The Molar Volume of Partially Associated Electrolytes Exemplified by CdSO₄ in H₂O  CH₃CN‐Mixtures

Abstract

Apparate und Methoden / Elektrocheniie / Elektrolytlosungen / Ionenpaare / Thermodynarnik Our model for the volume of partially associating electrolytes is based on the limiting law of Redlich and Rosenfeld for the electrolyte fraction of free ions and on the law of ideal mixing for the ion pairs. The resulting analytical density formula contains the partial molar volumes of the ions at infinite dilution VF and of the ion pairs V, . These two parameters can be evaluated by a least squares method, if the association constant is determined independently. This has been done conductometrically for the test system CdSO, in H,O-acetonitrile (AN)-mixtures. A magnetic suspension balance, reaching a sensitivity of 0, = 2 . lo-' g ~m -~, proved to be well suited for the density measurements. Data at 25 "C are presented for solutions with AN-contents up to 21.3 mole percent and CdSO,-concentraJions between 4 . and 4 . lo-' mol dm-3. -The values of V" (CdSO,) are not equal to the intercepts obtained by conventional 1 c-extrapolations.The low values of VF and of V, indicate volume saving solvation arrangements. A reevaluation of published density data of CaSO, and of MgSO, yields an almost identical value for the volume change of ion pair formation and agrees well with results calculated from the pressure dependence of the association constant. -Die Auswertung zeigt, da13 die Kurvenform des scheinbaren Elektrolytpartialvolumens in der konventionellen I//f-Auftragung keine Extrapolation zulabt. Die Grobe der Vr-und VA-Werte deutet auf enge Solvatpackungen. Die Volumenanderung der Ionenpaarbildung stimmt mit neu berechneten Literaturwerten von CaSO, und MgSO, sowie mit Resultaten der Druckabhangigkeit der Assoziationskonstanten iiberein. Im verwendeten Volumenmodell des teilweise assoziierten Elektrolyten ist angenommen, dab das Grenzgesetz nach Redlich und Rosenfeld fur den ionisierten Teil des Elektrolyten giiltig ist, wahrend dem Assoziat ideales

Electrostriction in Electrolyte Solutions

“…Another point to be considered is the determination of the theoretical slope, S V DH , of eq for the mixed solvent, requiring knowledge of (∂ln ε r /∂ P ) T and of κ T , for proper extrapolation of the apparent molar volumes calculated from the densities to infinite dilution. This was demonstrated for mixtures of acetonitrile and water by Schwitzgebel, et al, where S V DH is approximately linear with the mole fraction of the acetonitrile.…”

“…In the case of cadmium sulfate in aqueous acetonitrile, Schwitzgebel et al 154 used independently obtained association constants (from conductivity) to calculate the fraction associated with neutral ion pairs that presumably do not cause major electrostriction. The dissociated ions, however, cause large electrostriction, P V i ¥ being -64 ( 9 cm 3 3 mol -1 at a mole fraction of acetonitrile of 0.2132 (but at an only slightly lower mole fraction, 0.2092, P V i ¥ is much less negative, -36 ( 4 cm 3 3 mol -1 , a fact that was not explained).…”