The variation, with concentration and temperature, of the absorption, in aqueous solutions, of electromagnetic waves at ultra high frequencies

Abstract

A new, differential method is described, suitable for detecting small changes in the conductivities of dilute, aqueous ionic solutions at a frequency of 3x109c/s. The method essentially depends on measuring the change in 'q' of a resonant cavity containing a fine quartz tube, when the solution under investigation and water are passed through the tube in a continuously alternating stream, at a frequency of one complete change-over in eight seconds. The corresponding change in signal, picked up by a probe detector penetrating into the cavity, is viewed on a Cathode Ray Oscilloscope, after passing through suitable amplification and filtering processes. Measurements are described on a range of dilute solutions of potassium, sodium and lithium chlorides over a temperature range from 18°C to 25°C. For calibration purposes, the method has been used, in a modified form, for more concentrated solutions up to concentrations of 0.5 Normal. Assuming, from published results of other authors, that at high concentrations there is no difference between the ultra high frequency values of the ionic conductivities of these salts and the conductivities measured at low frequencies, the measured U.H.F. conductivities at low concentrations were compared with the corresponding D.C. values. They were found to exceed these by as much as 30%. This difference is in excess of the difference predicted by the theory of Falkenhagen, which has hitherto not been investigated at these frequencies. A new theory, which would explain the results obtained, is discussed in the final chapter; some justification for the new assumptions is obtained by using them to interpret the results of other workers for the variation of low-frequency dielectric constant of saline solutions with concentration.