5.20:
Colloidal precipitates
Colloidal precipitates have a large surface-to-mass ratio that encourages surface adsorption.
For example, colloidal silver chloride obtained by adding excess silver nitrate has a primary adsorption layer of silver ions and a secondary counter-ion layer of nitrate ions.
This electric double layer prevents particles from colliding and coagulating into larger particles, stabilizing the colloidal suspension.
Heating while stirring decreases adsorption and increases the kinetic energy of the particles. This enables them to overcome the electrostatic repulsion and coagulate.
Alternatively, adding an electrolyte shrinks the double layer, allowing particles to coalesce spontaneously at the critical coagulation concentration.
Washing the precipitate can decrease the electrolyte concentration and cause the particles to revert to their dispersed state in a process called peptization.
This can be prevented by washing with a non-interfering electrolyte.
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.