Temperature and Pressure Coefficients of Ionic Conductance in N ,N-Dimethylformamide

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Abstract
The limiting conductance of a number of salts in N ,N‐dimethylformamide (DMF) was determined from measurements of the electrolytic conductance, in the range 1–100×10−4M, 25°—65°C and 1–1900 atm. The results are discussed in terms of transition‐state concepts. An important parameter is the heat of activation at constant volume Ev, which is substantially smaller than the usually determined heat of activation at constant pressure Ep. Both Ev and Ep are independent of temperature. Ev, which decreases with increase in ion size, is sensitive to the density (packing) of the solvent and, in the range investigated, varies inversely with the free volume. A change in the dependences of Ev and of the pre‐exponent on solvent volume is observed at a volume of about 75 ml mole−1. At densities greater than this, Ev extrapolates to infinity (i.e., the conductance process ceases) at about the normal freezing point. Below this density, Ev would become infinite at about the close‐packed solvent density. In harmony with the behavior of Ev, the pre‐exponent is a simple function of ion size and solvent density at densities less than 75 ml mole−1; at greater densities, its behavior is more complex. The volume of activation ΔV‡, which is between 8 and 12 ml mole−1, increases with solvent volume and ion size and decreases with increases of temperature. The value of ΔV‡ for picric acid suggests that the H+ ion is monosolvated in DMF.