Purification of Colloidal Sols
Dialysis, Collodion Filters, and the Zero-Electrolyte Trap.
When a colloidal sol is synthesized (especially lyophobic sols via chemical methods or peptization), it usually contains large amounts of soluble impurities, primarily electrolytes (ions). While a tiny trace of electrolyte is essential to stabilize the sol, an excess of electrolytes will cause the sol to coagulate and precipitate out. Therefore, purification is necessary.
1. Dialysis and Electrodialysis
Dialysis is the process of separating particles of a true solution (crystalloids/ions) from a colloidal sol by means of diffusion through a suitable membrane.
Colloidal particles ($1-1000\text{ nm}$) cannot pass through parchment paper, animal membranes (bladder), or cellophane sheets. However, true solution ions ($\lt 1\text{ nm}$) can pass through easily. A bag made of this membrane is filled with the impure sol and suspended in a vessel of continuously flowing fresh water. The ions diffuse out, purifying the sol.
Electrodialysis (Speeding it up)
Ordinary dialysis is a very slow process. If the dissolved impurities are strictly electrolytes (charged ions), the process can be massively accelerated by applying an electric field. This is called Electrodialysis.
Figure 1: Electrodialysis speeds up purification by using an electric field to pull ions out of the dialyzing bag.
2. Ultrafiltration (The Collodion Trap)
Normal filter paper cannot stop colloidal particles because its pores are too large (much greater than $1000\text{ nm}$). To separate them, we must shrink the pores of the filter paper.
Preparing an Ultrafilter:
An ultra-filter paper is prepared by soaking standard filter paper in a specific chemical mixture to partially clog the pores, and then hardening it.
1. Collodion Solution: A $4\%$ solution of Nitrocellulose in a mixture of alcohol and ether. Soaking the paper in this shrinks the pores.
2. Hardening: The paper is then soaked in Formaldehyde (HCHO) to harden the nitrocellulose, making it rigid enough to withstand pressure.
3. Finally, it is dried.
Figure 2: Contrast between normal filter paper and Ultrafilter paper.
Process: The impure sol is poured onto the ultrafilter. The colloidal particles are retained on the paper as a slime, while the electrolytes wash through in the ultrafiltrate. Because pores are tiny, the process is very slow, so suction or pressure is usually applied to speed it up. The remaining slime is then stirred with fresh dispersion medium to recreate a pure sol.
3. Ultracentrifugation
Colloidal particles do not settle under normal gravity due to Brownian motion. However, if the sol is placed in a high-speed ultracentrifuge (spinning at immense speeds), the artificial centrifugal force overpowers Brownian motion.
- The heavier colloidal particles are forced to settle down at the bottom of the tube.
- The impurities (electrolytes) remain dissolved in the clear liquid above (the supernatant).
- The supernatant is poured off (decanted). The settled colloidal particles are then mixed with a fresh dispersion medium to regenerate the pure sol.
Mastery Check: Purification
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