Physicochemical Parameters Used in Evaluation of Drugs: Physicochemical parameters are measurable physical and chemical properties used to determine the identity, purity, quality, strength, and stability of crude drugs and natural products.
These parameters are very important in Pharmacognosy because they help in:
- Identification of genuine drugs
- Detection of adulteration
- Determination of purity
- Standardization of crude drugs
- Quality control of herbal materials
The important physicochemical parameters are:
- Extractive values
- Moisture content
- Foreign organic matter
- Ash values
- Bitterness value
- Foaming index
- Haemolytic potential
- Swelling index
- Viscosity
- Optical rotation
- Refractive index
- Acid value
- Saponification value
1. Extractive Values
Extractive value is the amount of active constituents or soluble substances extracted from a drug by a particular solvent.
It is generally expressed as percentage w/w of air-dried drug.
Principle
A known quantity of powdered drug is extracted with a suitable solvent. The solvent is evaporated and the weight of the dried extract is determined.
Important types
- Water-soluble extractive
- Alcohol-soluble extractive
- Sometimes other solvent-soluble extractives may also be determined.
Importance
Extractive values are useful for:
- Evaluating quality of crude drugs
- Detecting adulteration
- Detecting exhausted drugs
- Estimating soluble constituents
Example
If the normal extractive value of a drug is high but a sample shows a much lower value, it may indicate poor quality, adulteration, or removal of active constituents.
2. Moisture Content
Moisture content represents the amount of water present in a crude drug.
It is an important quality parameter because excess moisture can cause:
- Growth of bacteria
- Fungal growth
- Insect infestation
- Enzymatic degradation
- Chemical deterioration
- Loss of active constituents
Determination
A known quantity of drug is dried under specified conditions until a constant weight is obtained.
The loss in weight represents moisture and other volatile matter.
Formula
Importance
Too much moisture = poor stability + microbial growth.
Therefore, proper drying and storage are essential.
3. Foreign Organic Matter
Foreign organic matter refers to any plant or animal material that is not part of the genuine drug.
Examples:
- Weeds
- Other plant parts
- Insects
- Animal matter
- Other unwanted organic materials
Example
If the drug is supposed to consist of leaves, the presence of excessive stems, roots or leaves from another plant may be considered foreign organic matter.
Importance
Determination of foreign organic matter helps to:
- Detect contamination
- Detect improper collection
- Assess purity
- Identify adulteration
4. Ash Values ⭐
Ash value represents the amount of inorganic residue remaining after incineration of a drug.
It is useful for detecting:
- Inorganic impurities
- Sand
- Soil
- Silica
- Adulteration
- Contamination
Types of Ash Values
A. Total Ash: It represents the total inorganic residue obtained after incineration.
It consists of:
- Physiological ash – normal mineral content of the plant
- Non-physiological ash – extraneous inorganic matter such as soil and sand
B. Acid-Insoluble Ash: It is the portion of total ash that remains insoluble after treatment with dilute hydrochloric acid.
It mainly indicates:
- Silica
- Sand
- Earthy matter
Exam point ⭐
Acid-insoluble ash → mainly detects sand and silica.
C. Water-Soluble Ash: It is the portion of total ash that is soluble in water. It can be useful for detecting changes in the soluble inorganic salts of a drug.
D. Sulphated Ash: The sample is treated with sulphuric acid and then ignited.It is used for determining the amount of inorganic residue under specified conditions.
5. Bitterness Value
Bitterness value is a measure of the bitterness of a drug. It is mainly used for drugs containing bitter principles.
Principle
The bitterness of the test drug is compared with a standard bitter substance.
Traditionally, quinine hydrochloride is used as a reference standard.
Expression
The bitterness value is expressed in terms of the amount of standard bitter substance that produces a bitterness equivalent to that of the test drug.
Importance
It is useful for:
- Identification
- Standardization
- Quality control
- Evaluation of bitter drugs
Example
Gentian is a well-known bitter drug.
6. Foaming Index ⭐
Foaming index is used for the evaluation of drugs containing saponins. Saponins have surface-active properties and produce persistent foam when shaken with water.
Principle
An aqueous preparation of the drug is shaken and the height of the foam is measured.
Formula
For the pharmacopoeial method, the foaming index is related to the dilution of the drug preparation that produces a specified foam height.
A commonly used expression is:
Where a is the volume in mL of the drug preparation used to produce the specified foam height.
Importance
It helps in:
- Detection of saponins
- Standardization of saponin-containing drugs
- Quality control
Examples of saponin-containing drugs
- Liquorice
- Quillaia
- Dioscorea
7. Haemolytic Potential
Haemolytic potential is the ability of a drug or its constituents to cause haemolysis (rupture) of red blood cells (RBCs). It is particularly associated with saponin-containing drugs, because some saponins can damage the RBC membrane.
