The colloidal particles are electrically charged as a indicated by their migration towards cathode or anode under the applied electric field. In a particular colloidal system, all particles carry either positive charge or negative charge.
The electric charge on colloidal particles orginate in several ways. According to preferential adsorption theory, the freshly obtained precipitate particles adsorb ions from the dispersion medium, which are common to their lattice and acquire the charge of adsorbed ions. For example, For example, freshly obtained `Fe(OH)_(3)` precipitated is dispersed, by a little `FeCl_(3)`, into colloidal solution owing to the adsorption of `Fe^(3+)` ions in preference. Thus sol particles will be positively charged.
In some cases the colloidal particles are aggregates of cations or anions having ampiphilic character. When the ions posses hydrophobic part (hydrocarbon end) as well as hydrophilic part (polar end group), they undergo association in aqueous solution to form particles having colloidal size. The formation of such particles, called micelles plays a very important role in the solubilization of water insoluble substances, (hydrocarbon, oils, fats, grease etc.). In micelles, the polar end groups are directed towards water and the hydrocarbon ends into the centre. The charge on sol particles of proteins depends on the pH. At low pH, the basic group of protein molecule is ionized (protonated) and at higher pH (alkaline medium), the acidic group is ionized. At isoelectric pH, characteristic to the protein, both basix and acidic groups are equally ionized. The stability of colloidal solution is attributed largely to the electric charge of the dispersed particles. This charge causes them to be coagulated or precipitated. On addition of small amount of electrolytes, the ions carrying oppiste charge are adsorbed by sol particles resulting in the neutralization of their charge. When the sol particles either with no charge or reduced charge, come closer due to Brownian movement, they coalesce to form bigger particles resulting in their separation from the dispersion medium. This is what is called coagulating or precipitation of the colloidal solution. The coagulating power of the effective ion, which depend on its charge, is expressed in terms of its coagulating value, defined as its minimum concentration (m mol/L) needed to precipitate a given sol.
How would you obtain a sol of AgI, the particles of which migrate towards cathode under the electric field?

Options

A. By adding you obtain of KI to `AgNO_(3)` solution
B. By adding little excess of `AgNO_(3)` to KI solution
C. By mixing equal volumes of `0.010 M AgNO_(3)` and `0.010 M` KI
D. None of these

Explanation

To obtain a sol of AgI (silver iodide) where the particles migrate towards the cathode under an electric field, follow these steps:

### Step 1: Prepare the Potassium Iodide Solution
- Start by preparing a dilute solution of potassium iodide (KI). This will provide the iodide ions (I⁻) necessary for the formation of AgI.

### Step 2: Add Silver Nitrate Solution
- Gradually add a dilute solution of silver nitrate (AgNO₃) to the potassium iodide solution. This will lead to the formation of silver iodide (AgI) precipitate according to the reaction:
\[ \text{Ag}^+ (aq) + \text{I}^- (aq) \rightarrow \text{AgI} (s) \]

### Step 3: Control the Charge of the Colloidal Particles
- To ensure that the resulting AgI particles carry a positive charge, add a little excess of AgNO₃. The excess Ag⁺ ions will preferentially adsorb onto the surface of the AgI particles, giving them a positive charge.

### Step 4: Formation of Colloidal Solution
- Stir the mixture well to ensure that the AgI precipitate is dispersed into the solution, forming a colloidal solution of AgI with positively charged particles.

### Step 5: Verify the Charge
- Finally, you can confirm that the AgI sol particles migrate towards the cathode under an electric field, indicating that they are positively charged.

### Summary of the Process:
1. Prepare a KI solution.
2. Add AgNO₃ solution to form AgI.
3. Add excess AgNO₃ to ensure positive charge.
4. Stir to disperse AgI and form the colloidal solution.