4112
WILLIAM J. LIGHTFOOT AND CARLF. PRUTTON
[CONTRIBUTION FROM
THE
DEPARTMENT OF CHEMISTRY AND CHEMICAL ENGINEERING OF CASE
Vol. 70 INSTITUTE OF
TECHNOLOGY]
Equilibria in Saturated Solutions. 111. The Quaternary System CaC12-MgCl,KC1-H20 at 35' BY WILLIAM J. LIGHTFOOT AND CARLF. PRUTTON In previoiis papers,'V2 the isotherms for the ternary systems, CaCkMgCl*-H20, CaCl2-KC1H2O and MgC12-KCl-H20 were reported at 35' and 75'. These investigations were carried out in preparation for study of the quaternary system CaCl2-MgClrKCl-H20. Experimental Method The analytical methods and techniques were the same as those used in the investigation of the ternary systems. Solutions $nd solid phases were brought into equilibrium a t 35 * 0.02 It was found that six to eight hours was sufficient time for equilibrium to be established. In the identification of the solid phases present in a fourcomponent system, there are three methods which may be used. The application of the wet residue method used in ternary systems was extended by Schreinemakerss to quaternary systems. Igelsrud and Thompson4 in their study of this system a t 0 'presented an analytical method for the identification of the solid phases. Microscopic examination of the wet residue quickly establishes the identity oE the solid phases. All three of these methods were used. The Quaternary Isotherm.-In order to represent completely the compositions of the saturated solutions of a quaternary isotherm, a three dimensional coordinate system is required. The space model of the regular tetrahedron is usually employed. In Fig. 1, the upper portion of the quaternary tetrahedron is shown schematically, the relative proportions of the saturation surfaces being indicated.
.
The same characteristic of magnesium chloride solutions is noted in the quaternary system that was found in the ternary systems, carnallite, KC1.MgC12.6H20,is almost completely insoluble in solutions having MgCl2.6HpO as a solid phase. Tachydrite solutions dissolve more carnallite as the calcium chloride content of the solution is increased; with solutions saturated for CaC124H20 a considerably larger portion of carnallite is dissolved. The saturation surface of MgCl2~6HzOis extremely small, with tachydrite, CaC12.2MgC!g.l2H20, only slightly larger. The surface for CaC12.4HtO IS next in size with carnallite and KCl occupying the largest portions. The carnallite surface is the most significant in that it is a slice through practically the whole diagram, being in evidence as a solid phase from the ternary system MgCkKCl-H20 where there is no calcium chloride to solutions where the magnesium chloride content has fallen to 1.02% and the calcium chloride content has risen to 49.76y0. In discussing the associated ternary systems, it was pointed out that both carnallite and tachydrite were incongruently saturating compounds a t 35". No formation of a quaternary compound was found a t 35" either through analytical results or by microscopic examination.
A
M 10 Tach. N 2 0
30 MgC12.6Hz0
MgClz, %. Fig. 2.-Equilibria in the quaternary system, CaClzMgClz-KCI-H20 a t 35 ', projection on the MgC12-KC1HzO plane.
/
K CI
I
MgCl2
coc12 Fig. 1.-The
B
system CaC12-MgClTKCl-H20, tion of tetrahedron a t 35'.
upper por-
(1) Lightfoot and Prutton, THISJOURNAL, 68, 1001 (1946). (2) Lightfoot and Prutton, ibid., 69, 2098 (1947). (3) Schreinemakers, Z. Phys. Chsm., 19, 641 (1907). (4) Igelsrud and Thompsoq, THIS JOURNAL, 68, 2003 (1936).
