H. BRAUNSTEIN, J. BRAUNSTEIN, AND D. INMAN
2726
The Association of Cadmium Ion and Bromide Ion in Molten Potassium Nitrate and in Molten Sodium Nitrate’
by Helen Braunstein, Jerry Braunstein,2Eand Douglas Inma@ Department of Chemistry, University of Maine, Orono, Maine
(Received November 1, 1966)
The electromotive force was measured of cells consisting of a Pd-PdO-CdO indicator electrode and a reference half-cell in solutions of Cd(NO& and K(Na)Br in the molten solvents KN03, NaN03, and an equimolar mixture of K N 0 3 and NaN03. The association constant of CdBr+, calculated from the activity coefficients ( y ~ of) Cd(N03)2,was K1 (moles of solvent per mole): 650 in KNOs at 358”, 625 in NaN03 a t 331”, and 1500 in (Na,K)N03 a t 258”. The specific Helmholtz free energies of association, calculated from the association constant with the quasi-lattice model, were - AA (kcal per mole) : 5.88 in KNOa, 5.62 in NaNO3, and 5.83 in (Na,K)NO3. The results in (Na,K)N03were shown to be consistent with previous measurements of the activity coefficients of (Na,K)Br using an Ag-AgBr indicator electrode. I n the absence of diriuclear species formation and a t low solute concentrations the thermodynamic association constant may be estimated (if insufficient data are available for extrapolation to infinite dilution) by the relation K1 S se/(l - sORM)where so is the slope, at zero ligand concentration (RL)and a t the concentration RMof solute cation, of a plot of In l / y ~vs. RL.
Introduction Association constants and Helmholtz free energies of association of CdBr+ in molten nitrate are here reported as part of an investigation of solvent effects on association in molten salt s ~ l u t i o n s . A ~ ~s~y s k matic study of the solvent effect requires the use of a number of different experimental methods and a selfconsistent analysis of the data if significant results are to be ~ b t a i n e d . ~Electromotive ,~ force methods are among the most accurate and convenient when suitable reversible electrodes are available. In molten nitrates, the silver electrode has been the only suitable electrode of the first kind and has been used extensively Silverfor the study of the association of silver silver halide electrodes have been used for the study of associations of halide ions with lead or cadmium ions in solutions where the solubility of the silver halide is sufficiently l 0 ~ . 3 3 - 1 0 Inman5r1l has demonstrated the applicability of an electrode of the third kind-palladiuni-palladium oxide-cadmium oxide-to the study of association equilibria of cadmium ion with halide ions in molten sodium nitrate-potassium nitrate eutectic. The Journal of Physical Chemistry
In this paper, we present a detailed comparison of the evaluation of the association constant of CdBr+ in molten equimolar sodium nitrate-potassium nitrate ~~
(1) Based on an h1.S. thesis submitted by H. Braunstein, National Science Foundation Cooperative Fellow, to the Graduate School of the University of Maine. This work was supported in part under U. S. Atomic Energy Commission Contract No. AT(30-1)-2873 with the Vniverstty of Maine; Report No. NYO-2873-11. The work was
initiated during a stay at the Molten Salt Laboratory at Northampton College of Advanced Technology, London, by two of the authors (H. B. and J. B.). (2) (a) To whom correspondence should be addressed at Reactor Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. ( b ) Department of Chemistry, Northampton College of Advanced Technology, London. (3) J. Braunstein and A. S. LIinano, Inorg. Chem., 3 , 218 (1964). (4) C. Thomas and J. Braunstein, J. P h y s . Chem., 68, 957 (1964). (5) (a) D. Inman, Electrochim. Acta, 10, 11 (1965); (b) D. Inman, I. Regan, and B. Girling. J . Chem. Soc., 348 (1964). (6) J. Braunstein, 31. Blander, and R. Lindgren, J. Am. Chem. Soe., 84, 1529 (1962).
(7) M. Blander, F. F. Blankenship, and R. F. Newton, J . P h y s . Chem., 63, 1259 (1959). (8) J. Braunstein and R. & Lindgren, I. J . Am. Chem. Soc., 84, 1534 (1962). (9) R. W. Laity, “Electrodes in Fused Salt Systems,” in D. J. G. Ives and G. J. Janz, “Reference Electrodes,” Academic Press, New York, N. Y., 1961.
ASSOCIATIONOF CADMIUM AND BROMIDEIONS
~
Ag,AgBr
KNO3
(Xa,K)NOa, (Na,K)N03 (AgBr) (reference)
1
1 (Xa,I
