Q.By which of the following processes colloidal sols are purified?

Peptization converts a precipitate into a colloidal sol using a small amount of electrolyte that gets preferentially adsorbed on the precipitate particles.

When a precipitate is freshly formed, its particles can be dispersed into a stable colloidal sol by shaking it with a small quantity of a suitable electrolyte (called the peptizing agent), rather than water alone.

How it works:

• Ions of the added electrolyte are preferentially adsorbed on the surface of the precipitate particles.
• This adsorption gives every particle a common (like) surface charge.
• The particles now repel one another electrostatically instead of clumping together, so the precipitate breaks up into colloidal-sized particles that stay dispersed in the liquid. …

Bredig's Arc method disperses a metal into colloidal particles using an electric arc struck under the dispersion medium.

Two rods of the metal (e.g. gold, silver, or platinum) are used as electrodes and dipped into the dispersion medium (usually water), which is kept cold by surrounding the container in an ice bath. A high-voltage electric arc is struck between the two electrode tips just below the liquid surface. The intense heat of the arc vaporises a little of the metal at the electrode tips; these hot metal atoms then come into contact with the surrounding cold liquid and condense immediately, forming very fine colloidal metal particles dispersed throughout the medium. This method combines dispersion (arcing/vaporising the bulk metal) and condensation (rapid cooling of the vapour) in a single step, and is used to prepare sols of metals like Ag, Au, and Pt.

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Colloidal sols are purified by dialysis — option (C).

A freshly prepared sol often contains dissolved electrolyte impurities. Dialysis removes them: the sol is placed in a parchment/cellophane bag suspended in running water. The small ions and molecules diffuse out through the semi-permeable membrane, while the larger colloidal particles are retained, giving a purified sol. (Electro-dialysis speeds this up using an electric field.)

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Electrodialysis is ordinary dialysis assisted by an applied electric field, which makes the removal of electrolyte impurities from a colloidal sol much faster.

Apparatus/diagram description (labelled parts):

• A parchment paper (or cellophane) membrane bag containing the impure colloidal solution (sol + dissolved electrolyte ions) is suspended in a large vessel of pure/distilled water.
• Two metal electrodes (a cathode and an anode) are dipped into the outer water on either side of the membrane bag, connected to a DC source (battery).
• The membrane is a semi-permeable membrane: small electrolyte ions can pass through its pores, but the large colloidal particles cannot.
• When the electric field is switched on, the cations in the impure sol migrate through the membrane towards the cathode, and the anions migrate through the membrane towards the anode, both moving out into the surrounding water much faster than by ordinary diffusion alone. …

Electrical dispersion (Bredig's arc method) makes a metal colloidal sol by vaporising metal electrodes with an electric arc struck under the dispersion medium.

This method is used to prepare colloidal sols of metals such as gold, silver and platinum, and combines dispersion and condensation.

Setup (described, since only text output is possible here):

• A glass vessel is filled with the dispersion medium (usually water), containing a trace of an electrolyte such as KOH to stabilise the colloid.
• The vessel is kept in an ice bath to keep the liquid cold.
• Two metal rods (of the metal to be dispersed) are immersed in the liquid, close to each other, connected to a source of high-voltage DC current — these act as the two electrodes.
• An electric arc is struck between the submerged electrode tips.

Process: …

Bredig's arc method is an electrical dispersion method used to prepare sols of metals such as silver, gold and platinum.

In Bredig's arc method, an electric arc is struck between electrodes of the metal (e.g. silver) immersed in the dispersion medium (water), cooled in an ice bath. The intense heat of the arc vaporises some of the metal, and the vapour condenses immediately in the cold surrounding liquid to form colloidal-sized particles, giving a colloidal sol of the metal.

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Dialysis purifies a colloidal solution by letting dissolved crystalloid impurities (ions, small molecules) diffuse out through a semipermeable membrane while the larger colloidal particles are held back.

Principle: The pores of a semipermeable membrane (parchment paper or cellophane) are large enough to allow small ions/molecules (crystalloids, e.g. NaCl) to pass through by diffusion, but too small to allow the much larger colloidal particles to pass.

Set-up/diagram description (since no image can be drawn here):

• A bag made of a semipermeable membrane (parchment paper/cellophane) containing the impure colloidal sol is tied and suspended inside a larger vessel.
• Fresh water is continuously passed into the outer vessel (through an inlet near the bottom) and the water carrying the diffused-out impurities is continuously drained out (through an outlet near the top), i.e. water keeps flowing around the bag. …