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.