• Precipitation method:- In this method, the substance is firstly dissolved in an appropriate solvent and then after it is finely precipitated by the addition o another solvent. Both the solvents are miscible but the substance remains insoluble in the 2^nd This method is suitable for the production of raw materials and bulk drugs. Inorganic chemicals such as CaCO3 and MgCO3 are also prepared by this method.
• Mechanical method:- In this method, the substance is applied with mechanical force using grinding equipment like ball mill, hammer mill, cutter mill etc. Generally, milling or dry grinding is done for the production of tablets and capsule. Wet grinding is used for the production of suspensions, emulsions and ointments. This method is used either for the production of raw materials and is used during manufacturing cycle of dosage forms.

Advantages of size reduction

1. Content uniformity– Mixing turn out to be effective only when the ingredients are small and of uniform size. As the particle is of small size, the no of particles per unit dose is large. And greater the no. of particles, mixing becomes more efficient. This criterion is particularly more important for the formulations that contain low dose drugs.
2. Uniform flow– If the size of particle is small, the flow of powder into dies during the compression of tablets is effective.
3. Effective extraction of drugs– Penetration of solvent into tissue or cells of organic origin becomes rapid when the particle size is smaller.
4. Effective drying– If the size of granules is small, then the drying of granular mass becomes rapid.
5. Improved physical stability– In suspensions and emulsions, the rate of sedimentation decreases if the particles are of small and uniform size.
6. Improvised rate of dissolution– When the surface area is larger, than the dissolution of a substance increases. Thus size reduction facilitates in increasing the surface area.
7. Improved absorption rate– Smaller the particle, the faster is the absorption is the absorption because the dissolution is also enhanced.

Disadvantages of size reduction

1. Drug degradation– Decomposition of drug can occur due to the excessive heat production during milling. Another reason is increased surface area also contributes in drug decomposition.
2. Poor mixing– Generally, very fine particles are subjected to strong cohesive forces hence aggregation of particles takes place. But this aggregation inhibits the efficient blending of different ingredients.

Mechanism of Size Reduction

Mechanism of size reduction varies the nature of the particle. Hence different modes of stress are required for different substances.

Multiple Choice Questions (MCQs)

1. Size reduction is done by how many methods?
A. 2
B. 3
C. 4
D. 5

2. Which of the following substance is size reduced by precipitation method?
A. Fibrous material
B. Coal
C. Mercuric oxide
D. All of the above

3. What happens in attrition mode?
A. Blades are used for size reduction
B. Forceful object strikes the stationary object
C. Force is applied by the means of hammer
D. Rubbing the material between two surfaces

4. What type of materials is size reduced by compression?
A. Fibrous
B. Waxy
C. Brittle
D. None of the above

5. Match the following mill which is not used for following material-
A. Cutter mill 1. Soft material
B. Hammer mill 2. Sticky material
C. Ball mill 3. Friable material
D. End runner mill 4. Abrasive material

6. Which of the following is counted as the advantages of size reduction?
A. Content uniformity
B. Absorption is increased
C. Sedimentation rate decreases
D. All of the above

7. Brittle drugs are size reduced by which mill?
A. Ball mill
B. Hammer mill
C. Fluid energy mill
D. Both A and B

8. Which of the following mill works on the principle of both attrition and impact?
A. Ball mill
B. Hammer mill
C. Cutter mill
D. Pin mill

9. Which of the following statement is NOT true?
A. Penetration becomes slow when particles are large
B. Wet grinding is used for production of tablets
C. Colloid mill is not used for dry milling
D. Impact is done in 2 ways

10. Which type of particles is preferred for compound powders?
A. Very fine particles
B. Moderately coarse particles
C. Both can work
D. None of the above

ANSWERS:-
1. A
2. Mercuric oxide
3. Rubbing the material between two surfaces
4. None of the above
5. A – 3 B – 4 C – 1 D – 2
6. All of the above
7. Ball mill
8. Ball mill
9. Wet grinding is used for production of tablets
10. Very fine particles

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