9992
J. Phys. Chem. 1995, 99, 9992-9995
Solubility of CoC12 in Molten NaCl-AlCl3 Peter J. Tumidajski" Institute for Research in Construction, National Research Council of Canada, Ottawa, Ontario. Canada KIA OR6
M. Blander Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received: January 31, 1995; In Final Form: April 6,1995@
The solubility of CoC12 in molten NaCl-AlCl3 was determined by aliquot sampling at 255 "C for acidic and basic solvent compositions. There was a pronounced compositional dependence of the solubility with a minimum at approximately the equimolar solvent composition. The solubility minimum was 1.04 x mol fraction of CoC12. The solubility in the highly ordered NaCl-AlC13 solutions was described in chemical terms. The true solubility product for CoC12 was calculated as 1.50 x 10-l5. In basic and in somewhat acidic melts, the solubility was primarily related to the formation of the associated ionic species CoCl', CoC12, CoC13-, and C O C ~ ~ The ~ - . calculated formation constant of CoC1+ was 9.48 x lo6. Furthermore, the specific bond free energy for the associated complex, CoCl+, was -64.43 kJ mol-' for a coordination number of four.
Introduction
AlC1,-
Molten chloroaluminates are ordered ionic liquids with unusual physicochemical properties. For example, sharp solubility minima and activity coefficient maxima for solutes in NaC1-AlC13 exist at the equimolar composition^.^.^ The solute activity coefficients are very large and can decrease by more than 1 order of magnitude as the composition changes from 50 to 60 mol o/o AlC13. A large solute activity coefficient often indicates an insolubility. In fact, solutes which interact with NaCl and AlC13more weakly than NaCl and interact with each other, are often insoluble at the equimolar composition. The effect in NaC1-AlC13 is pronounced because the excess free energy of mixing of NaCl and AlC13 is negative and the absolute value is substantially greater than the excess free energy of mixing of the solute with NaCl or AlC13. This paper represents a further contribution on the nature of ordering in ionic liquids and its influence on the solubilities of metal chlorides in NaCl-AlCl3.'-" There is topological and chemical order in molten NaClAlC13. The liquid is most ordered at the equimolar composition where NaCl and AlC13 combine to form a low melting liquid composed mainly of Na+ and AlCld-. Except for the structural complexity of AI&-, the liquid is a simple molten salt with coulomb ordering (e.g., ...+-+-+-...), and cations and anions have a very strong tendency to be nearest neighbors almost exclusively. Structurally AICL- is a tetrahedron with a central A13+ and four C1- ions at the comers. Consequently, the topological order is defined by a repetitive sequence of next nearest neighbor Na-A1 pairs expressed in one dimension as (...Na-Al-Na-Al-Na-Al...). Chemical order is related to the structural definitiveness of the polyatomic anions present. As AlC13 is added to equimolar NaCl-AlCl3, AlzC17- forms, @
Abstract published in Advance ACS Abstracts, May 15, 1995
+ AlC1, = A1,C17-
K = 2.8 x lo4 at 255 0C435(1)
Ninety-nine percent of the AlC13 added to NaAlC14 is present as A12C1.1- at X N ~ C=I 0.49, well over 90% for X N ~ C=I 0.37, and about 80% for X ~ ~ = c l0.33. Consequently, NaC1-AlC13 is a solution of NaC1, NaAlC14, and NaA12C17 with some A12C16 that is negligible at X N ~ C>I 0.43 and 4% at = 0.375.4 As NaCl or AlC13 is added to NaAlC14, C1-, andor Al2C17- substitutes for AlC14. The chemical disorder is reflected by the extent of disproportionation of the polyatomic anion. For example, the equilibrium constant of eq 2 indicates that NaA1C14 2AlC1,- = C1-
+ Al,C17-
K = 9.08 x lo-' at 255 "C4 (2)
is a highly ordered liquid. The advantage of the chemical approach is that the ordering phenomenon is expressed by an additive ternary solution of NaC1, NaAlC4, and NaA12C17. Such a description of ordering is precise because of the near ideal behavior of mixtures of three salts with a common cation and different anions.
