Q.Write the usefulness of electro-dialysis.

Dialysis exploits a semi-permeable membrane's pores, which let small ions/molecules pass through but retain large colloidal particles, to purify a colloidal sol.

A colloidal solution (sol) is often prepared or contaminated along with small dissolved ions/molecules as impurities. A semi-permeable membrane (e.g. parchment paper, cellophane) has pores fine enough to let small ions and molecules diffuse through, but too small for the much larger colloidal particles to pass.

In dialysis, the impure sol is placed in a bag of this membrane and suspended in a stream of fresh water (or an electric field, for the faster variant "electrodialysis"). Small ions/molecules diffuse out through the membrane into the water, leaving the purified colloidal sol behind inside the bag.

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Dialysis is the process of removing dissolved ions/small molecules from a colloidal solution by selective diffusion through a semi-permeable membrane.

Colloidal particles are too large to pass through a semi-permeable (parchment/cellophane) membrane, while true dissolved ions and small molecules can pass through it. Placing a colloidal sol in a dialysis bag surrounded by flowing fresh water lets the unwanted small ions/molecules diffuse out, leaving a purified colloidal sol behind — this process is c …

Dialysis separates colloidal particles from soluble impurities using a semipermeable membrane.

A colloidal sol, when prepared, often contains dissolved electrolyte/small-molecule impurities. In dialysis, the impure sol is placed in a bag made of a semipermeable membrane (e.g. parchment or cellophane), which is suspended in a vessel of pure/running water. The membrane's pores allow small ions and molecules to diffuse out into the surrounding water, while the larger colloidal particles are retained inside the bag, so the sol is progressively purified. This is t …

Electrodialysis combines dialysis (separating dissolved ions from colloidal particles across a membrane) with an applied electric field, which speeds up the removal of the ionic impurities.

Description of the labelled diagram:

1. A U-shaped vessel (dialyser) — the central limb holds a parchment/collodion membrane bag containing the impure colloidal solution.
2. Pure water fills the outer compartment(s) on either side of the membrane, with an inlet and outlet for continuously flowing fresh water (to keep flushing out the ions that pass through).
3. Two metal electrodes (connected to a battery/DC source) are dipped into the water on either side of the membrane — label them anode (+) and cathode (−).
4. Label the arrows showing dissolved ions (e.g. small electrolyte ions) migrating out of the membrane toward the oppositely charged electrode, while the large colloidal particles stay trapped inside the membrane bag (the membrane's pores are too small for them to pass). …

Electrodialysis speeds up dialysis by applying an electric field across the semi-permeable membrane, so that charged impurities (ions/electrolytes) migrate rapidly to the oppositely-charged electrode and are removed — it purifies colloids of ionic impurities specifically.

A colloidal sol (e.g., ferric hydroxide sol) is often contaminated with an electrolyte like $FeCl_3$ left over from its preparation. In ordinary dialysis, this small ionic impurity diffuses out through the membrane down its concentration gradient; electrodialysis accelerates this by using an applied potential to actively pull the $Fe^{3+}$ and $Cl^-$ ions toward the cathode/anode respectively. Urea (a), glucose (c), and sugar (d) are neutral, non-ionic molecules — they diffuse out …