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C. Lochmiiller and M. Cefola Fordham Universitv New York, New York
Solubility in Mixed Solvents A radiochemistry experiment
There has been a trend in recent years to introduce experiments into undergraduate freshmen courses which illustrate both fundamental principles and advanced techniques. The following experiment illustrates the use of radioisotopes in one of the earliest classical applications, that of solubility measurement; more important, it demonstrates the effect of solvent dielectric on solubility. The relation of solvent dielectric to its dissociating power goes hack to the work of Thomson' and Nernst.% Denison and RamseyS have constructed a model which assumes that two oppositely charged ions exist either as free ions a t such a distance that the coulombic force between them is negligible or as an associated ion pair. The Bjerrnm-Fuoss ion pair model provides a theoretical approach to using the dielectric constant as a parameter in describing any solvent system. Recently Gentile, Cefola and Celiano' have demonstrated the existence of a linear correlation between pKD values for acetylacetone and the reciprocal of the dielectric constant. A similar relation involving log K,, versus 1 / D is shown by the experiment described herein. Cobalt mercurithiocyanate, CoHg(SCN)t, is prepared following the method of Cuveliers with the modification that W o tracer, sufficient to give 500 to 1000 cpm per aliquot of saturated solution, is added prior to precipitation. The solvent systems used are methanol-water and ethanol-water. The dielectric constants of the systems can be measured directly. If, however, each solvent system is made up to a known weight percent, the dielectric constant values are available in the work of Akerlof.B,7 The solutions are equilibrated in a constant temperature bath a t 25 0.5'C and aliquot portions of the supernatant counted in a well-type scintillation detector. The solubility can then be determined by specific activity measurements. A convenient method for determining the specific activity is to destroy a 100 mg sample of the solid salt with concentrated HNO1, diluted to 100 ml with HzO, and
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This work was supported in part by a grant from the U S . Atomic Energy Commission, under Contract AT(30-11906. THOMSON, J. J., Phil. Mag.,36,313 (1893). a NERNST, W., 2. Physik Chem., 13,535 (1894). a DENISON, J. T., AND RAMSEY, J. B., J. Am. Chem. Sac., 77, 2615 (1955). 4 GBNTILE, P. S., CEFOLA,M., AND CELIANO, A. V., J. Phys. Chem., 67, 1083 (1963). CWELIER,B. V. J., 2. A n d . Chemie, 101,108 (1935). ~ E R L ~G.,P J., Am. Chem. h., 54,4125 (1932). 7 . & K ~ R LG., ~ PAND , SHORT, 0. A., J. Am. Chem. 80%58, 1241 (1936).
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count an aliquot of the solution. The results of a typical experiment are found in the table. Solubilitv of Cobalt Mercurithiocvanate CoHdSCNh Solubility (all)
K,, X lo6
When the log K,, is plotted against 1 / D , shown in the figure, a linear relation is found to exist in the range of 40 to 80 dielectric units. Beyond these limits the simple relation of K,, = f(l/D) breaks down. This experiment was also performed in a radiochenlistry laboratory course with the modification that PhIz labeled with '3'1 was employed. Comparable results were obtained. There is no reason to believe that MgNH9Or6Hz0 labeled with 82P cannot be substituted for the systems mentioned above although its use would necessitate beta counting. SEMI-LOG PLOT K r p X lo6 v s l l D X lo3 AT 2 5 5 0 . 5 ' C .
