Preparation of Colloids

Colloids can be prepared by three broad methods. (a) Chemical methods: double decomposition, oxidation, reduction or hydrolysis reactions that produce a water-insoluble product whose molecules then aggregate together into a colloidal sol, rather than precipitating outright as a bulk solid. Three worked reactions are given: SO2 + 2 H2S --[oxidation]--> 3 S (colloidal sulfur) + 2 H2O; 2 AuCl3 + 3 HCHO + 3 H2O --[reduction]--> 2 Au (colloidal gold) + 3 HCOOH + 6 HCl; and FeCl3 + 3 H2O --[hydrolysis]--> Fe(OH)3 (colloidal ferric hydroxide) + 3 HCl. (b) Electrical disintegration -- Bredig's arc method (Fig 11.6, attached to this section): this method, used for metals such as gold, silver and platinum, involves both vaporisation and condensation. An electric arc is struck between two electrodes of the metal, both immersed in the dispersion medium (typically kept cold, e.g. with ice); the intense heat of the arc vaporises some of the metal, and that vapour then condenses back down through the cooled medium as fine colloidal particles. (c) Peptization: a precipitate is converted into a colloidal sol simply by shaking it with the dispersion medium in the presence of a small amount of an electrolyte, called the peptizing agent. During peptization, the precipitate adsorbs one of the electrolyte's ions onto …