Question

Given below are two statements:
Statement I: The permanent charge of silicate clay minerals results from isomorphous substitution within the clay lattice.
Statement II: The pH-dependent charges depend on the dissociation of $\text{H}^+$ ions from the edges of the crystal unit.
In light of the above statements, choose the correct answer from the options given below:

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Permanent charge $\rightarrow$ Caused by isomorphous substitution inside the clay lattice; constant across pH levels.
pH-dependent charge $\rightarrow$ Caused by protonation/deprotonation of $-\text{OH}$ groups at broken crystal edges; varies with soil pH.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Silicate clay minerals carry negative and positive surface charges that determine their cation and anion exchange capacities.
These charges are classified as permanent (constant) charges or pH-dependent (variable) charges based on their origin.

Step 2: Detailed Explanation:
Statement I describes the origin of permanent charge.
Permanent charge is created during mineral crystallization by isomorphous substitution, where a cation of similar size but lower valence replaces a structural cation in the tetrahedral or octahedral sheets.
For example, $\text{Al}^{3+}$ can substitute for $\text{Si}^{4+}$ in the tetrahedral sheet, or $\text{Mg}^{2+}$ can substitute for $\text{Al}^{3+}$ in the octahedral sheet.
This substitution leaves an unsatisfied negative charge within the crystal lattice that is unaffected by the pH of the soil solution. Thus, Statement I is true.
Statement II describes the origin of pH-dependent charge.
This charge occurs at the broken edges of clay crystals and on the surfaces of organic matter, iron, and aluminum oxides.
These surfaces contain exposed hydroxyl groups ($-\text{OH}$).
At high pH, these groups deprotonate ($\text{OH}^- \rightarrow \text{O}^{2-} + \text{H}^+$), creating a negative charge.
At low pH, they accept protons ($\text{OH}^- + \text{H}^+ \rightarrow \text{OH}_2^+$), creating a positive charge.
This charge depends on the concentration of $\text{H}^+$ ions in the soil solution. Thus, Statement II is true.

Step 3: Final Answer:
Both Statement I and Statement II are true, which corresponds to Option (A).