In Table I, the data for the quaternary system are given. The isotherm has been projected on the MgCls-KCl-Hz0 plane of the tetrahedron in Fig. 2 using the method suggested by Schreinemakers. This is a projection parallel to a particular and specified edge of the tetrahedron. Points E, F, G, Nand M are the invariant points of the associated ternary systems. Area IFQE is the saturation surface of KC1, FGOPQ that of carnallite, GBNO that of MgC12. 6Hz0, MNOP that of tachydrite, and AEQPM that of 'CaClt.4HZOa. The curve FQ is the intersection of the carnallite surface with the KC1 surface and all points on it represent equilibrium between the solid phases carnallite and KC1 with saturated solution given by some point on this curve. GO represents solutions saturated with MgC12.6Hz0and carnallite; OP with tachydrite and carnallite, PQ with CaC12.4H20aand carnallite; NO with MgC126H20and tachydrite; M P wih tachydrite and CaClz. 4Hz0a; and EQ with KC1 and CaClz.4HnOa.
SYSTEM CaC1~MLlgCl2KCl-H20
Dec., 1948
4113
TABLE I iARY THEQUATER~
Point or line
F FQ FQ FQ FQ FQ FQ FQ FQ FQ
Q Q Q Q Q mean G GO 0 0 0 0 mean OP OP OP OP OP P P
P P mean PQ PQ PL) PQ E EQ EQ M MP hf P
x
Saturated solution, weight per cent. MgClr CaClz
27.33 20.61 10.59 5.83 4.07 4.01 2.31 1.30 1.15 1.17 1.04 1.01 1.01 1.03 1.02 36.17 26.66 18.06 18.07 18.10 18.07 14.23 13.83 7.28 6.98 6.15 5.89 5.90 5.93 5.91 4.42 2.69 1.72 1.36
... 0.71 0.93 6.20 6.13 5.82 18.18
...
9.12 23.57 31.66 35.51 35.60 41.00 45.85 48.34 48.89 49.74 49.78 49.74 49.78 49.76
... 12.97 26.76 26.74 26.73 26.74 32.01 32.60 42.75 43.27 44.44 45.32 45.31 45.26 45.30 46.65 48.40 49.25 49.54 50.45 50.06 49.83 45.03 45.18 45.4.3 26.68
-
SYSTEM
MgClz-CaClz-KC1-HzO A T 35"
__ Wet residue
KCl
3.81 3.69 3.47 3.37 3.44 3.43 3.80 4.71 5.69 6.04 6.49 6.45 6.46 6.45 6.46 0.14 .I6 .23 .23 .23 .23 0.32 .33 .7l .76 .97 1.00 0.99 .99 .99 1.33 2.28 3.92 5.10 6.48 6.46 6.45
..
MgClz
...
CaCh
KC1
...
...
20.13
5.59
24.67
...
... ... ...
... ... ...
... ... ...
...
...
...
5.73
34.99
...
...
5.69 2.81
38.50 48.61
15.96 7.29
...
...
...
2.23
51.40
6.36
...
...
...
...
... 17.16
...
...
...
31.75 25.66
8.43 19.29
4.48
...
,..
...
25.39
21.15
3.35
...
...
...
21.40
24.24
5.67 ...
...
5.53
...
...
...
13.26 ... ... 9.06
34.98
5.11
...
...
...
...
...
...
42.50 ...
2.41
...
...
...
47.81 39.28 ...
3.96 8.29 ...
...
... 5.04 8.76
... ... 0.23 ...
...
...
...
51.57
9.72
...
...
...
...
0.33 0 70
5.75 4.53
51.34 51.99
.
...
...
... 0.13 0.31 . . . .
Metastable
OP' OP' OP' P' P' P' P' P' mean P'Q' P'Q' P'Q'
Q'
9' Q' Q' Q' Q' mean
4.34 4.22 3.97 3.38 3.38 3.38 3.42 3.39 3.06 2.05 1.51 0.86 .86 .92 .92 ,239
.89
48.18 48. e3 49.24 50.50 50.49 50.51 50.47 50.49 50.81 51.78 52.05 50.68 50.68 50.71 50.71 50.71 50.70
1.41 1.53 1.64 2.09 2.09 2.11 2.09 2.10 2.10 3.91 4.53 6.97 7.02 7.02 7.01 7.02 7.01
...
...
...
... ...
... ... ...
... ... ...
. .
...
. .
...
...
...
... ... ...
... ... . .,
...
...
... I
.
