Methods to Effect Coagulation

Lyophobic sols can be coagulated in five ways. (i) By electrophoresis: the charged colloidal particles migrate toward the oppositely-charged electrode, are discharged on contact, and precipitate. (ii) By mixing two oppositely-charged sols: mixed in roughly equal proportions, the opposite charges neutralise each other and both sols precipitate together -- e.g. mixing hydrated ferric oxide (a positive sol) with arsenious sulfide (a negative sol) precipitates both; this specific case, where two different sols mutually precipitate each other, is called mutual coagulation. (iii) By boiling: boiling a sol increases the frequency of collisions between the dispersed particles and the medium's molecules, which disturbs the particles' adsorbed (charge-carrying) layer, reduces their charge, and eventually causes them to settle out as a precipitate. (iv) By persistent (prolonged) dialysis: extended dialysis removes essentially all of the small amount of electrolyte that was actually needed to keep the colloid stable, and once that stabilising electrolyte is gone the colloid becomes unstable and eventually precipitates. (v) By adding electrolytes: adding an excess of electrolyte precipitates the colloidal particles directly. The Hardy-Schulze rule governs how effective a given electrolyte's ion is at causing this: in general, the greater the valence (charge) of the flocculating ion (the ion carrying charge opposite to the sol's particles), the greater its power to cause prec …