The effect of surface deprotonation of colloidal silica particles on settling mode transition has been studied quantitatively. We observed the settling mode transitions of silica aqueous suspensions by adding a pH controlled CaCl2 electrolyte solution as a coagulant. Depending on the solution pH, three types of settling modes were observed, i.e. "no settling mode", "free settling mode" and "hindered settling mode". In these observations, a large pH drop occurred immediately after mixing of the coagulant. To describe the pH drop, we developed a simple mathematical model consisting of a 1-pK Stern model and a mass balance equation of protons on silica particles. The model shows excellent agreement with experiments in the number of dissociated protons from a silica surface (n(p)). Furthermore, the boundaries of the three setting modes in the pH-phi phase diagram are also well explained by the value of n(p). Our results clarify that the extent of colloidal instability is quantitatively described by n(p.)