Extraction, Distillation, Evaporation and Drying
9 minute read
What you will learn
- Define extraction and name the main methods
- Explain distillation and its uses
- Explain evaporation and how it differs from distillation
- Describe drying methods and equipment
- Calculate the strength of a tincture, a concentration factor and a loss on drying
- State the precautions for heat-sensitive drugs
Key points
- Extraction removes the active parts of a drug, usually from plant material, using a solvent called the menstruum.
- Methods include maceration, percolation, infusion, decoction and, in the laboratory, Soxhlet extraction.
- Distillation separates liquids by boiling them and condensing the vapour. It is used for volatile oils and aromatic waters.
- Evaporation removes solvent from a solution to make it more concentrated.
- Drying removes moisture from solids, granules and extracts.
- Heat can damage drugs, so vacuum, low temperature and freeze drying are used for delicate ones.
- Moisture content is checked as 'loss on drying'.
Extraction
Extraction removes the useful constituents from a drug, most often a plant or animal material, by using a solvent. The solvent is called the menstruum. The part left behind is the marc. The liquid obtained is the extract, such as a tincture or liquid extract.
The choice of menstruum depends on what dissolves the wanted substance: water, alcohol, mixtures of alcohol and water, or others.
Methods
| Method | How it works | Notes |
|---|---|---|
| Maceration | The coarse drug is soaked in the menstruum in a closed container for several days, with stirring, then pressed and filtered | Simple, suits small scale |
| Percolation | The menstruum is allowed to trickle slowly through the drug packed in a cone-shaped vessel (percolator) | Gives a more complete extraction |
| Infusion | Hot water is poured on the drug and left for a short time | Fresh infusions for soft parts, used quickly |
| Decoction | The drug is boiled in water for a period | For hard parts such as bark and roots. Not for heat-sensitive substances |
| Digestion | Maceration at a gentle heat | Speeds up extraction |
| Soxhlet extraction | Solvent is boiled, condensed, and repeatedly passed over the drug | Efficient, used in laboratories |
Strength of an extract
Example. A tincture is made as 1 in 5. How much drug is needed for 500 mL?
- 500 ÷ 5 = 100 g of drug
- Strength = 100 ÷ 500 × 100 = 20% w/v
Distillation
Distillation separates liquids by boiling and condensing the vapour. The substance with the lowest boiling point turns to vapour first, then is cooled in a condenser and collected as the distillate.
| Type | Use |
|---|---|
| Simple distillation | Purified water from tap water, and liquids with very different boiling points |
| Fractional distillation | Separating liquids with close boiling points, using a column |
| Steam distillation | Volatile oils from plants, such as peppermint oil. Steam carries the oil over at a temperature below its boiling point, so it is not damaged |
| Vacuum distillation | Heat-sensitive substances, because lowering the pressure lowers the boiling point |
Products of distillation include purified water, aromatic waters, volatile oils and alcohol.
Evaporation
Evaporation removes solvent from a solution, usually water, by heating, so that the solution becomes more concentrated or a solid is left. Unlike distillation, the vapour is not collected.
- On a small scale, a dish on a water bath is used.
- On a large scale, evaporators with steam jackets, often under vacuum, are used.
- Vacuum evaporation works at a lower temperature, which protects heat-sensitive drugs.
Example. 200 mL of a solution is evaporated to 50 mL.
- Concentration factor = 200 ÷ 50 = 4 times, if no solute is lost.
Drying
Drying removes moisture from solids, granules, powders and extracts. It prevents decay, helps storage, and is needed before tabletting or filling.
| Method | How it works | Suits |
|---|---|---|
| Tray (oven) drying | Material on trays in warm air | Many powders, granules and plant materials |
| Fluid bed drying | Warm air blown up through granules, keeping them suspended | Granules, quick and even |
| Spray drying | A liquid is sprayed into hot air and dries to fine particles | Extracts, instant powders |
| Vacuum drying | Drying under reduced pressure at low temperature | Heat-sensitive materials |
| Freeze drying (lyophilisation) | Frozen material loses ice by sublimation under vacuum | Vaccines, antibiotics, proteins |
| Air or sun drying | Natural warmth and air | Plant material. Needs protection from dust, rain and sunlight damage |
Loss on drying
Loss on drying (LOD) is the percentage of weight lost when a sample is dried.
LOD (%) = weight lost ÷ original weight × 100
Example. A 10 g sample loses 0.5 g.
- LOD = 0.5 ÷ 10 × 100 = 5%
If the moisture is too high, the product may mould, cake or break down. Pharmacopoeias set a limit for each product.
Protecting the drug
- Low temperature, vacuum and freeze drying protect heat-sensitive drugs.
- Dry in clean air, and avoid contamination.
- Store dried products in tight containers away from moisture.
Exam focus
- Name and describe maceration and percolation.
- Define distillation and give an example product.
- Calculate tincture strength, concentration factor and loss on drying.
- Common mistake: using 'distillation' and 'evaporation' as if they were the same.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.Extraction is the process of:
2.In maceration, the drug is:
3.A tincture is made as 1 in 5 (20% w/v). How much drug is used to make 500 mL?
4.200 mL of a solution is evaporated down to 50 mL. By what factor is it concentrated?
5.A 10 g sample loses 0.5 g on drying. What is the loss on drying?