706 LEE AND EGERTON : HETEROGENEOUS EQUILIBRIA

LXXXIII. - Heterogeneous Equilibria between the Chlorides of Calcium, Magnesium, Potassium, and their Aqueous Solutions. Part I . By WILLIAM BELL LEE and ALFRED CHARLES EOERTON.

DURING the course of an investigation on the quinary system CaC12-MgC12-KC1-NaCl--H,0, i t became necessary to study in detail the ternary systems CaC12-KC1-H20, CaC12-MgCl,-H,O, and MgCI,-KCl-H,O.

Although a considerable volume of work closely bearing on these systems has been done, much of which was pioneer work of van't Hoff and his pupils, there appears to have been published no com- plete study of these ternary systems.

It is the object of the present paper to place on record the results obtained at 25" for the systems CaC12-MgC12-H,0 and CaCl,-KCl-H,O together with some remarks on the system MgC1,-KC1-H20.

I. The Ternary Xystem CaCI,-KCl-H,O at 25". Data for the solubility of potassium chloride and of calcium

chloride are available and, in addition, Mulder found that a t 7", a solution saturated simultaneously with calcium chloride hexahydrate and potassium chloride contained 2.9 per cent. of the latter (Seidell's " Solubilities," 2nd edition, p. 89). Van't Hoff (" Zur Bildung der Ozeanischen Salzablagerungen," 11, p. 11) also determined two points in the quaternary system with sodium chloride present as an additional solid phase. He found that the invariant solutions contained 4.5 and 3.5 per cent. of potassium chloride at 25" and 83", respectively.

The results of these investigators showed that the chlorides of potassium and sodium are comparatively insoluble in saturated solutions of calcium chloride.

The main purpose of the present work was to examine the form and extent of the saturation curves in the ternary system of the pure components (sodium chloride being absent) and also to obtain some idea of the form of the saturation surfaces in the quaternary system containing magnesium chloride as an additional component.

Table I gives the analytical results together with the density and viscosity data, and a graphical representation of the results obtained is given in Fig. 1.

E and P denote the solubility of pure potassium chloride and of calcium chloride hexahydrate, respectively, in water (grams per

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TA

BL

E I.

Solid

ph

ases

. K

C1 7

7

Y9

39

99

99

KC

1 +

CaC

l,,6H

20

19

9

9

99

Y,

CaC

I,,G

H,O

*

I

oFjc

25 -

1.18

2 1.

204

1.23

6 1.

273

1.34

9 1.

402

1.48

5 1.

485

-

1-4

i

The

Sys

tem

CaC

1,-K

CI-H

,O

at

25".

Solu

tions

. R

esid

ue.

- G

ram

s pe

r 10

0 gr

ams '

' G

ram

s pe

r 10

0 gr

ams

of s

olut

ioii.

of

res

idue

.

\

/-

h

7

Rel

. 7-

visc

osity

. C

aCl,.

K

Cl.

H,O

. C

aCl,.

K

C1.

H

,O.

1-07

2 -

26.7

4 73

-26

-

1.41

8.

53

17-6

3 53

.84

1.92

-

16.5

5 11

.64

71-8

1 4.

32

2.68

5 23

.15

7.52

69

.33

-

-

32.3

4 3-

72

63.9

4 9.

00

8.11

37

-82

3.15

59

.03

-

-

44.6

6 3.

05

52.2

9 25

.18

-

44.7

2 3-

06

52.2

2 47

-23

-

44.7

3 3-

28

51.9

9 38

.27

-

45.0

6 I

54.9

4 -

* Se

idel

l's " S

olub

ilitie

s,"

2nd

edit

ion,

19

20,

p.

196.

-

80.9

5 76

.33

-

53.0

0 -

47-5

2 2.

