Point of Zero Charge

Point Of Zero Charge

The point of zero charge (pzc), in physical chemistry, is a concept relating to the phenomenon of adsorption, and it describes the condition when the electrical charge density on a surface is zero. It is usually determined in relation to an electrolyte's pH, and the pzc value is assigned to a given substrate or colloidal particle. For example, the pzc of solid FeOOH is 9. In other words, pzc is (usually) the pH value at which a solid submerged in an electrolyte exhibits zero net electrical charge on the surface.

A related concept in electrochemistry is the electrode potential at the point of zero charge.

The value of pH is used to describe pzc only for systems in which H+/OH- are the potential-determining ions (which is the common case). Generally, pzc is the value of the negative decimal logarithm of the activity of the potential-determining ion in the bulk fluid. For example, the charge on the surface of silver iodide crystals may be determined by the concentration of iodide ions in the solution above the crystals. Then, the pzc value of the AgI surface will be described by the concentration of I- in the solution (or negative decimal logarithm of this concentration, pI-).

When the pH is lower than the pzc value, the system is said to be "below the pzc." Below the pzc, the acidic water donates more protons than hydroxide groups, and so the adsorbent surface is positively charged (attracting anions). Conversely, above pzc the surface is negatively charged (attracting cations/repelling anions).

Point of zero charge is of fundamental importance in surface science. For example, in the field of environmental science, it determines how easily a substrate is able to adsorb potentially harmful ions. It also has countless applications in technology of colloids, e.g., flotation of minerals.

At pzc, the colloidal system exhibits zero zeta potential (i.e., the particles remain stationary in an electric field), minimum stability (i.e., exhibits maximum coagulation/flocculation rate), maximum solubility of the solid phase, maximum viscosity of the dispersion, and other peculiarities.