Counterdiffusion and interaction of barium and sulfate ions in a gel

Abstract The conditions of precipitation and rate of growth of a BaSO 4 precipitate in a 1.5% agar gel were determined by a method involving the formation of Liesegang bands in a counterdiffusional system. In several experiments diffusion and precipitation were allowed to proceed for times ranging from several hours to two days after the appearance of the first BaSO 4 band. The values obtained for a calculated quantity c /Δt , which was directly related to the zeroth-order rate constant for the growth of the band, indicated that the growth of the band is diffusion-controlled rather than reaction-controlled. Data from experiments in which the gel was removed several hours prior to the appearance of the first BaSO 4 band revealed that barium ions and sulfate ions began to interact several hours before the appearance of the first band. The flux derivative ratios, calculated from the data obtained in these experiments, imply that the BaSO 4 precipitation is diffusion-controlled. The BaCl 2 and Na 2 SO 4 diffusion coefficients were determined in noncounterdiffusional systems.