Page 35 - Course Guide 2019

Several types of preservatives may be used for liquid preparations. Most commonly, benzoate-
or sorbate-based preservatives are preferred but at times, paraben are also used. The
following are commonly used preservatives with the acceptable concentration range:

Preservatives Concentration Type of preparations
Benzoic acid 0.1% w/v Acidic preparations
Benzoic acid solution B.P.C 1973 2% v/v Acidic preparations
Sodium benzoate 0.02% - 0.5% w/v All
Sorbic acid 0.1 – 0.2% w/v Acidic preparations
Potassium sorbate 0.1 – 0.2% w/v All
Methyl paraben 0.1 – 0.25% w/v All
Propyl paraben 0.05% w/v (aqueous) All
0.1% w/v (cream/emulsion)

9.2 SUSPENSION
Suspension is usually prepared for insoluble powders. The powder need to be finely sized
(dispersed phase) and will be suspended in a liquid vehicle (continuous phase). The
suspended particles need to be dispersed suitably to ensure consistent drug dose being
withdrew each time.
There are two types of insoluble powders – diffusible solids and indiffusible solids.

The formulation of suspension with diffusible solids do not need the presence of suspending
agent. They are light, easily wetted and hence are easily mixed with water. Upon shaking the
suspension made from this kind of powder, the powder could maintained their suspending
ability long enough to ensure an accurate dose could be measure out by patients.

Example of diffusible solids are:
- Calcium carbonate
- Light Kaolin
- Light Kaolin (Natural)
- Light Magnesium Carbonate
- Magnesium Trisilicate
- Rhubarb powder

Indiffusible solids on the other hand need the presence of suspending agent. They do not
easily wetted and could not suspend in a continuous phase long enough upon shaking.
Suspending agent is needed to help the dispersion of the powder in the preparation. Example
of indiffusible solids are mostly drug powder having low solubility and those that do not belong
to the diffusible solids.