Principle
The test sample is allowed to interact with red blood cells and the degree of haemolysis is determined.
The activity may be compared with a standard haemolytic agent.
Importance
It helps in:
- Evaluation of saponin-containing drugs
- Determining biological activity
- Quality control
- Safety assessment
8. Swelling Index ⭐
Swelling index is a measure of the volume occupied by a drug after it has absorbed water and swollen.
It is particularly useful for drugs containing:
- Mucilage
- Gums
- Pectin
- Other swelling substances
Principle
A known quantity of drug is placed in a graduated cylinder with water and allowed to swell for a specified period.
The volume occupied by the swollen drug is measured.
Expression: It is expressed as the volume in mL occupied by 1 g of the drug, including any adhering mucilage.
Importance
It is useful for:
- Identification
- Quality control
- Detection of adulteration
- Evaluation of mucilage-containing drugs
Examples
- Isabgol (Psyllium)
- Linseed
- Tragacanth
9. Viscosity
Viscosity is the resistance offered by a liquid to flow.
In simple words:
Viscosity tells us how thick or resistant to flow a liquid is.
For example:
- Water → low viscosity
- Glycerin → higher viscosity
- Honey → relatively high viscosity
Importance in Pharmacognosy
Viscosity is useful in evaluating substances containing:
- Gums
- Mucilage
- Fixed oils
- Liquid pharmaceutical preparations
Changes in viscosity may indicate:
- Adulteration
- Degradation
- Changes in concentration
- Poor quality
Measurement
Viscosity can be determined using instruments such as:
- Ostwald viscometer
- Brookfield viscometer
10. Optical Rotation
Optical rotation is the rotation of the plane of polarized light when it passes through an optically active substance. It is measured using a polarimeter.
Some natural compounds are optically active because of their molecular structure.
Types
Dextrorotatory (+): Rotates plane-polarized light to the right/clockwise.
Levorotatory (−): Rotates plane-polarized light to the left/anticlockwise.
Specific Rotation
Specific rotation is represented as:
[α]λT
It depends on:
- Temperature
- Wavelength of light
- Concentration
- Path length
Importance
Optical rotation helps in:
- Identification
- Purity testing
- Detection of adulteration
- Determination of concentration of optically active substances
11. Refractive Index
Refractive index is the ratio of the velocity of light in vacuum to its velocity in the substance. It is commonly used for evaluating oils and other liquid substances. It is measured using a refractometer.
Importance
It helps determine:
- Identity
- Purity
- Quality
- Adulteration
Example
Fixed oils and volatile oils have characteristic refractive-index ranges. A value outside the expected range may suggest adulteration or deterioration.
Factors affecting refractive index
- Temperature
- Wavelength of light
- Chemical composition
12. Acid Value ⭐
Acid value is the number of milligrams of potassium hydroxide (KOH) required to neutralize the free fatty acids present in 1 gram of fat or oil.
Formula
Importance
Acid value indicates the amount of free fatty acids present.
A high acid value generally indicates:
- Hydrolysis of fats/oils
- Rancidity
- Deterioration
- Poor storage
13. Saponification Value ⭐
Saponification value is the number of milligrams of KOH required to neutralize the free fatty acids and saponify the esters present in 1 gram of fat or oil.
In simple words
It indicates the amount of alkali required to saponify (convert into soap) a given amount of fat or oil.
Importance
It is useful for:
- Identification of oils and fats
- Detection of adulteration
- Quality control
- Determination of the approximate chain length of fatty acids
Important relationship
Generally:
Higher saponification value → shorter average fatty acid chains
Lower saponification value → longer average fatty acid chains
Acid Value vs Saponification Value ⭐
| Feature | Acid Value | Saponification Value |
| Measures | Free fatty acids | Free fatty acids + esterified fatty acids |
| Definition | mg KOH needed to neutralize free fatty acids in 1 g oil/fat | mg KOH needed to neutralize free fatty acids and saponify esters in 1 g oil/fat |
| Main use | Indicates hydrolysis/rancidity | Characterization of fats/oils |
| High value may indicate | Increased free fatty acids/rancidity | Relatively more low-molecular-weight fatty acids |
Easy trick:
Acid Value = Acid already FREE
Saponification Value = Soap-making requirement
Editorial Note
This article has been carefully researched and written by Deepak Rajput with a focus on accuracy, clarity, and evidence-based healthcare information. Pharmaacademias.com
