Experimental Study And Model Analysis Of Complex Precipitation Pattern

The transition of precipitation wave with stationary Liesegang rings could be performed in the sodium thiosulfate/silver nitrate system. Structural formation of moving precipitation pattern and Liesegang precipitation pattern can regulated.Novelty and variety precipitate patterns were observed for the first time in Na2S2O3-Ag NO3 reaction. Precipitate patterns evolved from Ag2S2O3 precipitate pulse to the coexistence of Ag2S2O3 precipitate pulse with stable Ag2 S precipitate, finally transformed to the Ag2S2O3 precipitate pulse with Liesegang type of Ag2 S precipitate. The precipitate patterns formed with different initial concentrations were studied in the thesis, in which the relationship of the position and width of the pulse type Ag2S2O3 precipitate with reaction time changed from linear to oscillated type. The linear relation of the time law of the precipitate pulse was deviated as decreasing the initial concentration of outer electrolyte. The migrating rate of precipitate pulse was increased with enhancing the concentrations of outer electrolyte and was decreased with the increasing concentration of the inner electrolyte.The effect of Gel types and p H value on the pattern formation of Na2S2O3-Ag NO3 reaction was studied. Thiosulfate becomes unstable as decreasing the p H value of Gel medium, and decomposed to element sulfur which could impact on the pattern forming process, such as the yellow dispersion of sulfur behind the migrating gray precipitation. Secondary precipitate structures were present as adjusting the p H 4.0 of the Gel. The precipitate pattern can convert to reaction front after we increase the initial concentration of Gel. Replacing of Gel by agar, homogeneous brownish black precipitate pattern was produced instead of traditional Liesegang rings.The effect of reaction temperature on the dynamic behaviors of the precipitate pulse was studied. The propagation rate of the precipitate pulse was increased at higher reaction temperature, and then the relevant constant K in the time law was increased. The radius of curvature of the groove can affect on the migrating rate of the precipitate pulse with direct ratio relationship. The diffusion of outer electrolyte from the out-lying of petri dish to the center of the dish containing Gel medium with inner electrolyte could performed the “precipitate eye” phenomena.Model simulations based on the Ostwald’s supersaturation theory could represent the pattern evolutions of the precipitation pattern observed in the experiments. Three type of precipitate patterns were performed through changing the initial concentrations of inner and outer electrolytes. The precipitate pulse followed the time law(Xn~tn1/2)and the spacing law in the experiments. The migrating rate of precipitate pulse was dominated by the initial concentrations of inner and outer electrolytes. the average speeds of the precipitation pulse could follow the()A BV μLn C C. There is weak influence of concentration gradient on the morphology of the precipitate patterns. The constant of the time law is different but the linear relationship still exist when the concentration gradient of inner electrolyte was exist.Model simulations containing two supersaturated loops and basing on the Ostwald’s theory to investigate all the features and behaviors of the precipitation patterns in chapters four were performed, which could represent the pattern evolution of the precipitation changes observed in the experiments.We investigated precipitation pattern formation in a medium in which the source substance was inhomogeneously distributed. Depending on the imposed concentration gradient and the location where the second substance was added, various new precipitation structures such as bifurcated segments, budding patterns, fragment patterns, dislocations and Liesegang rings segregated with broken circles were observed. Dependence of those new structures on the solubility of precipitate, supersaturation level, and nucleation and aggregation thresholds were systematically characterized. Our calculation illustrates that the presence of an imposed concentration gradient, the location of Xn of the nth band and its time of formation tn still satisfies a simple spacing and time law, Xn~tn1/2. Through the study of the influences of cutting, obstacles on the Liesegang patterns, we found the behaviors of Liesegang patterns, including self-cure, diffraction, interference phenomena.