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The solidus curve for the system H2O–NH4F was partly determined by means of chemical analyses, heating curves and a dilatometric method, and partly estimated from the liquidus curve using a thermodynamic relation. The solubility limit thus estimated is roughly 10%. The dielectric relaxation times, τ, of solid solutions of ice containing 0.002 to 10 weight percent NH4F were measured at temperatures of about —60° to —10°C. The activation energies for dipole rotation were only about 4 kcal/mole as compared with 13 kcal/mole in pure ice. At low concentrations the frequency factor increases with increasing concentration of NH4F. At high concentrations of NH4F this trend is reversed.

No abstract

Previous suggestions that dielectric relaxation in ice may occur by means of proton shifts have been refuted by Bjerrum. Yet, the role of ions in this relaxation process cannot be ignored in view of the more recent studies on ice-NH4F solutions, which indicate that solute ions can accelerate the rotation of ice molecules at a distance of 30 A or more from any ion. Simple electrostatic calculations show that proton jumps in ice would entail the formation of ``Bjerrum fault sites.'' In the case of pure ice, this might explain why: (a) the proton jump conductivity is much lower than in water at 0°C; and (b) the concentration of fault sites seems to be rather high, according to Bjerrum. Estimates of the frequencies of proton jumps and of activation energies show no disagreement of the proposed explanations with known facts. In the case of ice-NH4F solutions, proton jumps occurring between second-neighbor ion pairs, F—–H2O–NH4+, may also induce fault sites at the neighboring molecules. This ion-pair mechanism would agree with the low activation energy for these rates and with the peculiar concentration dependence of the frequency factor. Finally, the thickness of the ion atmosphere should, at equilibrium, be less in ice-NH4F solutions than in aqueous solutions of the same concentration. This, together with the above, might account for the changes with time in the dielectric relaxation rates of concentrated ice-NH4F solutions observed immediately after freezing.

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