The area N R UM represents mixtures of two liquids saturated with potassium iodide, the compositions of which are given by conjugate points on N R and MU.

The saturation surf ace of potassium mercuri-iodide is divided up in a similar way. The ethereal portion, UbcfmnpwT, is subdivided into the two portions U b c p T and nfm, separated by the two-liquid area pcfn. Any point on this area represents a mixture of two liquids, saturated with potassium mercari-iodide, lying on pc and

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POTASSIUM IODIDE-MERCURIC IODIDE. PART IT. 729

nf respectively. Both these curves correspond with solutions con- taining very little water. The former, however, are poor, and the latter rich, in dissolved salts.

The saturation surface of potassium mercuri-iodide hydrate is divided into a two-liquid area, SK,T, and a homogeneous area, HSR,TwJ, by the critical curve SII,T. Points on the portion SE,T represent mixtures of liquids on S K , and T K , respectively. It will be noticed that the homogeneous area is not divided into aqueous and ethereal portions, whence it follows that in contact with this phase water and ether are capable of mixing in all pro- portions. The solid potassium mercuri-iodide hydrate ceases, of course, to be stable in contact with solutions containing a larger proportion of ether than that corresponding with the solution w, which is in equilibrium with three solid phases.

The saturation surface of mercuric iodide is also divided by a critical curve, &li',P, into two-liquid and homogeneous areas, which have a similar significance to those described for the preceding phase. The homogeneous area is subdivided into the parts ZJwlJdePK2Q and nmg, separated by pngd, points on which repre- sent mixtures of liquids 011 pd and ng respectively. Both these curves correspond with solutions containing very little water, but the former are poor in dissolved salts and the latter rich.

The lines of intersection of these saturation surfaces represent solutions saturated with the two solid phases corresponding with the intersecting surfaces. A few points may be mentioned with regard to these. G R represents aqueous, and U b ethereal, solutions saturated with potassium iodide and solid potassium mercuri-iodide. Any point on the line U R gives a mixture of the two solutions U and R in equilibrium with the two solid phases. The position of the point on UR does not affect the composition of the two layers, but only their relative amounts. Similar remarks hold for solu- tions saturated with both solid potassium mercuri-iodide and its hydrate. The curve of solutions saturated with potassium mercuri- iodide hydrate and mercuric iodide, shown by Jw, is uninterrupted, since all solutions in equilibrium with these two phases are homo- geneous. The curve of solutions saturated with potassium mercuri- iodide and mercuric iodide is divided into two parts : wp represents solutions poor in dissolved salts, n m solutions rich in dissolved salts, whilst any point on pn represents a mixture of the two solutions n and m.

Since the saturation surfaces extend across the tetrahedron from the plane A B D to the plane ACD, they divide the tetrahedron into two portions. That lying between the saturation surfaces and tlie edge BC corresponds with unsaturated solutions. that lying between

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730 DUNNINGHAM : THE SYSTEM ETHYL ETHER-WATER-

the saturation surfaces and the edge A D t o supersaturated solutions or complexes of solid and solution. The latter space is bounded by the saturation surfaces and portions of the four side planes, namely, A P G H J Z D on A B D , AnbcfmgdeD on ,4CD, AFAI’f+ffl cn ABC, and DZQPe on DBC.

This space is divided up into a number of portions, the extent and significance of which are a t once clear from the following con- siderations. If any point on a saturation surface is joined t o the point representing the composition of the solid phase, the line so formed gives all possible mixtures of that particular solution and the solid phase. If, in the same way, all points on the saturation surface are joined to the point representing the solid phase, a space is formed which gives all possible mixtures of the solid phase and its saturated solutions. Such spaces exist for each of the four solid phases. It is t o be noted that each of these spaces is divided into two or more parts. Those arising from the homogeneous areas of the saturation surf aces represent complexes of homogeneous solu- tions and solid phase, whereas those arising from two-liquid areas represent complexes of two liquid layers and solid phase. Thus, for example, the space formed by joining d to all points on N M U R represents complexes of solid potassium iodide and two conjugate solutions on N R and MU respectively. It is not necessary to enumerate all these spaces, since they will be clear from the pre- ceding considerations.

