Which of the following is an example of a suspending agent?

• A. Glycerin
• B. Carboxymethylcellulose
• C. Sodium chloride
• D. Sorbitol

Answer: (B)

Suspending agents raise the viscosity of the liquid vehicle to slow down the settling of solid particles, helping keep a suspension uniform between shakes. Carboxymethylcellulose is a cellulose derivative that dissolves in water to form a thick, gel-like network. This increased viscosity reduces the rate at which particles settle (Sedimentation velocity decreases as viscosity increases, per the basic settling equation), and it also helps keep particles evenly dispersed and easier to re-disperse after shaking. In addition, such polymers can improve wetting and prevent flocculation, contributing to overall stability of the suspension.

Glycerin, while it can add some viscosity, is mainly a humectant and solvent rather than a primary suspending agent. Sodium chloride is used to adjust tonicity and electrolyte balance, not to suspend solids. Sorbitol is primarily a sweetener and humectant, not a standard suspending agent. Hence, carboxymethylcellulose best fits the role of a suspending agent in pharmaceutical suspensions.