Propagation Properties of the Precipitation Band in an AlCl₃/NaOH System

When inherently immobile solid particles collectively form precipitates in a reaction-diffusion system involving a redissolution reaction, a propagation phenomenon may occur in which a dynamic pattern of precipitation bands forms. This propagating precipitation phenomenon has been studied by many researchers. However, two completely different processes-i.e., the reaction-diffusion of reactants and the crystal growth of products-progress simultaneously in the system, thereby rendering the phenomenon complex. There are no well-established experimental laws for this propagating precipitation phenomenon, such as the spacing, time, and width laws associated with the well-known Liesegang phenomenon, which is static in the sense that precipitation bands form and remain at the same position. In fact, it has not been clarified which of the processes controls the propagation phenomenon. Accordingly, we have investigated the apparent diffusion coefficient associated with the dynamics of propagating precipitation band in an AlCl3/NaOH system for the case in which a large excess of outer electrolytes (i.e., OH(-)) diffuses into gel in which inner electrolytes (i.e.,Al(3+)) are homogeneously distributed. An isolated precipitation band of Al(OH)3 was formed horizontally in a test tube and propagated vertically in proportion to the square root of time. In our experimental results, we found that the apparent diffusion coefficient, D(p), possesses an exponential dependence on the initial concentrations of the outer electrolyte, and the inner electrolyte; the measured relation was D(p) = D[Al(3+)](-0.6)[OH(-)](0.6), where D = (0.63 ± 0.04) × 10(5) cm(2)/s. From our model equations based on the prenucleation theory, which take into account a redissolution reaction, we found that the dynamics of the reaction front of the outer and the inner electrolytes was an important factor in controlling the propagation of the precipitation band. In our simulation results, we obtained a similar dependence of the apparent diffusion coefficient on the electrolyte concentrations.