Spaces which correspond with complexes of solution and two solid phases are formed by joining all points on the saturation curve of the two solid phases t o all points on the line representing mixtures of the same, that is, the line joining the points which correspond with their respective compositions. Thus, on joining all points on G R U b to all points on A t , the space formed repre- sents all possible complexes of solid potassium iodide, solid potassium mercuri-iodide, and saturated solutions. The parts of this space arising from G R and U b represent complexes of homo- geneous aqueous and ethereal solutions respectively, and the two solid phases, whilst that arising from R U represents mixtures of the two solutions R and U and the two solid phases. Similar con- siderations hold for the spaces which arise from the curves HSTw, wpmm, and Jzu.

I n addition t o the spaees representing complexes of solutions and one or two solid phases, there is one space representing complexes of three solid phases and the saturated solution w. The area formed by joining r tD gives all solid mixtures of potassium mercuri-iodide, its hydrate, and mercuric iodide. If, therefore, all

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POTASSIUM IODIDE-MERCURIC IODIDE. PART 11. 731

points on this area are joined to w, the resulting space gives all complexes of these three solid phases with w.

The reactions in the system are best understood by studying the intersections of a series of planes with the saturation surfaces and spaces. I n Fig. 11, A L D is a plane on which all points represent mixtures containing ether and water in a definite ratio, which is given by the position of L on BC; quvxyhk is the curve of inter- section of this plane with the saturation surfaces. This section is

FIG 12.

(K1)

shown in detail in Fig. 12, which is lettered t o correspond with Fig. 11. S,, S2, S,, and S, represent solid potassium iodide, potassium mercuri-iodide, potassium mercuri-iodide hydrate, and mercuric iodide respectively. L, and L, represent two conjugate liquids, whilst L,, L,, L,s, L , L,, and L, represent thesolutions shown by those letters in Fig. 11. IVRSK,TUM and QR,P are the projec- tions of the binodal curves on the plane A L D , and are also lettered t o correspond with Fig. 11.

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73-2 DUNNINGHAM : THE SYSTEM ETHYL ETHER-WATER-

The curve of intersection, qwvxyhk, may be regarded as the isotherm in the pseudo-ternary system potassium iodide-mercuric iodideliquid L ; it can be followed by gradually increasing the ratio of mercuric iodide to potassium iodide.

The “ solution ” g in the three-component system potassium iodide-water-ether exists as the two layers N and M . When mercuric iodide is added, q follows the curve pu, the two layers the curves N R and MU respectively, until, when the ‘‘ solution ” reaches ZG, the layers reach R and U. These are in equilibrium with potassium iodide and potassium mercuri-iodide. When all the potassium iodide is converted into potassium mercuri-iodide, the “solution” follows uv, the layers RS and UT respectively. A t v the solid phase change from potassium mercuri-iodide to its hydrate, and the “solution” then follows the curve vy, while the layers follow S R , and T K , respectively. A t x , the curve vy cuts the binodal curve in the branch corresponding with the ethereal or upper layers. A t x, therefore, the lower or aqueous layer a entirely disappears, leaving only the upper or ethereal layer x. When the point x: falls on the branch SK,, the upper or ethereal layer disappears, leaving the lower or aqueous layer, and when, as only happens for one particular value of L, z coincides with K,, the two layers become identical.

The homogeneous solution x then follows the curve xy until a t y the solid potassium mercuri-iodide hydrate is replaced by mercuric iodide. The solution, which is still homogeneous, then follows yk , until a t h the curve cuts the binodal curve &R,P on the aqueous branch. An upper or ethereal layer b then commences to form, and as the “ solution” follows the curve hk, the two layers move along k Q and bP respectively, ending in the three-component system mercuric iodide-ether-water. When the curve y ll: cuts the binodal curve in the branch K,P, an aqueous or lower layer forms, and when h coincides with K,, the homogeneous liquid separates into two layers.

I n following the curve quvxyhk, i t must be borne in mind that in order to keep the ratio of water to ether constant in the solu- tions, i t is necessary to remove all potassium mercuri-iodide a t 91

and add solid hydrate to take its place. A conversion of one into the other changes the ratio of the liquid components in the solution. Similarly, a t y it is necessary to remove all potassium mercuri- iodide hydrate and replace it by mercuric iodide.

