Some liquid preparations containing resinous constituents, gums and small amounts of volatile components are precipitated on addition to water. Gum resins are oil soluble purified exudates of Myrrh, Benzoin, Galbanum, oppoponax, Tolu, Lobelia etc. The precipitated resin adheres to the side of the container and converted to non-dispersible form. To prevent this problem the pharmacist should adopt special precaution during dispensing. Two methods are commonly used for the prevention of precipitation of suspension.
A. Using Compound Tragacanth Powder
Mix finely powered substance, insoluble solid and gum intimately. Form a smooth cream with vehicle and dilute up to 50% of the final volume. Measure the precipitate forming liquid and pour in a slow stream into the center of the suspension with stirring. Avoid sticky liquid on the pestle and on the sides of the mortar. Precipitated resin particles are adsorbed by hydrocolloids over their surface conferring hydrophilic properties and preventing aggregation into clots. (Note – If electrolytes are present in the preparation, it should not be added until the resin has been protected by hydrocolloids)
B. Using Tragacanth mucilage
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This method is quite easy to dispense resin-containing substances. Mix mucilage with an equal volume of vehicle in beaker and add precipitate forming liquid into the center of the mucilage with constant stirring. If any electrolyte is present, add and dilute well with vehicle.
Example:
Prepare and dispense Mixture of Lobelia and Stramonium.
Lobelia ethereal Tincture 8.0 ml
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Tragacanth mucilage 10.0 ml
Potassium iodide 2.0 g
Stramonium Tincture 16.0 ml Double strength Chloroform water 10.0 ml
Purified Water q.s. to 100 ml
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Method of Dispensing:
Mix tragacanth with purified water and add Lobelia ethereal tincture and stramonium tincture with constant stirring. Add and mix potassium iodide and double strength chloroform water. Add sufficient purified water to make up the volume.