Question

Which of the following dispersing agent is used in determination of the particle size distribution of soils?

• A. Sodium hexametaphosphate
• B. Hydrogen peroxide
• C. Water
• D. Hydrochloric acid

Hint:

Sodium hexametaphosphate (Calgon) is chosen because ______Sodium ($\text{Na}^+$)______ is a highly dispersing cation.
It has a large hydrated ionic radius that keeps clay particles apart, preventing flocculation during sedimentation measurements.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Particle size distribution (mechanical analysis) of soil is used to determine soil texture.
Before conducting pipette or hydrometer sedimentation tests, soil aggregates must be completely dispersed into individual sand, silt, and clay particles.

Step 2: Detailed Explanation:
In natural soils, clay particles are bound together into aggregates by cementing agents, predominantly divalent and trivalent cations such as calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$).
To achieve complete dispersion, a chemical dispersing agent must be added.
______Sodium hexametaphosphate______ ($\text{NaPO}_3)_6$, commonly known as Calgon, is the standard dispersing agent used in soil laboratories.
It operates via two key mechanisms:
1. It replaces the strongly binding divalent and trivalent cations on the clay exchange complex with monovalent sodium ($\text{Na}^+$) ions.
2. The sodium ions cause the electrical double layer around the clay particles to expand, increasing electrostatic repulsion and causing the particles to disperse.
Hydrogen peroxide ($\text{H}_2\text{O}_2$) is used as a pretreatment to oxidize organic matter, and hydrochloric acid ($\text{HCl}$) is used to remove carbonates, but they do not act as the primary dispersing agent in the sedimentation suspension.

Step 3: Final Answer:
The dispersing agent used in soil particle size distribution is Sodium hexametaphosphate, which corresponds to option (A).