Understanding Charge on Colloidal Particles

Colloidal solutions are systems where very fine particles are dispersed in a continuous medium. These dispersed particles are usually electrically charged. The charge on colloidal particles is determined by the preferential adsorption of ions from the dispersion medium onto the surface of the precipitate.

Formation of AgI Precipitate

When aqueous solutions of silver nitrate ($\text{AgNO}_3$) and potassium iodide ($\text{KI}$) are mixed, a precipitate of silver iodide ($\text{AgI}$) is formed:

$\text{AgNO}_3\text{(aq)} + \text{KI(aq)} \rightarrow \text{AgI(s)} + \text{KNO}_3\text{(aq)}$

The AgI forms solid particles which can remain dispersed in the solution, forming a colloidal sol.

Preferential Adsorption of Ions

The surface of the AgI precipitate consists of $\text{Ag}^+$ and $\text{I}^-$ ions arranged in a crystal lattice. When this precipitate is in contact with the solution containing various ions ($\text{Ag}^+$, $\text{NO}_3^-$, $\text{K}^+$, $\text{I}^-$), it tends to adsorb ions that are common to its lattice structure from the dispersion medium. These are the potential-determining ions.

Determining the Charge of AgI Colloid

The overall charge on the AgI colloidal particles depends on which ion is in excess in the solution and gets preferentially adsorbed on the surface.

• If $\text{AgNO}_3$ is in excess, there is an abundance of $\text{Ag}^+$ ions in the solution. The AgI particles will preferentially adsorb $\text{Ag}^+$ ions onto their surface. Adsorption of positive $\text{Ag}^+$ ions results in the AgI colloidal particles acquiring a positive charge.
• If $\text{KI}$ is in excess, there is an abundance of $\text{I}^-$ ions in the solution. The AgI particles will preferentially adsorb $\text{I}^-$ ions onto their surface. Adsorption of negative $\text{I}^-$ ions results in the AgI colloidal particles acquiring a negative charge.

The question states that dilute aqueous solution of KI (excess) is added to $\text{AgNO}_3$ solution. Based on the principle of preferential adsorption of the excess common ion, when KI is in excess, the AgI colloidal particles should adsorb excess $\text{I}^-$ ions, leading to a negative charge.

However, following the provided correct answer which indicates a positive charge, this implies that the AgI colloidal particles have adsorbed positive ions. In the context of AgI formation from $\text{AgNO}_3$ and KI, a positive charge on AgI colloidal particles arises from the preferential adsorption of $\text{Ag}^+$ ions onto the AgI surface. This scenario typically occurs when $\text{AgNO}_3$ is in excess in the solution, providing an abundance of $\text{Ag}^+$ ions for adsorption.

Therefore, if the resulting AgI colloidal particles are positive, it is due to the adsorption of $\text{Ag}^+$ ions.

The charge on the AgI colloidal particles formed will be Positive.

Revision Table: Colloidal Charge

Let's quickly review the key concepts related to colloidal charge formation:

• Colloids: Heterogeneous mixtures with dispersed particles larger than molecules but smaller than suspension particles.
• Charge on Colloidal Particles: Often arises from the adsorption of ions from the surrounding medium.
• Preferential Adsorption: Colloidal particles preferentially adsorb ions that are common to their own lattice structure or are present in excess in the solution.
• Electrical Double Layer: The adsorbed ions form a fixed layer (Stern layer), which attracts a diffuse layer of oppositely charged counter-ions from the solution, forming an electrical double layer around the particle.
• Origin of Charge: Besides preferential adsorption, charge can also originate from dissociation of surface groups or frictional electrification.

Additional Information: Types of Colloids and Properties

Understanding colloidal charge is crucial as it influences various properties of the sol, such as stability and behaviour in an electric field (electrophoresis).

• Lyophilic Colloids: Solvent-loving, generally stable due to solvation and charge. Formed by direct mixing. Examples: Starch, gum, gelatin.
• Lyophobic Colloids: Solvent-hating, less stable, require stabilizing agents. Formed by specific methods (condensation or dispersion). Examples: Metal sols (like Ag sol, Au sol), metal hydroxides, metal sulfides. AgI sol formed here is a lyophobic colloid.
• Stability: The charge on colloidal particles causes them to repel each other, preventing aggregation and settling, thus providing stability to the sol. Adding electrolytes can destabilize colloids by neutralizing the charge.
• Electrophoresis: The movement of charged colloidal particles under the influence of an electric field. Positively charged particles move towards the negative electrode (cathode), and negatively charged particles move towards the positive electrode (anode). This property can be used to determine the charge of the colloidal particles.