pH plays a significant role in the electrokinetic phenomena in colloidal systems, particularly in affecting the zeta potential and electrophoretic mobility of colloidal particles.Zeta potential is the electric potential at the interface between the slipping plane shear plane and the bulk liquid surrounding a colloidal particle. It is an important parameter that influences the stability of colloidal systems. A higher absolute value of zeta potential indicates a more stable colloidal system due to the increased electrostatic repulsion between particles, which prevents aggregation.Electrophoretic mobility is the velocity of a colloidal particle under the influence of an electric field. It is directly related to the zeta potential and is used to determine the zeta potential experimentally.The effect of varying pH on zeta potential and electrophoretic mobility can be explained as follows:1. Surface charge modification: The surface charge of colloidal particles depends on the ionization or dissociation of functional groups present on their surface. When the pH of the solution changes, the degree of ionization of these functional groups also changes, which in turn alters the surface charge of the particles. For example, at low pH, acidic functional groups e.g., -COOH will be protonated, while at high pH, basic functional groups e.g., -NH2 will be deprotonated. This change in surface charge affects the zeta potential and, consequently, the electrophoretic mobility of the particles.2. Counterion distribution: The distribution of counterions in the diffuse layer surrounding the colloidal particles is also affected by pH. At low pH, there will be a higher concentration of positively charged counterions e.g., H+ in the diffuse layer, which can screen the negative surface charge of the particles, reducing the zeta potential. Conversely, at high pH, the concentration of negatively charged counterions e.g., OH- will be higher, which can screen the positive surface charge of the particles, again reducing the zeta potential. This change in counterion distribution affects the electrophoretic mobility of the particles.3. Isoelectric point IEP : The isoelectric point is the pH at which the zeta potential of a colloidal particle is zero. At this pH, the colloidal system is least stable due to the absence of electrostatic repulsion between particles, leading to aggregation. The IEP is specific to the type of colloidal particles and their surface chemistry. By varying the pH, one can determine the IEP and understand the stability behavior of the colloidal system.In summary, varying the pH of a colloidal system can significantly affect the zeta potential and electrophoretic mobility of the particles by altering their surface charge, counterion distribution, and stability behavior. Understanding these effects is crucial for optimizing the stability and performance of colloidal systems in various applications, such as drug delivery, water treatment, and food processing.