Sedimentation volume is a key parameter used to evaluate the physical stability of pharmaceutical suspensions. It is defined as the ratio of the final volume of sediment (Vu) to the original volume of the suspension (Vo), represented as:
F = Vu / Vo

A stable suspension exhibits a high sedimentation volume (F), indicating minimal settling and easy redispersion of particles. In a suspension, solid particles tend to settle over time due to gravity. To reduce this rate, suspending agents are incorporated to increase the viscosity of the continuous phase.

According to Stokes’ Law, the sedimentation rate is inversely proportional to the viscosity of the medium. Hence, increasing the concentration of a suspending agent generally enhances stability. However, excessively high concentrations can lead to gel formation and poor flow properties.

Effect of Suspending Agent Concentration:
● Low concentration: Low viscosity → Rapid sedimentation.
● Optimal concentration: Balanced viscosity → Slow sedimentation and good redispersibility.
● High concentration: Excessive viscosity → Difficult to pour or redisperse.

Determining the optimal concentration ensures the formulation is physically stable, easily pourable, and therapeutically effective.