Suspensions: Classifications, Preparation of suspensions, Additives
Mixtures containing diffusible solids
Diffusible solids are those substances that do not dissolve in water, but on shaking they can be mixed with it and remain evenly distributed throughout the liquid for a sufficiently long time allowing uniform distribution of the drug in each dose. However, on standing the insoluble solids settle at the bottom of the bottle which require shaking of the bottle each time whenever a dose is to be measured.
A preparation containing the following diffusible solids:
Bismuth carbonate Magensium carbonate
Light kaolin Magnesium trisilicate
Magnesium oxide Rhubarb powder
Method of dispensing
- Diffusible solids are finely powdered with a mortar and pestle. They are mixed thoroughly.
- 3/4th volume os vehicle is separated. From that portion small amount of vehicle is mixed and triturated to form a smooth paste. Remainder of the vehicle is added to that paste.
- The mixture is passed through a muslin cloth to remove any foreign particles.
- Liquid ingredients are added.
- Volume is made up with the vehicle.
- Mixture is transferred to a bottle, polished, labeled and dispensed. Label must contain “Shake the bottle before use”.
Mixtures containing indiffusible solids
Indiffusible solids are those substances that do not dissolve in water and they do not remain evenly distributed throughout the vehicle, even after shaking they immediately settle at the bottom – therefore, it becomes difficult to measure the dose.
Preparations containing: Aspirin Aromatic chalk powder
Chalk powder Phenobarbitone
To reduce the settling of particles the viscosity of the mixture is increased by adding some thickening agents like gum acacia, tragacanth or compound tragacanth powder or their mucilage.
Method of dispensing
- The indiffusible powders are triturated with a mortar and pestle to make them fine powder.
- Any diffusible or soluble powder substances are mixed with suspending agent (power). This powder mixture is triturated with a small amount of vehicle to form a smooth paste; then the remaining amount (3/4th of the vehicle) is added and mixed.
- Strained through a muslin cloth to remove any foreign particle.
- Requied volume is made up with the rest amount of vehicle.
- The mixture is transferred to bottle, labeled and polished. The label must contain “Shake well before use”.
Mixture containing precipitate forming liquid
Some liquid preparations contain resinous matter that is precipitated in addition to water.
Preparations containing any of the following preparations:
Compound Benzoin Tincture
Lobelia Ethereal Tincture
These preparations when mixed with water, resins precipitate out adheres to the side of the container and forms non-dispersible clots in the liquid. To prevent this a protective colloid is dispersed in the vehicle before the tincture is added.
Tragacanth mucilage or compound tragacanth powder is suitable.
As the resin particles are precipitated the hydrocolloids (acacia, tragacanth and starch) are adsorbed over their surface conferring hydrophilic properties and preventing aggregation into clots.
Method using compound tragacanth powder
This method is chosen when the vehicle is not water or chloroform water. [N.B to avoid displacement of part of the medicinally active vehicle by the mucilage]
- Any insoluble solid is powdered and mixed thoroughly with the gum (powder).
- The powder mixture is triturated withi small amount of vehicle to form a smooth cream and diluted gradually to about ½ the final volume.
- The precipitate forming liquid is measured in a dry measuring cylinder and pour in a slow stream into the center of the suspension, stirred rapidly.
- If electrolytes are present they can be added only after the resin has been protected (otherwise heavy clot will occur).
- The electrolyte are diluted with about ½ of the remaining vehicle.
- The electrolyte is added slowly with constant stirring to reduce local concentration.
- Volume is made up with the rest of the vehicle.
Method using tragacanth mucilage
This method is chosen when the vehicle is water or chloroform water.
- To mucilage is mixed with an equal volume of vehicle in a beaker.
- The precipitate forming liquid is measured in a dry cylinder and poured slowly into the center of the mucilage with constant stirring.
- Electrolyte is dissolved in ½ of the remaining vehicle and added slowly in the mixture with constant stirring.
