In the removal of silicate and impurities in the acid treatment of high-alumina fly ash, the transition from silica sol to gel in the presence of metal cations has a major role in filtration and product quality. In this work, an online physical analysis method that can continuously measure zeta potential, pH, and viscosity is established for low-concentration colloidal systems. The change in zeta potential can be explained by Derjaguin.Landau.Verwey.Overbeek (DLVO) theory and model of electric double layer, which is verified by the morphology of colloids in different periods. The influences of metal cations with different valences and contents on the transition from silica sol to gel are investigated in detail. Results show that trivalent ions have a positive role in the delayed coagulation because of their hydrolysis and complexation in relatively low concentrations. By contrast, divalent and monovalent ions have a negative role in the transition from sol to gel because of the bridging effect. In terms of concentration, divalent ions have greater influence on the transition from sol to gel than monovalent ions. This established method is adaptable for the analysis of a low-concentration colloidal system and could be applied in the acid treatment of silicon-containing solid wastes. (C) 2015 Elsevier B.V. All rights reserved.