Pipette/Hydrometer Analysis – Standard Dispersing Solution per IS 2720 As per IS: 2720 (Methods of Test for Soils), the recommended composition of the dispersing solution used in pipette analysis is which of the following?

Introduction / Context:
Accurate sedimentation analysis requires deflocculating the particles so that Stokes’ law assumptions are approached. A standard dispersing agent prevents clays and silts from aggregating, ensuring that measured settling velocities correspond to individual or small-clustered particles.

Given Data / Assumptions:

• Testing follows IS: 2720 guidance for particle-size analysis by sedimentation.
• The solution must promote dispersion without altering specific gravity significantly.
• Solution volume standardized to 1 litre for reproducibility.

Concept / Approach:

Sodium hexametaphosphate acts as a peptizing/dispersing agent, while a small amount of sodium carbonate helps adjust alkalinity and enhance dispersion. The commonly cited IS composition is 33 g of sodium hexametaphosphate plus 7 g sodium carbonate in distilled water to make 1 litre.

Step-by-Step Solution:

Verification / Alternative check:

Equivalent guidance appears in many soil mechanics laboratory manuals aligned with IS methods.

Why Other Options Are Wrong:

The other mass combinations invert or alter the recommended proportions and would not match the IS reference composition.

Common Pitfalls:

Using tap water (ionic content affects dispersion), inaccurate weighing, and not allowing sufficient mixing time reduce test reliability.

Final Answer:

Sodium hexametaphosphate 33 g, sodium carbonate 7 g, distilled water to 1 litre