Question

The reagents of Cation Exchange Capacity are:
(A) Sodium Acetate (1N)
(B) Ammonium Acetate (1N)
(C) Isopropyl alcohol(90%)
(D) The sodium bicarbonate
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (B), (C) and (D) only.
• C. (A), (C) and (D) only.
• D. (A), (B) and (D) only.

Hint:

In CEC testing, washing with water instead of isopropyl alcohol causes clay dispersion and hydrolyzes the adsorbed sodium, leading to inaccurate, low CEC measurements.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Cation Exchange Capacity (CEC) is a standard laboratory measurement of the quantity of exchangeable cations a soil can retain.
The classic sodium acetate-ammonium acetate method is used to determine CEC in neutral and alkaline soils.

Step 2: Detailed Explanation:
The standard laboratory procedure for measuring CEC involves three key steps:
1.Saturation (A): The soil sample is treated with a \( 1\text{N} \) Sodium Acetate solution (pH 8.2).
This saturates all cation exchange sites with sodium (\( \text{Na}^+ \)) ions, displacing all other cations.
Thus, Sodium Acetate (A) is a required reagent.
2.Washing (C): The excess unabsorbed sodium ions must be removed from the soil pore space without displacing the adsorbed exchangeable sodium.
This washing is performed using an organic solvent, typically \( 90\% \) to \( 95\% \) Isopropyl alcohol.
Thus, Isopropyl alcohol (C) is a required reagent.
3.Displacement (B): The exchangeable sodium on the soil complex is then displaced by washing the soil with a \( 1\text{N} \) Ammonium Acetate solution (pH 7.0).
The ammonium (\( \text{NH}_4^+ \)) ions replace the sodium on the exchange complex.
The concentration of displaced sodium in the collected extract is then measured using a flame photometer to calculate the CEC.
Thus, Ammonium Acetate (B) is a required reagent.
Sodium bicarbonate (D) is not used in this standard CEC procedure; instead, it is a reagent used for extracting available phosphorus (Olsen's method).

Step 3: Final Answer:
The reagents used for determining Cation Exchange Capacity are Sodium Acetate (1N), Ammonium Acetate (1N), and Isopropyl alcohol (90%).