54

20.5

3 -

-

17.1

3 19

.35

-

18.0

0 -

27.3

0 50

.23

41.2

0 -

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708 LEE AND ECERTON : HETEROGENEOUS EQUILIBRIA

100 grams of saturated solution). ED is the saturation curve of potassium chloride, that is, it represents the composition of all possible saturated solutions containing potassium and calcium chlorides in solution, with one solid phase present, namely, potassium chloride in contact with solution. Similarly, FD denotes the composition of the solutions saturated with respect to calcium chloride hexahydrate only. The isothermal invariant point D represents the composition of the solution saturated with respect to both potassium chloride and calcium chloride hexahydrate. The area BED is the KC1-field of complexes, and DFN is the CaCl2,6H2O-field. Points in of solid potassium chloride,

the area BDN represent complexes solid calcium chloride hexahydrate,

FIG. 1.

and invariant solution D. The area BNC denotes mixtures of solid potassium chloride and calcium chloride, the latter not containing sufficient water to form only hexahydrated calcium chloride. AEDF is the area of unsaturated solutions.

Fig. 1 shows that the solubility of potassium chloride is relatively greatly depressed by the presence of a small concentration of calcium chloride, and that as the latter increases the potassium chloride- content rapidly decreases until it attains the order of from 3 to 4 per cent. of KCI, after which further addition of calcium chloride does not cause the percentage of potassium chloride to change appreciably. From the form of the saturation curves it is evident that if an unsaturated solution containing calcium chloride and potassium chloride is evaporated at 25", potassium chloride will, in general, be deposited first, and further that as soon as the solution,

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BETWEEN THE CHLORIDES OF CALCIUM, ETC. PART I. 709

has been concentrated to the point of crystallisation (which may vary widely according to the composition of the original unsaturated solution), potassium chloride will separate in relatively large quantity a t first.

Cameron, Bell, and Robinson (J. Physical Chem., 1907, 11, 396) determined solubilities in the system CaCJ-NaCl-H,O a t 25", and their results are plotted in Fig. 2 for comparison with Fig. 1. It is seen that the form of the saturation curves is very similar in the two systems : CaCI,-NaCl-H,O and CaC1,-KCl-H,O, both at 25".

FIG. 2.

Further, both potassium chloride and sodium chloride are com- paratively insoluble in very concentrated calcium chloride solution, so that in the general case of the evaporation of unsaturated solu- tions in either of these ternary systems the bulk of the sodium chloride (or potassium chloride) will be readily recoverable.

Density Determinations.-The relative density (OX;) of the mono- variant solutions (KCl solid phase) may conveniently be plotted as a function of the composition (ratio CaCl,/KCl), as it will be seen that the KCI saturation curve extends almost right across the ternary diagram. The empirical equation p = 1 el82 + 0.0444R - 04048R2 + 0-000219R3, where p is the density and R the ratio CaCl,/KCl, gives the density of any solution on the KC1 saturation curve. The results of Cameron, Bell, and Robinson (Eoc. cit.) for the densities of the corresponding monovariant solutions in the system CaCl, NaCl-H,O at 25", when plotted, give a curve which is of the same general form.

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710 LEE AND EGERTON : HETEROGENEOUS EQUILIBRIA

Viscosity Determinations. The relative viscosity of solutions of pure potassium chloride at

25" passes through a minimum value with increasing concentration (compare Herz, 2. anorg. Chem., 1917, 99, 132.)

In the system CaC1,-KCl-H,O a t 25" there is a very striking increase in the viscosity of the saturated solutions wibh increasing concentration of calcium chloride. The effect of adding calcium chloride to a saturated solution of potassium chloride at 25.1" is to increase the viscosity until, at the " break-point," the saturated solution is more than twenty times as viscous as pure water at the same temperature.

Practical Details.-The salts used were pure potassium chloride and pure calcium chloride. The working temperature was 25.1" & 0-05". The solid phases occurring in the systems investigated were characterised by the well-known " residue " method of Schreine- makers. All analyses were carried out in duplicate and in several cases the analytical work was checked by starting with a weighed complex of pure components, and after equilibrium had been reached, analysing the solution and residue. The analyses were usually carried out by precipitating calcium as oxalate and titrating the washed calcium oxalate with standard permanganate according to Peters's method (Amer. J. Sci., 1901, [iv], 12, 216), which gives results in good agreement with those obtained by the usual gravi- metric method. Chlorine was estimated by titrating the neutral solution against silver nitrate. Analyses were carried out in duplicate and potassium was found by difference. A direct estima- tion of potassium was made at the " break-point " by the cobalti- nitrite method. Several estimations indicated that the potassium chloride content a t the " break-point " was about 3.4 per cent. ; the value 3.1 per cent. of potassium chloride was obtained by difference. For the relative viscosity determinations an Ostwald viscosity tube was used, the time of outflow for water being about two and a half minutes. The results given were calculated by the usual Ostwald formula. Duplicate or triplicate measurements were made.

