Justification for the colloid explanation of Liesegang ring formation

 The persuasive evidence for the role of colloid in the formation of Liesegang rings is nullified by the low diffusion constants (less than 2 × 10⁻¹¹ m² s⁻¹) of sol particles; however, those values were obtained for sols suspended in otherwise homogeneous solutions. The essential randomness of Brownian motion in such situations is absent in Liesegang experiments, where the large excess of outer electrolyte diffusing into the gel creates a bias in molecular bombardment resulting in sol particles moving a given distance in fewer steps, hence in a shorter time. From Einstein's equation (D=x ²/2t) values for D of 2–4 × 10⁻¹⁰ m² s⁻¹ have been calculated for Liesegang experiments in the literature. It is suggested that such values could well pertain to sol particles diffusing in the heterogeneous conditions existing in those experiments. Received: 13 April 1999 Accepted: 16 November 1999