Electric Dipole Moments in Polar Solvents. I. Audiofrequency Measurement with Slightly Conducting Solutions

Audiofrequency methods were used to measure dielectric constants of dilute solutions containing electrolytes up to free-ion concentrations of 10(-4) M. Using a calibrated transformer bridge, capacitance was measured with an accuracy of 0.03 pF at a conductance of 100 mu mho and within 0.3 pF at 800 mu mho. Evaluation of the double-layer capacitance from the frequency dependence of the data is described. The effect of the free ions on the dielectric constant is found to be relatively large and in reasonable agreement with the prediction of the Debye-Falkenhagen theory. The calculation of the electric dipole moment for polar solutes, including ion pairs, is discussed in terms of Kirkwood's theory. Experimental tests are described for finding out whether possible deviations of Kirkwood's correlation factors from unity may be neglected. These tests involve changing the solvent and the temperature. The tests were satisfied for the ion pairs of tetraisoamylammonium nitrate and for nitrobenzene in chlorobenzene and acetic acid.