11. The Ternary System CaC12-MgC12-H,0 a t 25". Data for the solubility of the hexahydrates of calcium and mag-

nesium chlorides separately in water are available (Seidell, Landolt, etc.). If the effect of saturation with sodium chloride be neglected, one may say that van't Hoff determined the composition of the two invariant points at 25" (op. cit., 11, p. 10). The results of the present work are represented graphically in Fig. 3 and in Table 11.

In this system a double salt, tachhydrite, CaC1,,2MgC4,12H20,

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BETWEEN THE CHLORIDES OF CALCIUM, ETC. PART I. 711

occurs and Fig. 3 also differs from Fig. 1 in that both salt components exist as stable salt hydrates a t this temperature. D denotes the solubility of pure magnesium chloride hexahydrate in water, and E its composition. H represents the solubility of calcium chloride hexahydrate, and K its composition. DF is the saturation curve of magnesium chloride hexahydrate, FG and GH are the correspond- ing curves for tachhydrite and hydrated calcium chloride, respect- ively. The area DEF represents complexes of magnesium chloride hexahydrate and satu- rated solutions on curve DF, and FGT denotes complexes of tach- hydrite + saturated solutions on curve GF. EFT is the area of complexes of solid MgCl,,GH,O + tachhydrite + solution F , and

FIG. 3.

ADFGH is the area of unsaturated solutions.

TQK the complex area tachhydrite + hydrated calcium chloride + solution G. BETKC is the area of solid mixtures of calcium chloride, magnesium chloride and water, the latter component not being in sufficient amount to form only the hydrated solid salts. The composition of the invariant solution F cannot be expressed in terms of positive amounts of water and the solid phases tach- hydrite+MgCJ,6H20 with which it is in equilibrium. F is therefore an incongruently saturated solution, and G denotes a con- gruently saturated solution.

On concentrating an unsaturated solution of the chlorides of magnesium and calcium in water, different phenomena occur accord- ing to the composition of the original solution. All unsaturated solutions lying to the right of the line AT will on evaporation ulti- mately dry up at the point P, and solutions on the left of this line

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712 LEE AND EGERTON : HETEROGENEOUS EQUILIBRIA

will dry up a t the point G. In the former case, sufficient magnesium chloride hexahydrate will be deposited during the course of the evaporation to keep the final composition of the solution at t,he point F, and in the latter case all magnesium chloride hexahydrate deposited during the first part of the crystallisation will be redissolved owing to the phase reaction which occurs at the point F, where the separation of tachhydrite begins. If the magnesium chloride hexahydrate is removed as fast as it is deposited, then all unsaturated solutions on evaporation will finally dry up at the point G. An unsaturated solution lying in the area between A T and AP cannot deposit sufficient magnesium chloride hexahydrate during the evaporation to keep the resulting incongruently saturated invariant solution a t the point F. This may, however, be done by adding an excess of the hexahydrate during the progress of the evaporation.

The work of van't Hoff and his collaborators has shown that tachhydrite cannot have a stable existence in contact with solution in this system a t temperatures below 22", so that an isotherm a t a somewhat lower temperature than this would be of a type similar to that in the system CaC1,-KC1-H,O at 25", showing only one invariant point.

Van't Hoff, Kenrick, and Dawson (2. physikl . Chem., 1902,39,27) found that the transition point CaC1,,6H20 + CaC&,4H20 (29.2") is lowered to about 25" in the presence of tachhydrite. Consequently, a t temperatures just above 25O, a very small additional field, namely, that of CaC1,,4H,O, should appear. The present work, which was carried out a t 25.1" 0.05", neither disproves nor confirms the exact temperature at which calcium chloride tetrahydrate makes its appearance in this system; to establish this point by solubility measurements it would be desirable to obtain data at temperatures both appreciably above and below 25".

