Chemistry · Ch 5 — Surface Chemistry
Purification of Colloidal Solutions
Purification of Colloidal Solutions
A freshly made colloidal solution usually carries excess electrolyte and other soluble impurities. A trace of electrolyte is needed for stability, but too much coagulates the sol, so the impurities must be reduced to a minimum. This purification is done in three main ways.
(i) Dialysis
Removing a dissolved substance by diffusion through a suitable membrane. Ions and small molecules of a true solution can pass through parchment, cellophane or animal membrane, but colloidal particles cannot. As Fig. 5.9 shows, a membrane bag of colloidal solution is suspended in a vessel of continuously flowing fresh water; the small molecules and ions (crystalloids) diffuse out, leaving pure sol behind. The apparatus is a dialyser.
(ii) Electro-dialysis
Ordinary dialysis is slow. If the impurity is an electrolyte, applying an electric field speeds it up. As Fig. 5.10 shows, electrodes are fitted on either side of the membrane bag, and the ions migrate out to the oppositely charged electrodes — the anode drawing the anions, the cathode the cations.
(iii) Ultrafiltration …
What this figure shows. Apparatus diagram of dialysis. An outer beaker holds water (light blue hatched region). Water enters the beaker at the lower left (a leader line labelled 'Water' with an inward arrow at the bottom left) and exits at the upper right (an outlet spout with an arrow, labelled 'Water + Crystalloid'). Suspended inside the beaker is a bag/vessel made of dialysing membrane (a rounded flask/bag shape, its neck at top) — a leader line at the top labels it 'Dialysing membrane'. Inside the bag are the colloidal contents shown as small circles: larger/ open circles labelled 'Sol particle' (leader line on the right) that stay inside, and small dots labelled 'Crystalloid' (leader line at the bottom) that diffuse out through the membrane …
What this figure shows. Apparatus diagram of electro-dialysis. An outer beaker holds water (light blue hatched region); a leader line labelled 'Water' with an inward arrow enters at the lower left. Suspended inside is a bag of dialysing membrane (rounded flask/bag shape with neck at top) — labelled 'Dialysing membrane' by a leader line at the top; it contains the colloidal solution shown as circles ('Sol particle', leader line bottom-left) and small dots ('Crystalloid', leader line bottom-right). At the top right an outlet is labelled 'Water + Electrolyte' with an outward arrow. Two electrodes are fitted, one on each side of the membrane bag inside the outer water compartment: the left electrode carries a '+' and is labelled 'Anode', the right electrode carries a '–' and is labelled 'Cathode'. Horizontal arrows point outward f …