![]() Values of IEP are obtained by measuring the zeta potential as a function of pH and identifying the pH at which the zeta potential value crosses zero. In most cases this is achieved by interpolating the experimental data. Textbook values of IEP are often not useful for practical work since the value of IEP can change dramatically with even a small amount of impurity that is driven to the sample surface. At pH values lower than the IEP, the particle surface charge is positive and at pH values higher than the IEP, the particle surface charge is negative. One rule of thumb for stable suspensions is to ensure that the pH is one full pH unit away from the IEP. Typically, and for good reason, H + concentration is discussed in terms of pH. pH has a strong effect on the surface charge of many types of particles. In addition, pH is a parameter that is often and readily changed in a formulation. For these reasons, the effect of pH on particle surface charge is often studied. One number that characterizes a surface is the isoelectric point, IEP, or point of zero charge, PZC, which refers to the conditions, often pH, at which the particle surface charge is zero. Zeta potential is a function of both the particle surface chemistry and the suspending medium chemistry (1). The ions that are at the particle surface and controlling surface potential are a function of the concentration and nature of the ions in the bulk liquid. In addition, the concentration of ions affects the distance over which charge effects persist. For example, a significant amount of dissolved salt will shield the electrostatic interactions between particles. Some ions, known as specific ions will prefer to stick to the particle surface as the concentration of these ions increases. Examples of specific ions include H + and polyvalent ions. In this work, the effect of H + concentration on particle surface charge is studied. Other examples of the effect of various ion concentrations can be found in (2) and (3). ![]()
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