Density Determinations.-The densities of the various saturated solutions occurring in this system are complex functions of the composition. The equilibria may consequently be shown graphic- ally by plotting the density against the ratio CaCI,/MgC12, since this ratio uniquely determines the composition of the various saturated solutions. The density becomes a maximum when the solution is simultaneously saturated with tachhydrite and hydrated calcium chloride.

Practical Details.-Blasdale's oxalate process for the separation of calcium from magnesium ( J . Amer. Chem. Xoc., 1909, 31, 917) was adopted in the investigation of the system CaCb-MgCl,-H,O. The precipitated calcium oxalate after washing was titrated in hydrochloric acid solution in presence of a little manganous chloride according to the method of Peters (loc. cit.). Chlorine was estimated

The data are given in Table 11.

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TA

BL

E 11

.

Solid

ph

ases

. M

gC1,

,6H

20

’9

9’

9,

’2

’9

9’

9)

’9

MgC

~2,6

H,O

+ ta

chhy

drit

e T

achh

ydri

te

Cal

cium

chl

orid

e +

tach

hydr

i te

CaC

I,, 6

H2

0

’9

’9

9’

99

’9

The

Sys

tem

CaC

1,-M

gC1,-

H,O

at 2

5”

Solu

tion.

R

esid

ue,

r

Gra

ms

per

100

gram

s of

G

ram

s pe

r 10

0 gr

ams

of

solu

tion.

re

sidu

e.

r

I

Dti.

. C

aC1,

. M

gC1,.

H

,O.

CaC

1,.

MgC

1,.

H,O

. 1.

341

-

35-5

4 64

-46

-

-

-

1.37

1 10

.33

27-6

1 62

-06

3.41

40

.72

55.8

7 1-

391

16.0

5 23

.33

60-6

2 8.

61

33.7

3 57

-68

1.42

8 25

-09

18-1

3 56

.80

7.89

37

.95

54.1

6 1-

44 1

28.1

2 16

.31

55.5

7 12

.65

33.2

8 54

.07

1-45

5 31

.17

14.5

4 54

.29

15.4

1 35

.28

49-3

1

1.46

0 32

-82

13-5

5 53

.63

25.1

0 28

.60

46-3

0 1.

473

36-3

7 10

.78

52-8

5 -

-

-

-

-

1.48

6 38

-70

9.43

51

-87

-

1.47

2 38

.95

7-93

53

.12

-

1.46

5 41

.87

4-06

54

-07

48.8

2 0.98

50.2

0 1-

47 *

45.0

6 -

54.9

4 -

-

-

-

-

H E ”2 M H

Q T: H

* Se

idel

l’s “ S

olub

ilitie

s,”

2nd

edit

ion,

19

20,

p.

196.

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714 LEE AND EGERTON : HETEROGENEOUS EQUILIBRIA

by titration against, silver nitrate, and occasionally tests for magnesium were made by treating the filtrate from the calcium estimation with disodium hydrogen phosphate.

111. The System MgCl2-KC1-H,0. Although the two invariant points in this system have been

determined a t different temperatures by several investigators with results in good agreement, the same cannot be said in the case of the investigation of the exact form of the isothermal saturation curves. Uhlig (Cen.fr, Min., 1913, 417) has arrived a t the con- clusion that van't Hoff's two invariant point,s at 25" are slightly

FIU. 4.

incorrect. Precht and Wittgen (Ber., 1881,14, 1667) made a series of determinations of the solubility of potassium chloride in 11, 15, 21-2, and 30 per cent. solutions of magnesium chloride which also contained some sodium chloride. Their determinations were carried out a t intervals of 10" between 0" and 100". These investigators were unaware of the fact that in the case of the 30 per cent. solution of magnesium chloride,, carnallitc could exist as a solid phase. Van't Hoff and Meyerhoffer (2. physikal. Chem., 1899, 30, 84), in determining the composition of the two invariant solutions a t a series of temperatures, pointed out that a t all temperatures other than 10" a t which these points were determined their values appear to lie on curves corresponding to a lower potassium chloride content than those of Precht and Wittgen. Feit and Przibylla (2. Kali, 1909, 3, 393) give more recent data on the solubility of potassium chloride in solutions of magnesium chloride which also

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TA

BL

E 11

1.

