Determination of Adsorption Isotherm of Lead (II) Ion onto the (100) Face of a Sodium Chloride Crystal by the Measurement of Growth-Starting Supercooling

The growth-starting supercooling, Δ T G, a supercooling at which a crystal starts to grow when the solution is cooled continuously after an impurity has been adsorbed onto the surface of the crystal, was measured for the (100) face of a sodium chloride crystal in the presence of a lead (II) impurity. From the data of Δ T G measured when adsorption was in equilibrium, the Langmuir constant was deduced as K = 9.35 × 10 5 ± 1.57 × 10 5 (mol/L) -1, which was in good agreement with a literature value ( K = 9.93 × 10 5 ± 2.6 × 10 5 (mol/L) -1 ) obtained from ordinary direct adsorption measurements. This agreement indicates that the measurement of Δ T G can be a convenient method to study adsorption behavior of an impurity onto a specific surface of a crystal in supersaturated solution. This agreement also corroborates the soundness of the Kubota-Mullin model for the impurity action on crystal growth [Kubota, N.; Mullin, J. W. J. Cryst. Growth 1995, 84, 509−514], on which the theoretical treatment in this study is based. The average spacing L between the active sites for impurity adsorption was evaluated to be about 40 times the linear dimension of the growth unit.