Preparation of Colloids

1. Chemical Methods

• Oxidation: \[\mathrm{SO}_2+2\mathrm{H}_2\mathrm{S}\xrightarrow{\mathrm{Oxidation}}3\mathrm{S}\downarrow+2\mathrm{H}_2\mathrm{O}\] (sulphur sol)

• Reduction: \[2\mathrm{AuCl}_{3}+3\mathrm{HCHO}+3\mathrm{H}_{2}\mathrm{O}\xrightarrow{\mathrm{Reduction}}2\mathrm{Au}\downarrow+3\mathrm{HCOOH}+6\mathrm{HCl}\] (gold sol)

• Hydrolysis: \[\mathrm{FeCl}_3+3\mathrm{H}_2\mathrm{O}\xrightarrow{\text{Hydrolysis}}\mathrm{Fe(OH)}_3\downarrow+3\mathrm{HCl}\] (iron hydroxide sol)

2. Electrical Disintegration — Bredig's Arc Method

• An electric arc is struck between electrodes of the metal immersed in the dispersion medium
• The intense heat produces metal vapour that condenses to form colloidal particles

3. Peptization

• A precipitate is converted into colloidal sol by shaking it with the dispersion medium in the presence of a small amount of electrolyte (the peptizing agent)