Chemistry · Ch 10 — Surface Chemistry
Purification of Colloids
Purification of Colloids
Whatever method is used to prepare a colloidal solution, the resulting sol commonly carries dissolved electrolyte and other soluble impurities left over from the preparation process, and if these impurities are not removed, they tend to destabilise the sol and cause it to coagulate (precipitate) prematurely. Purification aims to remove exactly these impurities — mainly electrolytes — while leaving the genuinely colloidal particles behind, and rests throughout on one simple physical fact: colloidal particles are far too large to pass through a semipermeable membrane, while dissolved ions and small molecules pass through it freely. Three standard techniques are used.
- Dialysis. In 1861, T. Graham separated dissolved electrolyte from a colloidal solution using a semipermeable membrane (a dialyser): the colloidal solution is placed inside a bag made of this membrane, and the bag is suspended in a trough of continuously flowing water. The small electrolyte ions diffuse out through the membrane (down their own concentration gradient) into the surrounding water and are carried away by the flow, while the much larger colloidal particles are retained inside the bag. A medically important application: kidney malfunction lets electrolyte concentration build up to toxic levels in the blood, and dialysis treatment recycles a patient's blood through a considerable length of semipermeable tubing immersed in an isotonic saline solution, removing the excess electrolyte in exactly the same way.
- Electrodialysis. Applying an electric field considerably speeds up the removal of electrolyte impurities compared with ordinary dialysis. The impure colloidal solution is placed in a central compartment bounded on either side by dialysing membranes, with two outer compartments filled with water and fitted with electrodes; passing a current drives the impurity's cations and anions to migrate, through their respective membranes, into the two separate water compartments, from which they are periodically flushed out. Because the applied electric field actively pulls the ions out, rather than relying purely on passive diffusion as in plain dialysis, electrodialysis achieves the same purification considerably faster. …
What this figure shows. A central compartment holding the impure colloidal solution is bounded on either side by dialysing (ion-selective) membranes — a cation-transfer membrane on one side and an anion-transfer membrane on the other — beyond which sit two outer compartments filled with pure water, each in contact with an electrode (cathode on one side, anode on the other) connected to a power supply. When current is passed, the cations of the dissolved impurity (e.g. ) migrate through the cation-transfer membrane toward the cathode into one water compartment, while the anions (e.g. ) migrate through the anion-transfer membrane toward the anode into the other water compartment; both are then periodically flushed out with the flowing water. Because the applied electric field actively drives the impurity ions out rather than relying on passi …