April, 1958
COMPLEX IONS IN FUSED SALTS
value. At a temperature of 25’ the values of k‘ and k“ have been determined and the values of lcl are known with a precision of *2%; hence, by a trial and error procedure extreme values which E /and E” can have were found. From these calculations it has been possible t o show that E’ = 15.8 f 0.5 kcal. The uncertainty in the value of E“ is considerably larger; the value obtained was 16.0 f 2.5 kcal. From expressions of the transition state theoryjBvalues of the heat, free energy and entropy of activation were computed for the reaction between plutonyl ion and trivalent vana(6) S. Glasstone, K. Laidler and H. Eyring, “The Theory of Rate Processes,” MoGraw-Hill Book Co., Inc., New York, N. Y., 1941, p. 417.
417
dium ion in terms of the principal species, i.e., PUO~+~ V+3+ (activated complex) H+. These quantities are AF* = 17.03 f 0.01 kcal./mole, AH* = 15.5 f 0.4 kcal./mole and AS* = -5* 2 cal./deg. The magnitudes of the rate constants k’ and k“ show the relative importance of the two paths which appear to be indicated in the reaction between plutonyl ion and vanadium(II1). Thus a t 25’ about 90% of the over-all reaction occurs through the path in which a single hydrogen ion is liberated. Acknowledgment.-The author wishes to express his appreciation t o Drs. J. F. Lemons, Charles E. Holley, Jr., and Thomas W. Newton for valuable discussions and interest shown in this research.
+
+
CQNPLEX IONS I N FUSED SALTS BY F. R. DUKEAND M. L. IVERSON Contribution No. 545 rom the Institute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Zowa. A r k was performed in the Ames Laboratory of the U.8. Atomic Energy Commission Received October 31, 1967
The complex formation constants for a series of bivalent metal halides were determined using fused KN08-NaNOs eutectic as solvent. The method involved measuring the increase in solubility of the slightly soluble metal chromate as halide ion was added.
The existence of complex ions in fused salts has been fairly well substantiated by previous studies. The main sources of evidence have been freezing point depression,’ conductance,2 spectrophotometric3 and electrometric titration4 studies in fused salts. Solubility data have been used in aqueous solutions to study complex ions, e.g., cadmium chlorides and silver chloride6 complex ions. In the present research the effect of alkali halides upon the solubility of slightly soluble chromate salts in molten sodium nitrate-potassium nitrate eutectic is interpreted in terms of complex ion formation. Experimental ACS reagent grade chemicals were used. The chromate salts of barium, calcium, lead and magnesium were precipitated from aqueous nitrate solutions with a potassium chromate solution. Cadmium chromate was prepared by mixing solutions of stoichiometric amounts of cadmium nitrate and potassium chromate dissolved in the eutectic solvent of KNOrNaNOs. The solvent was then drawn off and the precipitate equilibrated with a second portion of the fused solvent. After the second portion had been drawn off, the eutectic solvent-cadmium chromate residue was analyzed and used for the cadmium chromate solubility studies. The solubilities of the chromate salts in the fused sodium nitrate-potassium nitrate eutectic were determined in the following manner. An amount of the chromate salt in excess of its solubility was added to 50 g. of the fused solvent in a test-tube. The test-tube was immersed in a fused salt bath maintained a t the desired temperature &lo. The -
(1) E . Kordes, W. Bergmann and W. Vogel, Z. Elektrochem., 55, 600 (1951); E. Kordes, G. Ziegler and H. Proeger, ibid., 58, 168 (1954); E . Van Artsdalen, THISJOURNAL, 60, 172 (1956). (2) H. Bloom and E. Heymann, Proc. Roy. SOC.(London), A M s , 392 (1947). (3) D. hf. Gruen and P. Graf, -4bstract of Papers, 131st meeting of the -4merican Chemical Society, April, 1957. 52R. (4) 9. N. Flenglaas and E . K. Rideal, Proc. Roy.SOC.(London),8233, 442 (1956). (5) E . King, J . Am. Chem. Soc., 71, 319 (1949). (6) J. H. Jonte and D. S. Martin, ibid., 74, 2052 (1952).
solutions were stirred from 15 to 30 minutes and then the excess chromate salt was allowed to settle. Usually one hour was sufficient settling time. A medicine dropper was used to withdraw samples which were then analyzed for their chromate and chloride content. The absence of undissolved particles of the chromate salts in excessive amounts was reasonably assured by visual observation of the molten sample. The solutions were then stirred again for one-hour periods and samples again withdrawn. The constancy of the chromate concentration within the experimental error of the measurement indicated that the desired equilibrium was attained. The equilibrium temperatures were approached from above and below for the PbCrOa determinations. All the other values for solutions containing halide ions were obtained approaching the equilibrium temperature from below. The chromate content of the samples was determined following the diphenylcarbazide colorimetric method.? For the high concentrations of chromate in the more soluble cases, the iodine-thiosulfate methods was used.
