Molar conductivity of sodium fluoride in aqueous solution at 25°C. Applications of Pitts' conductivity equation

Abstract

The conductivity of aqueous solutions of sodium fluoride at 25°C has been measured over the concentration range 103c= 0.8–200 mol l.–1 The results for this salt and for five other electrolytes have been analyzed by means of Pitts' full equation, modified to allow for possible incomplete dissociation of the electrolyte. The dissociation constant of a weak electrolyte such as acetic acid can be determined uniquely, independent of the value of the distance parameter a, assigned to it in Pitts' equation. But with less associated electrolytes the dissociation constant KD depends on the value of a which is used; reasonable values of a(3–7 A) give values of KD for iodic acid between 0.165 and 0.155 mol l.–1 Potassium hexafluorophosphate and sodium fluoride, and even hydrochloric acid and potassium chloride, can be treated as associated electrolytes; only for an electrolyte such as hydrochloric acid, for which there is independent evidence of virtually complete dissociation, can an unequivocal assignment to the class of “completely dissociated electrolytes” be made. For potassium fluorophosphate and sodium fluoride, activity coefficient data suggest that treatment as moderately associated salts is to be preferred.