.
...
... ...
... ...
...
... ...
...
... ...
...
... ...
...
... ...
...
... ...
...
...
...
+ + +
Solid phase
Carnallite KCI Carnallite KC1 Carnallite KCI Carnallite KCI Carnallite KCI Carnallite f KCI Carnallite KCI Carnallite KCI Carnallite KCI Carnallite KCI Carnallite KCI CaCI2.4HzO~ Carnallite KCI CaC12.4HzOa Carnallite KCl CaC1~.4H~0a Carnallite KCl CaC12.4Hz0a Carnallite KCI CaCla.4HzOa Carnallite MgC1,.6HrO Carnallite MgClz-6H20 Carnallite f MgC12.6HzO Tachydrite Carnallite MgC12.6HzO Tachydrite MgClz.6HzO Tachydrite Carnallite Carnallite MgC12.6H~0 Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite CaCL.4H20m Tachydrite Carnallite CaC12.4HzOa Tachydrite Carnallite CaC12.4HZOa Tachydrite Carnallite CaCl2.4H~Oa Tachydrite carnallite CaC12.4H2Oa CaClz.4H20a Carnallite CaCls.4H2Oa Carnallite Carnallite CaC1~.4HaOa KCl CaCIz.4HzOa CaC12.4HtOa KCl KCI CaClp.4HZOm Tachydrite CaCl2.4H~Oa CaC12.4HzOa Tachydrite Tachydrite CaC12.4HzOa Tachydrite MgCl2.6HrOa
+ +
+ + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + +
+ + + +
+ + + +
+ + + + Carnallite + Tachydrite Carnallite f Tachydrite Carnallite + Tachydrite Carnallite iTachydrite + CaClz.4HZOy Carnallite + Tachydrite + CaCIa.4HzOy Carnallite + Tachydrite + CaC12.4H20y Carnallite + Tachydrite f CaClz.4HzOy Carnallite + Tachydrite + CaClz.4H~Oy Carnallite + CaClt.4HzOy Carnallite f CaC12.4HzOy Carnallite + CaClt.4HzOr Carnallite + KCl + CaC12.4HzOr Carnallite + KCl + CaC12.4HzOy Carnallite + KCl + CaCIz.4HzOy Carnallite + KCl + CaClz.4HzOy Carnallite + KC1 + CaCle.4HzOy Carnallite + KCl + CaClt.4HnOy
WILLIAMJ. LIGHTFOOT AND CARLF. PRUTTON
4114
There are three quaternary isothermal invariant points in the system : P, Q and 0. At Q the three solid phases in equilibrium with the fixed composition of saturated solution are CaCl2.4HzOa, KCl, and carnallite; a t P, CaCl2. 4H20a, tachydrite, and carnallite. Actually the carnallite area is not pinched off as is shown in this projection. It is due to the fact that the left-hand portion of FQ superimposes part of the curve PQ. In Figs. 3 and 1,the quaternary isotherm has been projected OII thc CaC12-KCl-HZO plane and the CaC12MgC12-H20 plane, respectively. A consistent notation of the curves and points has been followed to facilitate the location of the same point on the different projections. In the projection on the CaCl2-KCI-H20 plane, the saturation curve for KCl in the ternary system superimposes the saturation curve for carnallite and KCl, FQ. Recognition of this aids in visualizing the surfaces in the space model. When the isotherm is projected on the MgCl&aC12-H20 plane, a considerable portion is superimposed by the ternary curves. This is probably the least useful diagram of the three, but it presents relationships which are not so easily seen froin the other two projections.
-
\
i, $>&,$,i.
50 M
40
COW$
Q‘
30 N
20
indicated on microscopic examination that CaCl~.$H,0y was the solid phase. Analyses of these samples were made and the invariant points and saturation curves for the metastable form in relation to the quaternary system were determined. The results are plotted in Fig. 3, the projection on the CaClTKCl-H20 plane. Comparison with the same projection showing stable equilibrium show that a marked change has taken place. The left-hand portion of the carnallite area has been broadened and thearea of existence of tachydrite as a solid phase increased. Points Q’ and P’ are now the invariant points, where CaC12.4H20a is replaced with CaC12.4H20y. Points M and E represent the iso-
TABLE I1 THE QUATERNARY SYSTEM MgC12-CaClZ-KCl-H20 AT 35O, BASISOF ZEROPERCENT.WATERCONTENT Point or
line
Saturated solution, weight per cent. MgCh CaClz
.. .