The

Sys

tem

MgC

I2-K

C1-H

,O

at 2

5".

W

M 3 M x el

Res

idue

. /

- \

Gra

ms

per

100

gram

s of

2 9

resi

due.

No.

1

2 3 4 6 i) 7 8 9 10

Solid

KC

1 1.

182

phas

es.

DZi

.

¶, 1.

201

9)

1.

234

KC

1 +

carn

alli

te

-

-

9,

$3

9,

99

-

Car

nall

ite +

MgC

l,, 6

H,O

-

-

YY

1

1

-

99

9

)

MgC

1,,6

H,O

1.

341

Sol

utio

ns.

Gra

ms

per

100

gram

s of

so

luti

on.

,

..

/

,-. 1

KC

l. M

gCl 2

. H

,O.

26.i

4 -

73.2

6 13

-56

12.1

1

74.3

3 7.

90

19.8

3 72

.27

3.19

26

.66

i0-1

5 3-

20

26.7

9 70

.01

3.33

26

.81

69.8

6

0.52

38

.47

64.0

1

0-38

35

.13

64.4

9 0.

53

35.1

4 64

.33

-

35.5

4 64

.46

I

.

ri F

KC

l. M

gCl,.

H

,O.

3

1,

2,

3. N

ew

dete

rmin

atio

ns;

4 an

d 9.

G

ross

, D

iss.

, E

rlan

gen,

19

14;

5 an

d 8.

Deb

ler,

D

iss.

, E

rlan

gen,

19

13;

6. D

'Ans

, 2. K

uli,

1915

; 7.

L6v

-enh

erz,

2.

phys

ikal

. C

hem

., 18

98, 12,

481;

10

. N

ew

dete

rmin

atio

ns.

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716 HETEROGENEOUS EQUILIBRIA, ETC. PART I.

contained sodium chloride, their results being expressed as volume percentages. No complete investigation of this ternary isotherm (sodium chloride absent) appears to have been published.

In the present work, two points on the potassium chloride satura- tion curve (sodium chloride being absent) at 25" were determined. These new points help to fix the form of the potassium chloride saturation surface in the solid model for the quaternary system CaC1,-MgC1,-KC1-H,O a t 25", and also suffice for the quantitative application of the diagram to the case of the splitting of carnallite. They have a slightly lower content of potassium chloride than the corresponding points on a curve, giving interpolated values of the results of Precht and Wittgen (Zoc. cit . ) , and agree more closely with the prediction of van't Hoff and Meyerhoffer.

A determination of the solubility of magnesium chloride at 25.1" gave a result rather lower than that of van't Hoff (op . cit., I, p. 17), but agreeing closely with that of Biltz and Marcus (2. anorg. Chern., 1911, 71, 187) for 25". The solubility of pure potassium chloride was determined a t 25-1", giving as the result 26.74 per cent., in good agreement with the values of Armstrong and Eyre (Proc. Roy. 8oc., 1911, [A]: 84, 123; 1913, [A] , 88, 234-26.73 and 26.89 per cent.) for a temperature of 25" and with that given by Reinders (2. anorg. Chem., 1915, 93, 202-26.46 per cent. a t 25"). The solubility and the density of the saturated solution of potassium chloride were in close agreement with the values interpolated from the results of Berkeley (Phil. Trans., 1904, 203, [A] , 189. The density (0%) of a saturated solution of magnesium chloride was found to be 1.341 (Table 111); the addition of magnesium chloride to a saturated solution of potassium chloride causes the resulting density to increase progressively.

The curve of Precht and Wittgen does not differ greatly from the results obtained in the absence of sodium chloride.

Xummary . 1. Solubilities in the ternary system CaCI,-KCl-H,O at 25"

have been determined. The diagram given represents in a com- plete manner the equilibria in this system. Viscosity and density data are also given.

2. Similarly, the system CaC1,-MgCl2-H,O, in which the double salt, tachhydrite, occurs, has also been studied at 25".

3. The system KC1-MgC12-H20 is discussed and new points determined which render the data for this system more complete.

CLARENDON LABORATORY, OXFORD. [Received, January 22nd, 192 3.1

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