Question

The wet digestion method is followed for the estimation of

• A. pH
• B. E C
• C. Organic Carbon
• D. Cation Exchange Capacity

Hint:

The Walkley-Black method is a wet oxidation/digestion technique where potassium dichromate and sulfuric acid are used to estimate soil organic carbon.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Analytical soil testing uses specific extraction, digestion, and measurement techniques to quantify soil properties.
Wet digestion (or wet oxidation) methods use strong chemical oxidants and acids to decompose organic matter, allowing for the quantitative analysis of organic constituents.

Step 2: Detailed Explanation:
The standard procedure for estimating soil organic carbon is the Walkley-Black wet oxidation method.
In this method, soil organic matter is digested using a mixture of potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) and concentrated sulfuric acid ($\text{H}_2\text{SO}_4$).
The heat of dilution generated by mixing the acid and aqueous salt solution drives the rapid oxidation of organic carbon to carbon dioxide ($\text{CO}_2$), reducing the dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$, orange) to the chromic ion ($\text{Cr}^{3+}$, green).
The remaining unreacted dichromate is back-titrated against ferrous ammonium sulfate (FAS) to calculate the concentration of organic carbon oxidized.
Let us review the other options:
1. pH: Measured electrometrically in a soil-water suspension using a glass electrode, without any digestion.
2. EC (Electrical Conductivity): Measured in a soil-water extract using a conductivity meter to estimate soluble salt concentration.
3. CEC (Cation Exchange Capacity): Determined by saturating the soil exchange complex with an index cation (like ammonium), washing out the excess, and displacing the index cation for analysis.

Step 3: Final Answer:
The wet digestion method is used for the estimation of Organic Carbon, which corresponds to Option (C).