Experimental Section The experimental procedure was described p r e v i o ~ s l y . ~ . ~ Polarographic grade NaAlC14, AlCl3, NaC1, and CoC12 were obtained from Anderson Physics Laboratory, Inc. (Urbana, IL). The experiments were performed in an argon-filled glovebox where the oxygen and water contents were maintained below 5 ppm. The fumace was a Glascol pot furnace (Model TM572) fitted to an Omega power proportional (Model CN-2011) temperature controller. The solubility apparatus consisted of a 1 L pyrex resin reaction jar. The ground glass cover had ports for an alumelchrome1 thermocouple, a fused silica stirrer, materials addition, and aliquot sampling. Intially, about 200 g of NaAlC14 were melted in the resin reaction jar. An excess amount of CoC12 was added to the melt, and the solution was stirred for several
0022-3654/95/2099-9992$09.00/0 Published 1995 by the American Chemical Society
J. Phys. Chem., Vol. 99, No. 24, 1995 9993
Solubility of CoC12 in Molten NaCl-AlC13
TABLE 1: Composition of the CoC12-NaCl-AIC13 System Saturated with Solid CoClz at 255 "C
of formation reactions is generated: Co2+
0.5151 0.5113 0.5072 0.5027 0.4999 0.4639 0.4308 0.3964 0.3668 0.3377 -4
I
10-3 10-3
lo-' 10-3 lo-? lo-?
I
I
0.4824 0.4872 0.4920 0.497 1 0.5000 0.5354 0.5674 0.6007 0.6293 0.6572 I
"
CoCl, coc1,-
+ C1- = C0C13+ c1- = coc1,2-
K, I
(34
KI2
(3b)
K13
(3c)
K14
KllKl2Kl3 =
(44
Z(Z - 1)(Z - 2 ) c o l l P I 2 P l 3 - 3PllPl2 + 3!
Kl lKl2KI3Kl4 =
-
(34
The free Co2+ion is solvated by nearest-neighbor AlCL- anions in this formalism. The concentration dependence of the solubility of CoC12 depends only on the formation constants and stoichiometries of the defined chloro species of cobalt. The formation constants are related to the specific bond free energies, AA I,, through the following statistical mechanical relations: l 4 Kll =z(plI - 1)
-7-
-8
+ C1- = CoCl,
CoCl'
-
-6
E
I
loW3
-
-5
-
4.84 x 3.07 x 1.57 x 2.51 x 1.04 x 1.48 x 4.11 x 7.41 x 1.05 x 1.51 x
0.4801 0.4857 0.4913 0.4970 0.4999 0.5346 0.565 1 0.5962 0.6227 0.6472
+ C1- = CoC1'
Z(Z - 1)(Z - 2)(Z - 3) 4! (PllP12P13P14 -
-9
where Z is the coordination number for the cobalt cation, and
;
-10 0.45
/31nis related to MI,,,through the relation I
t
I
I
0.50
0.55
0.60
0.65
solvent 'AICI,
Figure 1. CoCh solubilities at 255 "C.
hours. Stirring was stopped 1 h before sampling which allowed the solids to settle. Subsequently, aliquots of the melt were taken for chemical analysis. Then NaCl or AlC13 was added to the melt, and the sampling process repeated. The concentration of cobalt in the sample was determined by inductively coupled plasma-atomic emission spectroscopy.
