Understanding Charges of Colloidal Sols

Colloidal particles in a sol always carry an electric charge. This charge is usually acquired by the preferential adsorption of ions from the dispersion medium onto the surface of the colloidal particles. The adsorbed ions are typically those common to the colloidal particle material or specific ions present in excess in the solution. The nature of the charge (positive or negative) depends on the specific substance forming the sol and the method of preparation.

Analyzing Each Sol and Its Charge

Let's examine each statement to determine the correct charge associated with the given sols:

1. (A) Cr(OH)₃ sol : negatively charged sol
   Cr(OH)₃ (Chromium(III) hydroxide) sol is typically prepared by peptization of freshly precipitated Cr(OH)₃ with a small amount of HCl. In this process, the colloidal particles preferentially adsorb Cr³⁺ ions from the solution. Therefore, Cr(OH)₃ sol is a positively charged sol. This statement is incorrect.
2. (B) TiO₂ sol : positively charged sol
   Metal oxide sols like TiO₂ (Titanium dioxide) and Al₂O₃ are generally positively charged. They tend to adsorb H⁺ ions from acidic solutions or adsorb metal ions preferentially. TiO₂ sols formed by hydrolysis methods are known to be positively charged. This statement is correct.
3. (C) CdS sol : positively charged sol
   Metal sulfide sols like CdS (Cadmium sulfide), As₂S₃ (Arsenic(III) sulfide), and Sb₂S₃ (Antimony(III) sulfide) are typically negatively charged. This is due to the preferential adsorption of sulfide ions (S²⁻) from the solution. Therefore, CdS sol is a negatively charged sol. This statement is incorrect.
4. (D) Gum : negatively charged sol
   Organic sols like gum, starch, gelatin, and albumin are often negatively charged. This charge can arise from the ionization of acidic groups within the organic molecules or the preferential adsorption of anions from the solution. Gum sols are generally known to be negatively charged. This statement is correct.
5. (E) Silver sol : positively charged sol
   Metal sols of gold, silver, platinum, etc., prepared by dispersion or reduction methods, are typically negatively charged. This is often due to the adsorption of anionic impurities or OH⁻ ions from the dispersion medium, or electrons during the reduction process. Silver sol is generally a negatively charged sol. This statement is incorrect.

Identifying the Correctly Matched Pairs

Based on our analysis:

• Statement (A) is incorrect. Cr(OH)₃ sol is positive.
• Statement (B) is correct. TiO₂ sol is positive.
• Statement (C) is incorrect. CdS sol is negative.
• Statement (D) is correct. Gum sol is negative.
• Statement (E) is incorrect. Silver sol is negative.

The correctly matched pairs are (B) and (D).

Checking the Options

We look for the option that includes only (B) and (D).

1. (C) and (E) only - Incorrect.
2. (A) and (C) only - Incorrect.
3. (B) and (E) only - Incorrect.
4. (B) and (D) only - Correct.

Thus, the correct answer is the option that lists (B) and (D) only.

Revision Table: Sols and Their Charges

Additional Information on Sol Charges and Stability

The presence of charge on colloidal particles is responsible for the stability of sols. Since all particles of a particular sol carry the same charge (either all positive or all negative), they repel each other. This mutual repulsion prevents them from coming together and forming larger aggregates that would settle down, thus maintaining the dispersion. If the charge is neutralized (e.g., by adding an electrolyte with oppositely charged ions), the repulsion decreases, leading to coagulation or flocculation of the sol.

Different types of sols and common examples based on charge:

• Positively Charged Sols:
  1. Hydrated metal oxides (e.g., Fe₂O₃.xH₂O, Al₂O₃.xH₂O, CrO₃.xH₂O)
  2. Basic dyes (e.g., Methylene Blue sol)
  3. Haemoglobin
  4. Oxides like TiO₂
• Negatively Charged Sols:
  1. Metal sols (e.g., Au, Ag, Pt, Cu sols)
  2. Metal sulfide sols (e.g., As₂S₃, Sb₂S₃, CdS sols)
  3. Acid dyes (e.g., Eosin, Congo Red sol)
  4. Starch, Gum, Gelatin, Clay, Charcoal

The charge of a sol can sometimes depend on the pH of the dispersion medium. For example, a protein sol can be positively charged in acidic medium (adsorbing H⁺) and negatively charged in alkaline medium (dissociating H⁺ from acidic groups or adsorbing OH⁻). The pH at which the net charge is zero is called the isoelectric point.