Question

Given below are two statements:
Statement I: $\text{Al(OH)}^{2+}$ ion is predominant in soil between pH 4.7 & 6.5
Statement II: $\text{Al}^{3+}$ ion is predominant in soil pH below 4.7
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:

Soluble aluminum speciation and pH:
$\text{pH} < 4.7 \rightarrow \text{Al}^{3+}$ (highly toxic)
$\text{pH } 4.7 - 6.5 \rightarrow \text{Al(OH)}^{2+}$ and $\text{Al(OH)}_2^+$
$\text{pH} > 6.5 \rightarrow \text{Al(OH)}_3$ (insoluble precipitate)

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
In acidic soils, aluminum ($\text{Al}$) undergoes a series of pH-dependent hydrolysis reactions.
The resulting monomeric aluminum species vary in charge and solubility, influencing their toxicity and reactivity in the soil solution.

Step 2: Detailed Explanation:
The concentration and speciation of soluble aluminum in the soil solution are regulated by soil pH.
As water molecules surrounding the aluminum ion lose protons ($H^+$), the ionic form changes as follows:
1. Under strongly acidic conditions where the soil pH is below $4.7$, aluminum exists primarily as the trivalent monomeric cation, $\text{Al}^{3+}$ (specifically as the hydrated $\text{Al(H}_2\text{O)}_6^{3+}$ ion).
This free $\text{Al}^{3+}$ form is highly soluble and is the primary source of aluminum toxicity in plants. Thus, Statement II is true.
2. As the soil pH rises between $4.7\text{ and }6.5$, aluminum undergoes progressive hydrolysis, releasing hydrogen ions.
In this intermediate pH range, the first hydrolysis product, $\text{Al(OH)}^{2+}$, becomes the dominant monomeric species in solution, followed by $\text{Al(OH)}_2^+$ as the pH approaches neutrality. Thus, Statement I is true.
3. As the pH rises above $6.5$, aluminum precipitates out of solution as insoluble aluminum hydroxide, $\text{Al(OH)}_3$ (gibbsite).
Since both statements accurately describe the behavior of aluminum species in relation to soil pH, they are both true.

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