The areas representing complexes of solid and solution are now intelligible without further explanation.

The actual figures obtained experimentally are given in table V.*

* For table IV, see this vol., p. 376.

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POTASSIUM IODIDE-MERCURIC IODIDE. PART 11. 733

TABLE V.*

at 20°. The System Ether-Water-Potassium Iodide Hereuric Iodide

Percentage composition Percentage composition of upper layer. of lower layer.

No. KI. HgI,. Et,O. 12 26.7 50.3 - 34 22.1 50.7 9.4 35 17.0 50.1 19-1 36 12.1 46.0 35.4 37 11.2 44.0 39.0 38 9.5 41.0 45.1 39 8.6 34-6 52.6 40 4-5 19.2 73.8 41 1-5 12.0 84.6 42 1.1 4.3 93.8 43 1.0 3.2 95.4 3 3 - - -

*27 - 4 96-1 44 0-2 1.6 96-2 45 0.3 1.9 95.8 46 1.4 8.7 86.1 47 2.5 14.0 77.0 48 3.0 15.6 73.5

t49 3.1 17-9 69.4 28 0.4 - 99.2 50 1.2 3.0 94.7 51 3.6 9.3 83.7 52 5.3 14.0 76.6 53 14.1 33.8 44.5 54 17-6 39.6 32.7 55 19.7 42.8 23.7

t56 26.6 47.2 9:0

-0. 23.0 17.8 13.8 6-5 5-8 4.4 4.2 2-5 1.9 0.8 0.4

3.9 2.0 1-9 3.8 6.5 7.9 9-6 0.4 1.1 3-4 4.1 7.6

10.1 13.8 17.2

-

KI. HgI,. - - - - - - - - - - - - - - - - - - - -

15.6 54.3 17.0 58.2 - 4 3.5 10.5 4.4 15.5 5.2 26.5 4.3 22.9 4.0 21.5 3.1 17.9

55.6 - 38.5 51.1 36.2 50.2 36.4 51.0 33.7 51.7 32-4 50.3 31.7 49.5 26.6 47-2

Et20. H20. Solid Phase. - - Hg12+ KHg13YH20

- - Hg12 + KHg13,H20 - - Hg12+ KHg13,%0 - - Hg12f KHg13,&0

- - HgI2 + KHgI,,H20

- - HgI2 + KHg13,H.,0

- - HgI,+KHgI,,q?O - - HgI2+ KHgI3S2O

- - Hg12+KHgI,,H;0

- - Hg12+KHg13(?) 28-8 1-3 HgIz+KHg13 24.8 - Hg&+KHg13

7.0 93.0 HgI2 12.4 73.6 HgI2 18-3 61.8 Hg12 60.7 17.6 Hg12 60-0 12.8 Hg12 63.0 11.5 Hg12 69.4 9.6 Hg12 3.7 40.7 KI 0.6 9.8 KI+KHgI

0.6 12.0 KHgI,,sO

2-4 14.9 KHg13,q?0 2.9 15.9 KHgIJ&O 9.0 17.2 KHg13,H20

0.9 12.7 KHgI, or KHg13,&0

1.4 13.2 KHgI3,%O

* The solubility of mercuric iodide in water and ether is too small to estimate. t Nos. 49 and 56 are critical points.

Owing to the very slight difference in composition between potassiuni mercuri-iodide and its hydrate, it has not been found possible t o determine the curve of solutions saturated with respect to these two solid phases, nor the solution w (Fig. 11) saturated with respect to these phases and mercuric iodide. The probable relationship existing between the saturation surfaces of potassium mercuri- iodide and its hydrate has therefore been deduced from a con- sideration of the isotherms in the three component systems potassium iodidemercuric iodide-water, and potassium iodide- mercuric iodide-ether.

The author wishes to acknowledge his indebtedness to the Chemical Society for a grant towards the expenses of this research.

* See footnote, p. 732.

SIR JOHN DEANE’S GRAMMAR SCHOOL, NORTHWICH, CHESHIRE.

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