F FQ FQ FQ FQ FQ FQ FQ FQ FQ Q mean
87.76 61.67 28.14 14.27 9.46 9.32 4.90 2.51 2. 08 2.09 1.78
G GO Q mean
99.61 . 67.00 32.60 40.12 59.37
OP OP OP OP OP P mean
30.56 29.58 14.35 13.68 11.93 11.32
68.75 69.72 84.25 84.83 86.19 86.87
8.44 5.04 3.13 2.43
89.03 90.69 89.72 88.46 88.62 87.47 87.10 87.90 87.49 87.44 59.47
27.29 62.64 77.48 82.54 82.71 87.03 88.41 87.60 87.15 86.93
..
10
CaC12, %. Fig. 3.-Equilibria in the quaternary system CaC12MgC12-KCl-H 20 a t 35”, projected on the CaC12-KClHzO plane.
Vol. 70
PQ PQ PQ PQ E EQ EQ M MP MP
h’
...
1.24 1.63 12.10 11.87 11.20 40.53
Solid phase
+ + + + + + + + + +
Carnallite KCl Carnallite KC1 Carnallite KCl Carnallite KCl Carnallite KC1 Carnallite KCl Carnallite KCl Carnallite KCl Carnallite KCl Carnallite KCl KC1 Carnallite CaC1~.4H~0 Carnallite MgC12.6Hz0 Carnallite MgC12.6Hz0 MgCI2.6Hz0 Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite CaC1t.4H2Oor Carnallite CaCls.4HzOa Carnallite CaCl2.4HzOa Carnallite CaC12.4H~Oa Carnallite CaClz.4HzOor KC1 CaCl2~4HtOa KCl CaCl~.4H2Oa KCl CaC12.4HzOor Tachydrite CaClz.4HzOa Tachydrite CaC12.4HtOa Tachydrite CaClz.4H20a Tachydrite MgClf.6Ha0
+ + +
+ + + + + + + + + + + + + +
+
+
+
+ + + +
Metastable equilibrium 10
30 40 CaClz, %. Fig. 4.--Fquilibria in the quaternary system CaC12hlgClr--KCl-€I?O a t 35”, projection on the CaC12-MgC12H20 plme. 20
I n the experimental work, there was some difficulty a t first in obtaining solutions saturated with the stable form of calcium chloride tetrahydrate. Samples quite frequently
OP’ OP’ OP’ P‘ mean
8.05 7.76 7.24 6.04
89.34 89.43 89.77 90.21
P’Q‘ P’Q’ P’Q’ Q’ mean
5.47 3.55 2.60 1.49
90.78 89.68 89.60 86.55
+ + + + + + + +
Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite Carnallite Tachydrite CaCl2.4H2Oy Carnallite CaC12~4H20y Carnallite CaClr.4H~Oy Carnallite CaC12.4H20y Carnallite KCl CaC12.4H~Oy
+
+
Dec., 1948
THE- 2 STATES OF SELENIUM AND TELLURIUM BY POLAROGRAPHY
thermal invariant points in the ternary systems where the stable form of C a C b 4H20 is one of the solid phases. The effect of metastable form in these ternary systems was not determined. Hence MP’ and EQ’ are not true saturation curves for the metastable equilibria. However, it is possible to make an assumption as to the general effect of CaCls.4H20y on the ternary systems. In the ternary system CaCl*-KCl-H20 the invariant point would be of slightly higher calcium chloride content with the potassium chloride content remaining about the same. The effect on the CaC12MgClZ-HgO system would be more pronounced in that the tachydrite area would be larger, occupying some of the space now covered by CaC12.4Hz0 a. The CaC12.4H20 y field would be very small. In Table I1 the data for the quaternary isotherm are given based on the A M ?u’ D Tach. salt content of the saturated solution exclusive of water. The alot of the Fig. 5.-Quaternary system CaC12-MgC12-KCl-H~O a t 35’, projected on base data is shown in Fig. 5: Coexistof tetrahedron. ent solid phases have been connected by straight lines and the triangles outlined by them Summary represent the three solid phases Dresent a t the invariant points. The lines and points foilow the consistent noIsotherms have been determined a t 35’ for 1. tation used in the other projections. The Janecke the quaternary system MgClaCaClz-KC1-HzO, diagram for the metastable relations is very similar to Fig. 5 . RECEIVED AUGUST23, 1948
