The zeta potential of colloidal silica nanoparticles is highly dependent on the pH of the surrounding medium. The zeta potential is a measure of the electrostatic potential at the slipping plane of a particle, which is the boundary between the particle's surface and the surrounding liquid. It is an important parameter in determining the stability of colloidal systems, as it influences the degree of repulsion between particles.Silica nanoparticles have silanol Si-OH groups on their surface, which can ionize depending on the pH of the medium. At low pH, the silanol groups are protonated Si-OH2+ , giving the particles a positive surface charge. At high pH, the silanol groups are deprotonated Si-O- , resulting in a negative surface charge. The isoelectric point IEP of silica nanoparticles, where the zeta potential is zero, typically occurs around pH 2-3.The effect of pH on the zeta potential of colloidal silica nanoparticles can be measured using electrophoretic techniques, such as capillary electrophoresis or laser Doppler electrophoresis. In these techniques, an electric field is applied across a suspension of the nanoparticles, causing them to migrate towards the electrode with the opposite charge. The velocity of the particles is proportional to their zeta potential, and can be measured using either a detector in capillary electrophoresis or by analyzing the Doppler shift of scattered laser light in laser Doppler electrophoresis.To determine the effect of pH on the zeta potential, the electrophoresis experiment is performed at different pH values. The zeta potential is then plotted as a function of pH, allowing for the identification of the isoelectric point and the overall trend of zeta potential as a function of pH. This information can be used to optimize the stability of colloidal silica suspensions by selecting the appropriate pH for a given application.