Additivity relationships in the magnetic susceptibility of diamagnetic salts

Abstract

The magnetic mass susceptibilities of salts of ammonium and potassium have been measured in the solid state and in a series of solutions, as well as the single salts of magnesium, zinc, cadmium, lithium and thallium. Additivity relationships suggested by earlier workers for the potassium, ammonium and magnesium halides and sulphates were confirmed both in the solid state and in solution. The molar susceptibilities of these salts in solution are generally about 1 to 2% higher than for the solid state. The molar volumes and molar refractions were also determined from density and refractive index measurements on most of the solutions investigated. 2. Susceptibility measurements on double salts of ammonium and potassium with magnesium and zinc were made. Additivity relationships in these salts in the solid state and in solutions were found to be valid. Comparing the molar susceptibility of the double chlorides of ammonium and potassium with magnesium at the same ionic strengths, it was found that in the case of the double salts containing the potassium ion the calculated molar susceptibility and molar volume are slightly greater than the experimental. 3. The susceptibility of the salt was found to varywith the concentration of the solution for most of the salts studied. In the case of ammonium, potassium and magnesium salts the susceptibility of the solute showed only a slight change, a fall in susceptibility with increasing concentration being observed. The susceptibility of the salt calculated from the susceptibility of solutions of zinc, cadmium, lithium and thallium salts showed a marked variation with the concentration, the susceptibility concentration curves passing through a minimum after which the susceptibility of the salt rises again. A marked change in molar volumes with increasing concentration for this group of salts was also noticed. 4. These changes with increasing concentration ofthe solute have been correlated with changes in the mean stoichiometric activity coefficient of the salt and resemblances between the changes in the magnetic susceptibility and the activity coefficient noted. The effect of ion pair formation and complex ion formation has been considered and the influence of ionic hydration on the activity coefficients has been estimated in some cases. By correction of the activity coefficient of the anhydrous salt to that for the solvated salt the curves of molar susceptibility and activity coefficient when plotted against the ionic strength of the solutions were closely parallel.