Results and Discussion The solubility of lead chromate in a fused KN03n’aNO8 eutectic solvent increases with increasing sodium chloride content up to the limiting solubility of sodium chloride as shown in Fig. 1. A similar increase in solubility with addition of sodium bromide is shown by the data of Table I. The solubility of cadmium chromate jn the same solvent also increases with the addition of either chloride or bromide as shown in Table 11. The solubilities of the chromate salts of magnesium, calcium and barium are not affected significantly by the addition of halide ions as shown by the data in Table 111. The increase in solubility of a salt such as lead chromate in fused I(N03-NsN03 eutectic solvent on adding chloride can be interpreted on the basis (7) Snell and Snell, “Colorimetric Method of Analysis,” Vol. 11, D. Van Nostrand Co., Inc., New York, N. Y., 1926, p. 274. (8) Kolthoff and Sandell. “Textbook of Quantitative Analysis,” The Macmillan Co., New York, N. Y., 1943, p. 624.
F. R. DUKEAND M. L. IVERSON
418
Vol. 62 TABLE I11
SOLUBILITY OF ALKALINEEARTHCHROMATES IN FUSEI) KN03-Naso3 EUTECTIC COXTAINIXG HALIDEIONS Solubility, m X 102
Chromate salt
ihCrOa
CaCrOl
MgCrOa
250' 0.55 & 0.09 .44
-J m
Solvent
300 ' 0.20 & 0.02 .25 .27 1 . 3 i: 0 . 1 1.4
.67 .47 .55
0.8-
3
Pure eutectic 0 . 4 1 m NaCl I . 00 m NaCl Pure eutectic 0.64 m NaBr Pure eutectic 0.36 m KC1 0.71 m KCl 0.23 m NaBr Saturated NaBr soh.
chloride concentration according to the expression Ks' = Ks I1
+ Pi(C1-) + Pz(Cl-)' +
-
~ ~ ( ~ 1 - 1 3...I
6-25OoC
where Ks' is the apparent sohbiiity product of lead chromate, Ks is the solubility product of lead chromate in the pure solvent, and Pn is the constant.
e-275 X
-300
=
o
0.1 02 0.3 0.4 0.5 0.60.7 0.8 N a C l (molality).
a9 1.0
1.1
(1)
1.2
Fig. 1.-Effect of added sodium chloride on the solubility of lead chromate in fused sodium nitrate-potassium nitrate eutectic.
(PbCl,' -") (Pb++)(Cl-)n
The curves of Fig. 1represent the equation S = Ks'*/2
{ K s [I
+ Pi(C1-) + P*(Cl-)*
+ ... I ) '1%(2)
where X is the solubility of the chromate salt. The change in solubility with respect to chloride LEADCHROMATE SOLUBILITY IN SODIUM NITRATE-POTAS- concentration is given by the derivative TABLE I
SIUM
NITRATEEUTECTICCONTAINIKG SODIUMBROMIDE
Sodium bromide,
m
PbCrOa solubility0 (mg./g. solvent) 2600 2750 300'
- dS d(Cl-)
7
Ks [PI
+ 2Pz(C1-) + ...I 2s
(3)
It is apparent from the curves shown in Fig. 1 that one must start with MX+ as the first complex ion species since the slope of the curve as the chloride concentration goes to zero is some positive value. The highest complex species which needs to be considered to explain the data is the MXSion. This does not necessarily rule out the existence of others nor prove absolutely the existence of MX+, MX2 and MXa-. Exceedingly accurate data are required to settle this as an absolute and unique answer because of the numbers of adjustTABLE I1 able parameters involved in such a solution. The CdCrOl SOLCBILITY DEPESDENCE ON TEMPERATURE AND values for stability constants of the various comHALIDECONCENTRATION IN KNOa-NaNOa EUTECTIC plex ions shown in Table IV were determined by the NaBr (and NaC!) graphical method of obtaining successive interCdCrO4 solubility ( m X 102) conrn. In eutectic solvent, m 250' 300' c e p t ~ . The ~ estimated probable errors in the 0.00 0.59 f 0.07 0.80 f 0.07 values for MX+, MX2 and MX3- are 20, 50 and .10 1.2 1.7 100~o, respectively. .20 1.7 2.6 The effect of halides on the solubility of lead 3.5 .30 2.3 chromate and cadmium chromate in moiten alkali .40 2.9 4.4 nitrates is good evidence for the formation of .50 3.5 5.3 stable complexes of the type MXn2-" where n is one, a Values taken from a smoothed curve through the origitwo and three. The data show that the cadmium nal data. Accuracy of data is approximately f150j,. ion forms slightly more stable complex ions with of complex ion formation. If one assumes that chloride and bromide .than does lead. It is evistable complex ions of the type PbC1n2-nare formed, dent that the trend is toward a decrease in stability (9) I. Leden, Z. physik. Chem., A188, 160 (1941). the solubility of lead chromate should vary with the 0.0 0.071 0.13 0.21 .05 .10 .18 .27 .10 .14 .22 .32 .20 .22 .30 .44 .30 .29 .39 .55 .40 .38 .48 .69 .50 .48 .59 .85 .60 .59 .72 1.04 Values taken from smoothed curves through original data.
