Abstract: Using HOAc and H₂SO₄ as acidifying agents, the relationship of the gelation time and pH of the solution containing SiO₂/Na₂O ratio of 1.0, 2.0 and 3.0, has been studied.The curve which expresses the relationship is of N-shape. At the minimum point of the curve, the pHis equal to pK₁,at maximum point, the pHis equal to (pK₀ + pK₁)/2 and on the right side of the minimum point where , the pHis equal to (pK₁+pK₂)/2. Therefore, from the structure of N-shape curves, dissociation constants of various species of silicic acid can be obtained.The results indicate that the dissociation constants of silicic acid in gelation process vary with the acidiffying agent, total salt concentration and the degree of polymerization of silicic acid, that when the total salt concentration of the solution maintained constant and the acidfying agent is same, the dissociation constants of silicic acid would not change with the concentration of the acid, and that the pK₁ of silicic acid and ionic strength of its solution bear a linear relationship, in which the slop represents coordination energy of anion of the corresponding salt with silicic acid and the intercept represents dissociation constant of silicic acid in the gelation process when the salt effect is eliminated.