Stability of suspensions: theoretical and practical considerations before compounding.

Suspension stability can be theoretically estimated prior to the beginning of the formulating process based on the solid phase particle size, liquid phase density, and viscosity. Stokes equation can be used to predict suspension stability in order to save time and resources. The examples of these calculations for the assessment of suspension physical characteristics are given in this article. One parameter that cannot be theoretically estimated with precision is flocculation/deflocculation. Flocculation can be experimentally determined using the "jar test," and it is a critical parameter for the substances showing inclination toward caking. Suspensions will sediment in time; however, it is their key feature to be able to redisperse in order to preserve the efficacy and proper dosage. Bismuth subnitrate is practically insoluble in water, which makes it convenient for oral pharmaceutical suspensions, rather than the other pharmaceutical forms. Like the other bismuth compounds, it tends to cake in aqueous medium. In order to prevent formation of the solid sediment, controlled flocculation of the suspended bismuth subnitrate particles is recommended. The effect of the excipients (sodium citrate, Tween 20, propylene glycol, microcrystalline cellulose) on the transmittance of the prepared suspensions and the quantity and characteristics of the formed sediment were evaluated. Suspensions containing sodium citrate, as well as the formulations with sodium citrate and microcrystalline cellulose, based on their transmittance characteristics, were determined to be flocculating suspensions, regardless of the sodium citrate concentration used. The highest affinity towards formation of flocculating suspensions, with the highest transmittance value had microcrystalline cellulose with 15% (w/w) sodium citrate.