SPECIFICCONDUCTANCE AND DENSITYIN FUSED ALKALI METALNITRATES
mained constant. According to the model considered, the chloride ion occupies an anionic hole, in other words, a hole in the anionic cluster surrounding the conducting species is substituted by a chloride ion. This substitution results in a tighter “grip” of the ionic atmosphere on the central metal ion. The escape of the conducting species from its anionic cluster becomes more difficult, and as a result a decrease in the specific conductance is observed. The density, on the other hand, remains constant because the chloride ion forces the nitrate ions further apart. This increa,se in volume is compensated by
2433
the increased mass. Hence, no density change is observed. Effects observed in the other systems can also be explained in the above fashion if one considers not only sizes but relative polarizabilities as well.
Acknowledgment. It is a pleasure to acknowledge the support of the United States Atomic Energy Commission for this investigation. The authors also wish to thank Mr. M. Goffman, of this laboratory, for sacrificing time from his own work to help verify the results of this study.
Specific Conductance and Density Measurements in Fused Alkali Metal Nitrate Systems.
11.
Conductometric Titrations
by Plutarchos C. Papaioannoula and George W. Harringtonlb Department of Chemistry, Temple University, Philadelphia 28, Pennsylvania (Received January 87, 1964)
Conductometric titrations using alkali halides as titrating agents were performed on solutions of Co(I1) and Ni(I1) in LiNOcKN03 and NaNO3-KNO3 eutectics, respectively. The solutions were quite dilute but nevertheless conduction minima were obtained for the expected ratios of transition metal ion :halide ion. I n addition a precipitate was obtained from solutions of CoClz in LiN03-KN03 in the presence of excess chloride. The precipitate analyzed to LirCoClz(N03)4.
Introduction Conductometric titrations have long been used in aqueous solutions to study complex ion formation. I n recent years, the technique has been extended to molten salt solutions.2-s Most of these investigations, however, have centered on systems in which the solvent salt also acts as it complexing agent, i e . , CdC& in KC1. In this inv,estigation the solvent may supply Only a very weak ligand, the nitrate ion* The stronger ion, is added Only complexing agent, the titrating agent. Transition metal complexes have also been investigated s p e c t r o s ~ o p i c a ~ ~ yThe . ~ - ~ re-
sults of these investigations indicate that Co(I1) and Ni(I1) form fourfold complexes in halide melts and (1). (a) Part of the work submitted by P. Papaloannou to Temple University in partial fulfillment of the requirements for the degree of Doctor of philosophy; (b) author to whom all inquiries should be addressed. (2) H. Bloom, D i s c u s s i o ~ sFaraday SOC.,32, 7 (1961). (3) H. Bloom and E. Heymann, Proc. Roy. Soc. (London), A l 8 8 , 3 9 2 (1947). (4) N. Greenwood and I. Worrall, J . Chem. SOC.,1680 (1958).
(5) E. Riebling and C. Erickson, J . Phys. Chem., 67, 307 (1963). (6) D . Gruen and R. MeBeth, &id., 6 3 , 3 9 3 (1959). (7) D. Gruen, J . Inorg. Nuci. Chem., 4 , 74 (1954).
Volume 68, Number 9
September, 1964
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PLUTARCHOS C. PAPAIOANNOU AND GEORGE W. HARRINGTON
excess halide environments. The present investigation essentially supports these results. Experimental The experimental procedures were basically those described in the previous paper. lo The operating procedure was the same except that increments of alkali halide were added to a melt already containing a known amount of cobalt or nickel salt. After each addition the melt was stirred vigorously by bubbling with dry PIT2 to ensure dissolution and homogeneity. During this agitation the platinum bob and capillary arm of the conductance cell were raised out of the melt. When the capillary was returned to the melt it was rinsed several times. Readings were not taken until temperature equilibrium was re-established. Conductivity and density were then measured as a function of temperature for each increment of alkali halide added. A given run was not accepted as valid unless the initial readings reproduced to within O.lyo after each temperature cycle. The appearance of oxide film was also considered to invalidate a given experiment. The results presented below are average values for no less than three runs each.
~~~
~
SOURCE O F C l - : KCI OR
Na CI
SOURCE OF c o ( r ~ i ~ c o c i ~ , ~ MOLE ~ x i o%' ~
0
SPECIFIC CONDUCTANCE
A
DENSITY 185.0'C
ISOTHERMS
1.990
0.240-
1.980
0,230 I
0.220-
0.210 -I
F
1.970
- Co(II):Cl-
1:2
1:3
1:4
RATIO
1:6,
1:7
I:8
1:s
Figure 1. Specific conductance and density us. Co(I1):Cl- ratio in the (Li,K)NOs eutectic.
