Methods of Physical Evaluation of Crude Drugs

Drug evaluation is an important step that helps identify and determine the purity as well as quality of a drug. It is crucial due to the biochemical variation, adulteration, effect of treatment, etc.

There are different types of evaluation methods such as microscopic, physical, chemical, biological, organoleptic, etc.

Criteria for Physical Evaluation of Crude Drugs

The physical evaluation of crude drugs is done to determine the solubility, optical rotation, refractive index, specific gravity, melting point, viscosity, fiber elasticity, water content, etc., among other physical characteristics.

Solubility

Each drug responds differently toward each solvent and this behavior is considered while examining the properties of oils, resins, etc. It is insoluble in water but soluble in different solvents such as alcohol, light petroleum, ether, etc.

Pure castor oil is soluble only in 3 volumes of 90% alcohol. However, contaminated castor oil exhibits better solubility in alcohol, which helps differentiate them.

Optical Rotation

Some substances are said to be optically active when they can rotate the plane-polarized light towards the left or left when passing through them. This phenomenon is called optical rotation. When the light is rotated towards the right or in the clockwise direction, it is called dextrorotatory and when it is toward the left or anticlockwise direction, it is called levorotatory.

Each drug in its pure form will have a specific optical activity level. By examining this physical phenomenon, one can determine the purity of the crude drug under question.

Pure blossom honey is levorotatory and in the adulterated form, it will be dextrorotatory.
The optical rotation of eucalyptus oil is between -2.0 and +4.0.
Pure chenopodium oil is levorotatory.

Refractive Index

The refractive index is the property of a material to change the angle of incident light when passing through it. When light passes between two materials, it travels at an angle specific to the materials. The resulting refractive indices are used to calculate the angle of transmission refraction of light. Thus it can be used as a parameter to evaluate the quality of drugs as well.

Each crude drug will have a specific range of refractive index in its purest form which helps determine its purity. The refractive index range of a few crude drugs is given below.

Castor oil: 1.4758-1.527
Clove oil: 1.527- 1.535

Specific Gravity

Specific gravity is the ratio of the mass of a solid or liquid to the mass of the equal volume of distilled water, at a specific temperature of 4⁰C. For gases, it is the ratio of the mass of gas to the equal volume of air or hydrogen gas under a specific temperature and pressure. Each crude drug will have its range or specific gravity in pure form.

Castor oil: 0.95
Cottonseed oil: 0.86-0.93
Coconut oil: 0.925

Viscosity

Viscosity is the resistance of a fluid against its flow. The viscosity of a liquid will be constant at a given temperature so it is used as an index of its composition. In Pyroxylin, the kinematic viscosity is around 1100-2450 centistokes.

Melting Point

The melting point is the temperature at which a solid is converted into liquid. Each plant constituent has a specific and constant melting point. For crude drugs, the melting point range is considered due to its chemical composition. Thus each crude drug has a specific value of range which is considered to evaluate the purity of the crude drug.

Beeswax: 62-65⁰C
Woodfat: 34-44⁰C
Agar: 85⁰C

Moisture Content

It is important to control the moisture content of a crude drug as it can cause chemical change, microbial growth, or decomposition. The moisture content of a drug is constant or should not be more than a specific percentage. During physical evaluation, the drug is heated at 105⁰C to a constant weight, in an oven. For Digitalis, the moisture content at the end of the process should not be more than 5% W/W, and for Ergot, it is 8% W/W.

Ultraviolet Light

Some drugs exhibit fluorescence when exposed to ultraviolet radiation. It can happen in its powdered form or with its cut surface. This helps identify the varieties of the same drug which exhibit undistinguishable characteristics. But the fluorescence reveals distinguishable characters thereby helping their identification.

Ash Values

Here, different types of ash values such as total ash value, water-soluble ash value, acid-insoluble ash value, sulfated ash value, etc are used to detect crude drugs from mixtures.

Total ash value is used for crude drugs mixed with mineral or inorganic substances. Here, the use temperature higher than 450⁰C should not be used as it can cause the loss of volatile oils.
Acid insoluble ash value means the insolubility of ash in dilute hydrochloric acid. Since most crude drugs contain calcium oxalates which are converted into calcium oxide after incineration, they will be soluble in HCl. The remaining insoluble ash is weighed to determine earthly matter. This method is widely used for rhizomes, roots, and densely pubescent leaves.
Water soluble ash helps detect materials that are exhausted by water.
Sulfated ash is made by mixing with sulfuric acid to get sulfate salts. The percentage of this ash is calculated against air-dried drugs.

Extractive Values

Different solvents are used to exhaust crude drugs to obtain their extracts. The solvent is chosen according to the chemical constitution and target constituent. A water-soluble extrctive is chosen for water-soluble constituents such as tannins, mucilage, glycosides, etc. Ether is chosen for volatile oils and alcohol is used for resins.

Foreign Organic Matter

Foreign organic matter refers to the organs of the drug other than the ones mentioned in its description such as earthly materials, insects, animal excreta, molds, etc. Each drug has a limit to such contents that helps determine its purity.

References

https://www.jiwaji.edu/pdf/ecourse/pharmaceutical/Evaluation%20of%20crude%20drugs.pdf
Shah, Biren N, Avinash Seth. (2010). Textbook of Pharmacognosy and Phytochemistry. Elsevier.