Concentration dependent solute redistribution at the ice–water phase boundary. III. Spontaneous convection. Chloride solutions

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Abstract
Spontaneous convection in the ice–water system was investigated with temperature and concentration measurements. Earlier work had shown that mild convection causes an apparent concentration dependence of the distribution coefficient that masks any real concentration dependence. Convection was found to be determined primarily by the density inversion at the 4 C isotherm. A simple method of evaluating the height of convection cells was developed which enables one to obtain an order‐of‐magnitude estimate of the distribution coefficient. Forced convection (stirring at 300 or 1000 rpm) and improved sampling and analysis techniques were used to redetermine the distribution coefficients of chlorides in ice. Solutions of HCl, LiCl, NaCl, KCl, RbCl, CsCl, and NH4Cl were investigated in the concentration range 10−6–10−2M. For the hydrogen and alkali chlorides no effect of the different cations on the chloride distribution coefficient was evident. Its average value is 2.7×10−3. For the ammonium chloride, the distribution coefficient is 1.4×10−2. The chloride distribution coefficient is not affected by solution pH, nor by an electrical interface potential. The chloride distribution coefficient is nearly or completely independent of concentration. This contrasts with the distribution coefficients of alkali fluorides which are strongly concentration dependent and much greater than those of the chlorides. These results point to different structural relationships of chloride and fluoride impurities with the ice lattice. However, dielectric relaxation and electrical conductivity of ice containing chloride and fluoride, respectively, are quite similar. According to a classical concept, each solute particle introduces into the ice lattice electrical point defects of specific type and number. The distribution coefficient enables one to determine these defects. We conclude from our results that this concept needs to be re‐examined.