5.20: Colloidal precipitates
The high insolubility of some precipitates can result in an unfavorable relative supersaturation. This can lead to colloidal particles with a large surface-to-mass ratio, where adsorption is promoted. For instance, in the precipitation of silver chloride, silver ions are adsorbed on the surface of the colloidal particles, forming a primary layer. This layer attracts ions of opposite charge (such as nitrate ions), forming a diffuse secondary layer of adsorbed ions. This electric double layer prevents colloidal particles from colliding and coagulating into larger particles and stabilizes the suspension.
The coagulation of particles in a colloidal suspension can be enhanced by heating with stirring. This decreases adsorption and increases the kinetic energy of particles to overcome the electrostatic repulsion, enabling coagulation. Alternatively, the addition of an electrolyte can shrink the electrical double layer. At a critical coagulation concentration of electrolyte, the particles can coalesce spontaneously.
Following filtration of a coagulated colloid, washing with pure solvent can decrease the electrolyte concentration below the coagulation value, causing the particles to revert to their dispersed state. This process is called peptization and can be prevented by washing with a non-interfering electrolyte that can be removed by volatilization, such as nitric acid for silver chloride.
