Nitrates in ice: uptake; dielectric response by the layered capacitor method

Abstract

Ice columns of 0.20–0.25 m length and 0.038 m diameter were grown in the laboratory from dilute solutions (10–5 to 10–2 N) of potassium, sodium, or ammonium nitrate at a constant freezing rate of 0.002 m h–1 and stirring at 300 rpm. The distribution coefficient was computed at ~0.015 m intervals as the ratio of nitrate concentration in the melted ice and in the liquid phase as a function of interface position. The average distribution coefficients were (2.25 ± 0.55) × 10–4 for the potassium and sodium nitrate, and (6.1 ± 1.4) × 10–3 for the ammonium nitrate samples — about a 27-fold increase. These results are in line with other large anions such as sulfate and methanesulfonate that were previously investigated. The dielectric relaxation spectrum of ice slices (~0.012 m thick and sandwiched between thin fluoroplastic foils) was measured between –1 and –85°C at frequencies from 1 Hz to 100 kHz with a lock-in amplifier technique. First, the ice response was recovered from the (Maxwell–Wagner) layered capacitor. The dielectric relaxation ranges were then separated and their characteristic parameters computed. The complex conductivity (Grant plot) and the conductivity frequency-response plot have been the most useful tools for this purpose. Both (alkali-metal and ammonium nitrate) sample groups exhibit the Debye dispersion of polar molecules so characteristic for ice regardless of impurity content. There is also a dispersion range at lower frequencies, and a static or quasi-static conductivity. In the alkali-metal nitrates the low-frequency dispersion is a prominent space-charge dispersion, and the temperature-dependent interaction between orientational and ionic point defects in the ice lattice leads to the conductivity crossover phenomenon. Ammonium greatly reduces the ionic lattice defects responsible for space charge and static conductivity; there is no crossover. Both the Debye dispersion and the crossover support the concept of co-operative responses by the polar molecules making up the ice substance to physico-chemical stimuli. PACS Nos.: 61.72, 77.22G, 77.22J, 81.30F