Purification of Colloidal Solution

A freshly prepared colloidal solution usually contains an excess of electrolytes and other soluble impurities left over from its preparation. A small amount of electrolyte is actually necessary to keep a colloidal solution stable, but too much electrolyte causes coagulation (Section 11.9.5), so purification -- reducing the amount of impurities present to the minimum needed for stability -- is an essential step. This is carried out by dialysis: the colloidal solution is placed inside a bag made of a suitable membrane (such as parchment paper or an animal membrane) and suspended in a vessel through which fresh water is kept continuously flowing (the apparatus is called a dialyser, shown in Fig 11.7, attached to this section). The small dissolved ions and molecules of the impurity electrolyte diffuse out through the membrane into the surrounding flowing water, while the larger colloidal particles themselves cannot pass through the membrane and are left behind, purified, inside the bag. This same size-based sieving is one of three general properties of colloidal systems noted here: (i) a colloidal system is heterogeneous, made of two distinct phases (dispersed phase and dispersion medium); (ii) dispersed-phase particles pass only slowly through parchment paper or an animal membrane, yet …