Values of the specific bond free energies for each of the four successive chloride additions to the coordination shell of cobalt cation are different. The total cobalt dissolved in the NaCl-AlCl3 melt is the sum of the metal concentrations tied up in the various ionic species and any uncomplexed free metal ions: XCo(tota1)= @Z+ + ~ C o c l + ? ~cocI,-+ ~coc1,2-
(6)
where N represents ionic fractions. The apparent solubility product of CoC12 is given by
Results and Discussion The mol fractions of NaCl, AlC13, and CoC12 at 255 "C are given in Table 1. The logarithm of CoC12 solubility is plotted against solvent composition in Figure 1. The CoCl2 solubilities show a pronounced compositional dependence. The CoC12 at ~~~~~~t = 0.50 and increases solubility is 1.04 x dramatically as the solvent becomes more basic or acidic. The CoC12 solubilities increase by a factor of 150 when the solvent composition is made 15% more acidic and by a factor of 50 for a basic shift of 2%. The pronounced dependence of the CoC12 solubilities with composition is related to the relatively strong bonding interactions between NaCl and AlC13. A chemical model is described below to interpret the results. The UV-vis spectra of the CoC12-NaCl-AlCl3 system was reported by Newman et a1.I2 and for the CoC12-KCl-AlC13 system by Oye and Gruen.I3 The presence of associated species was indicated in both studies. The chemical description considers all four of the formation constants for the associated 4. Consequently, a series species, CoCln2-" where, n = 1
-
pparent
K'p
2 - XCo(tota1FCi-
(74
Combining eq 2 and eq 7 gives an alternative expression for
1vA12C1,-
The true solubility product is given by Ksp = hfE+Nc,-2
(8)
Combining eqs 4 and 6-8 gives the following expression for the ratio of g r tto Ksp: Kapparent FP i~~~ =
1
+ K , , N ~ ,+_ K , , K , , N ~ , -+~ K, IK12K13NC1-3 + Kl IK12K,3K14NCl-4 (9)
Tumidajski and Blander
9994 J. Phys. Chem., Vol. 99, No. 24, 1995 TABLE 2: Ionic Fractions for the CoC12-NaCl-AIC13 System at 255 "C NC1In Kappare"' NhaNco2NNCI,- NAI~CI~CP 8.51 x lo-' -9.605 0.9907 9.31 x lo-' 0.9149 8.93 x 6.13 x lo-? -10.704 0.9940 5.96 x lo-' 0.9386 1.30 x 0.9969 3.09 x lo-' 0.9626 2.25 x 3.74 x IO-? -12.351 0.9875 7.13 x 1.25 x lo-' -16.380 0.9995 4.98 x 0.9981 7.67 x lo-' 1.18 x lo-' -21.968 0.9998 2.07 x 4.57 x lo-' -30.340 0.9968 3.18 x lo-' 0.8549 0.1451 1.61 x -31.344 0.9906 9.45 x lo-' 0.7128 0.2872 6.11 x lo-' -32.616 0.9817 1.83 x lo-' 0.5500 0.4500 2.34 x lo-' -34.120 0.9721 2.79 x lo-? 0.3946 0.6054 0.2409 0.7591 6.94 x lo-' -36.114 0.9571 4.29 x 1
8.0X10'~
.
1
.
1
.
1
TABLE 3: Chemical Model Parameters
CoClz 1.50 x FeClf 4.78 x AgClb 7.35 x IO-'
9.48 x lo6 1.55 x 10' 29.9 7.8 4.03 x lo6 3.13 x lo4 89.8 8.1 1.36 x lo6
-64.43 -60.67 -47.50
"Reanalysis of data in ref 9. T = 175 "C." 8.0Xl0.5
.
1
6.0x10"
-
4.0~10"
m
i
p' I
x
P
,,
-
,
ci ,
P
?
::
Y
-
2.0~10"~
,'
KSvapparent=1 . 5 0 1 ~ 1 0 "+~ (1.4223x10')Nc,' l
0.00
.
1
0.02
.
1
.
1
0.04
.
0.06
1
.
0.08
1
0.10
NU' 0.0k '0
I
0
-
l
1x10'
.
l
2x10'
-
l
3x10'
.
l
4x106
=
5x
4,Figure 2. Plot of versus Ncl- at 255 OC. Broken line represents a linear representation of the data according to eq 10. Acidic compositions only.
