Based on Type of Solids

Learning Objective

• Differences between diffusible and indiffusible suspensions

1. Diffusible suspensions: These are suspensions containing diffusible solids. These are light powders, which are insoluble or only very slightly soluble in the vehicle but readily mix with water and remain suspended throughout the liquid for a sufficient time after shaking, allowing an accurate dose to be withdrawn. On standing, the insoluble solids settle at the bottom of the bottle, which requires the shaking of the bottle every time a dose is to be withdrawn.Examples of diffusible solids are calcium carbonate, light magnesium carbonate, magnesium trisilicate, rhubarb powder and light kaolin.
2. Indiffusible suspensions: These are suspensions containing indiffusible solids. These are heavy powders, which are insoluble in the vehicle and on shaking do not remain uniformly distributed in the vehicle for a long time to allow an accurate dose to be withdrawn. Indiffusible suspensions contain a suspending agent or a thickening agent to delay the sedimentation rate and prevent the formation of aggregates.Examples of indiffusible solids are calamine, hydrocortisone, precipitated sulfur, zinc oxide, aspirin, chalk powder, zinc oxide and phenobarbitone.