Lobelia ethereal tincture – precipitate forming liquid
Tragacanth mucilage – mucilage, viscosity building agents
Potassium iodide – electrolyte
Chloroform water D.S. Vehicle
- Chloroform water D.S. + equal volume of water → Chloroform Water (Vehicle)
- Tragacanth mucilage + equal volume of vehicle → Mixture 1
- Lobelia ethereal tincture + Stramonium tincture are poured slowly in mixture –1
- Potassium iodide + ½ of the remaining vehicle → Solution-1
- Solution-1 is poured slowly in Mixture-1 with constant stirring.
- Vehicle is added to make up to the volume. The mixture is transferred in a bottle, labeled, polished.
Mixture containing colloidal particle
The particles are insoluble, but are in colloidal range hence, remain stable in suspension.
Magnesium hydroxide mixture (Milk of magnesia)
- Method: By precipitation
A solution of NaOH is added to a solution of magnesium sulfate. The precipitate of magnesium hydroxide is washed and allowed to stand. The clear supernatant liquid is decanted to remove SO42–.
MgSO4 + 2NaOH = Mg(OH)2 ↓ + Na2SO4.
This suspension sediments quickly but can be easily redispersed by shaking.
- Method: By hydration of MgO
In presence of water magnesium oxide becomes hydrated to produce magnesium hydroxide.
MgO + 2H2O = Mg(OH)2↓
This method produces a viscous and unpourable suspension on keeping.
3) Method : Precipitation and hydration
- A solution of NaOH is triturated with light magnesium oxide to form a smooth cream.
- The cream is diluted and added slowly to a stirred solution of magnesium sulphate.
- After the Mg(OH)2 has settled the supernatant liauid is decanted and the precipitation washed with purified water unless if is virtually free from SO42–.
- The precipitation is then mixed with purified water, chlroform water is added to give a final concentration of 0.25% v/w. The preparation is adjusted to final volume.
Additives used in suspension dosage forms
1. Wetting agents:
Some substances (e.g. sulfur, hydrocortisone etc.) are both insoluble in water and are poorly wetted by it. During preparation, it is difficult to disperse the clumps and the foam produced on shaking. So wetting agents are used to reduce the interfacial tension between the solid particle and the vehicle and increase wetting of the particle.
e.g. alcohol, glycerin, propylene glycols, saponins of quillaia extract etc.
2. Flocculating agents:
On standing for a long period the suspension may become difficult to redisperse on shaking. At that time, controlled flocculation is required to prevent compact sediment, which is difficult to redisperse. Controlled flocculation can be produced either by,
(i) electrolytes (e.g. potassium citrate, phosphate salts), (ii) surfactants, and (iii) polymers.
3. Suspending agents / Thickening agents:
Suspending agents are the substances, which are added to a suspension to increase the viscosity of the continuous phase so that the particles remain suspended for a sufficiently long time and it becomes easy to measure an accurate dose.
Due to the increase in viscosity of the vehicle, the particles sediment at a much slower rate.
e.g. methylcellulose, hydroxypropyl methylcellulose, sodium carboxymethylcellulose, bentonite, veegum etc.
The aqueous vehicle may be liable for bacterial growth, so a preservative should be used. e.g. benzoic acid, sodium benzoate, methylparaben, propylparaben etc. may be used.
5. Organoleptic additives:
Colours, sweeteners and flavouring agents may be used to make the oral suspensions more palatable.
Colours: e.g. Amaranth, Tartrazine, Caramel, and other approved colours
Sweeteners: e.g. Sucrose
Flavours: e.g. Peppermint oil, Chocolate flavour, Raspberry syrup etc.
Biphasic Liquids: Suspension: Definition, advantages, and disadvantages, Classifications, Preparation of suspensions, Flocculated and Deflocculated suspension Emulsions: Definition, Advantages & Disadvantages, Classification, Emulsifying agent, Test for the identification of the type of Emulsion, Methods of preparation, Stability of emulsion
First Year B Pharm Notes, Syllabus, Books, PDF Subjectwise/Topicwise