[CONTRIBUTION FROM
THE
DEPARTMENT O F CHEMISTRY, HARVARD UNIVERSITY]
Polarography of Selenium and Tellurium. I. The - 2 States BY JAMES J. LINGANE AND LEONARD W. NIEDRACH’ Investigations of the polarographic behavior of the - 2 oxidation states of selenium and tellurium have not been reported in the literature. In the case of sulfide ion in 1 N sodium hydroxide RevendaIzaand Kolthoff and Miller,z observed a single, well-developed anodic wave due to the reaction Hg S= = HgS 2e. We have found that selenide and telluride ions produce similar anodic waves. The reaction of selenide ion is analogous to that of sulfide ion, i. e., primary oxidation of the mercury of the dropping electrode and the subsequent precipitation of mercuric selenide. In the case of telluride ion, however, we have obtained evidence that the anodic wave results from oxidation of the telluride ion itself to the element, rather than “depolarization” of the dropping mercury anode.
+
+
Experimental The selenide and telluride solutions used for these experiments were obtained by controlled potential electrolytic reduction at a mercury cathode of the purified dioxides whose preparations have been previously described . 3 (1) Allied Chemical and Dye Corporation Fellow, 1947-48. (2) (a) J. Revenda, Coll. Csechoslou. Chem. Commun., 6 , 453 (1934), (b) I. M. Kolthoff and C. S. Miller, THIS J O U R N A L , 63, 1406 (1941). (3) J. J Lingane and L. W. Niedrach, ibid., 70, 1997 (1948).
The reductions were performed in the diaphragm cell shown in Fig. 1 and the cathode potential was controlled against a saturated calomel reference electrode by an automatic potentiostat.‘ The reduction of tellurium(1V) was performed in 1 N sodium hydroxide at a potential of - 1.7 v. vs. the S.C.E., while the selenium(1V) was reduced in a 1 M ammoniacal chloride buffer a t pH 8.0 and a potential of - 1.8 v. The reduced solution in the cell was sometimes used directly for the polarographic measurements, while in other cases aliquots were transferred to supporting electrolytes in a conventional H-type cell. The diaphragm cell in Fig. 1 has been modified from that previously described6 to incorporate a reference electrode as an integral part so that polarograms could be recorded without disturbing the contents of the cell. The cathode chamber is a 250-cc. wide-mouthed erlenmeyer flask, which is joined to the anode chamber and the reference electrode through 30 mm. and 10 mm. medium porosity sinteredglass disks, respectively. Provision is also made for sweeping the solution with an inert gas and for electrical contact with the mercury cathode. The four-hole rubber stopper accommodates the dropping mercury electrode, a propeller type glass stirrer which agitates the mercury-solution interface as well as the solution, an additional gas entry, and a salt bridge to an external reference electrode for use during the electrolysis. To minimize the inclusion of ili drop in the measured cathode potential the tip of the salt bridge must be as close to the mercury as is physically possible.5 A carbon rod served as anode. A layer of agar saturated with potassium chloride was placed on the anode side of the (4) J. J. Lingane, Ind. Eng. Chem., Anal. E d . , 17, 332 (1945). ( 5 ) J. J. Lingane, C. G . Swain and M. Fields, THIS JOURNAL, 66, 1348 (1943).