Results and Discussion The results are presented in tabular and graphic form. The graphs show typical isotherms and the tables give data concerning the temperature dependence. The equations and symbols have been given previously. lo 1. CoClz in Li-K Nitrate Eutectic at l70-,9OO0. Identical results were obtained whether NaC1 or KC1 was used as a source of added chloride. The results are given in Fig. 1and Table 1." At Co :C1 ratios of 1:3.5 to 1:4 a dark blue crystalline solid precipitated out of solution. Further additions of chloride did not cause the formation of more solid. I n order to obtain meaningful results beyond this point and to analyze the solid, the supernatant was analyzed for its Co and C1- content. The solid could not be analyzed directly because it was coated with solvent. Attempts to separate the crystals from frozen solvent by various dissolution techniques were unsuccessful in that both appeared to be equally soluble in the various organic and inorganic solvents tested. Weighed samples of the supernatant were analyzed spectroscopically for cobalt and gravimetrically for chloride. For the cobalt determination, the sample was dissolved in 12 n/r HC1 and its absorbance was measured using a Model DU spectrophotometer a t a wave length of 690 mp. The concentration was then determined The Journal of Physical Chemistry
from a Beer's law plot obtained from standard cobalt solutions in 12 M HC1. The presence of varying amount of LiN03 or K N 0 3 did not interfere with the determination. The supernatant was found to contain 0.5 mole less of cobalt per mole of cobalt originally present and 1.0 mole less chloride per mole of cobalt originally present. It was found that the solid did not form when just K N 0 3was used as solvent. Consequently, solutions of CoClz in pure KN03 were prepared. Excess KC1 was added and the melt titrated with pure LiIS03. The amount of LiN03 necessary to cause precipitation was thus established. The supernatant in this case gave only a faint positive flame test for lithium ion indicating that essentially all the lithium had precipitated in the solid. (8) C. Boston and G. Smith, J . Phys. Chem., 62, 409 (1958). (9) G. Harrington and B. R. Sundheim, Ann. N . Y. Acad. Sci., 950 (1960). (10) P. Papaioannou and G. W. Harrington, J . Phys. Chem., 68, 2424 (1964). (11) Table I and Tables 111-XI have been deposited as Document Number 7997 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the Document Number and by remitting $1.25 for photoprints or $1.25 for 35-mm. microfilm. Advance payment is required. Make checks or money orders payable to: Chief, Photoduplication Service, Library of Congress.
SPECIFICCONDUCTANCE AND DENSITYIN FUSED ALKALIMETALNITRATES
2435
SOURCE'OF ecould not be used as a solute in the Na-K nitrate eutectic. Oxidation to X i 0 occurred so rapidly that meaningful measurements could not be obtained. The salt was, however, considerably more stable in the LiN03-Kn'Oa eutectic. The results of these titrations are presented in Fig. 8 and Tables IX, X, and XI.ll It will be observed that again a certain uniformity occurs regarding the ratio at which the conductance minima occur. The densities vary slightly from one halide to another. The regions beyond the minima are quite different for each of the three halides. Since the solvent is different for the titrations using N i x z and Ni(N03)2as solute, it is not completely reasonable to compare the two as was possible in the case of cobalt. There is, however, much more uniformity in over-all behavior in the Nix, systems than in the Ni(X03)zsystems. The general interpretation given to the data obtained in this study has been in terms of complex ion formation.
OR KX
SOURCE OF N i ( I t ) I N i Xe,8O X I0"MOLE %
1.925
DENSITY I @ ,X-:CI',Bi,F-
3.5301
0.270
-
I.980
ISIS
0.260-
1.970 w
z
c a >.52c
4
0 0
f
z 0.250-
8 Y
L
n 0.SlO-J
0.240
1:2
1'4
1:s
I:'a
-
I:lo
190.7'C ISOTHERMS
0.230-
N I ( a ) : X' RAT.10
-
280.0'C ISOTHERMS
Figure 7 . Specific conductance and density us. Ni(I1):X- ratio in the (Xa,K)Pu'Oaeutectic.
I
1:o
I
1:s
1:10
Figure 8. Specific conductance and density us. Ni(I1):X- ratio in the (Li,K)?JOs eutectic.
Volume 88, h'umber 9
September, 1984
2438
PLUTARCHOS C. PAPAIOANNOC A N D GEORGE W. HARRINGTON
This would appear to be the most reasonable approach in view of the usual significance attached to conductance minima in titrations of this type. Conductivity, however, measures a bulk property, unlike other physical measurements, i.e., spectral, which may be designed to measure only a property of the solute. Consequently, the question which remains to be answered is whether complex ions at the concentrations of this investigation could cause the changes which were observed. It has been shown*O that in simpler systems, ie., soluteeutectic, the partial equivalent conductance of Cox2 is quite large with the sign of this quantity highly temperature dependent. If these result$ have any significance, one of the things they mean is that species such as Coxz will have a relatively large effect on conductivity even a t low concentrations. It seems quite reasonable, therefore, that larger, multiply charged species such as C o X c 2 might have an even greater effect. It will also be noticed that in most of the systems presented, the density changes observed are rela-
The Journal of Physical Chemistry
tively small compared to the changes in conductivity. The exception here is the CoC12-KC1titration in LiIYOsKNO, eutectic. As mentioned above, however, it is possible that gross structural changes may explain the nature of this data. In the other systems the small density changes suggest that no extensive structural changes have occurred. This would further seem to lead one to the conclusion that the ionic mobilities on the principal ions present must be affected rather profoundly by the presence of bulky, multiply charged species such as COX^-^ or NiX4-2. A final answer will have to wait until other types of precise physical measurements are made on systems of this type.
Bclcnowledgments. It is a pleasure to acknowledge support of the United States Atomic Energy Commission for this investigation. The authors also wish to thank Mr. Goffman, of this laboratory, for sacrificing time from his own work to help verify the results obtained in this study.
