Interdiffusion of aqueous silver nitrate and potassium chromate and the periodic precipitation of silver chromate Liesegang bands

Abstract

AbstractPeriodic bands of crystals (Liesegang rings) are formed when a pair of soluble salts interdiffuses and reacts to form an insoluble salt. A classic example is the banded precipitation of silver chromate produced by the interdiffusion of aqueous silver nitrate and potassium chromate: 2AgNO3(aq)+K2CrO4(aq) = Ag2CrO4(s)+2KNO3(aq). This process is studied by using Taylor dispersion to measure interdiffusion coefficients, including cross‐coefficients, for aqueous AgNO3+KNO3 and K2CrO4+KNO3 solutions found on opposite sides of AgNO3/K2CrO4 diffusion boundaries. Silver chromate readily supersaturates to 5 times its solubility. This information together with the measured diffusion coefficients and previously reported rates of precipitation are used to develop a diffusion‐precipitation model for the formation of silver chromate Liesegang rings. Numerical integration of the equations employing a Gaussian distribution of critical supersaturation values shows that a sharp supersaturation limit is not essential for periodic precipitation, and secondary band structure is predicted.