9. FILTRATION

Filtration is one of the most important unit operations in pharmaceutics, used to separate solid particles from liquids or gases using a filtering medium. It ensures that pharmaceutical products such as injections, eye drops, and syrups are clear, free from suspended impurities, and safe for use.

What is Filtration?

Definition:
Filtration is the process of separating solids from liquids or gases by passing the mixture through a porous medium (filter) that retains the solid particles and allows the clear fluid (filtrate) to pass through.

The process is commonly used in pharmaceutical, chemical, and food industries to obtain clear solutions and sterile products.

ORIGINAL NOTES:

Theory of Filtration

Filtration operates on the principle of pressure difference across the filter medium.

When a mixture of liquid and solid is passed through a filter, solid particles are retained on the surface or within the pores of the filter, forming a filter cake, while the liquid (filtrate) passes through.

The rate of filtration depends on:

• Pressure difference across the filter
• Viscosity of the liquid
• Thickness and permeability of the filter cake
• Surface area of the filter medium

The basic equation representing filtration rate is derived from Darcy’s Law, which relates flow rate to pressure, viscosity, and resistance of the filter medium.

Types of Filtration

Filtration can be classified into:

1. Simple filtration: Removes coarse particles using filter paper or cloth.
2. Vacuum filtration: Uses suction to increase the rate of filtration.
3. Pressure filtration: Uses applied pressure to speed up the process.
4. Sterile filtration: Removes microorganisms using bacterial filters such as membrane filters.

Membrane Filter

Principle

Membrane filters operate on the principle of microfiltration.
The liquid is forced through a microporous membrane, which traps particles and microorganisms larger than its pore size (usually 0.22 µm).

Construction

• The filter membrane is a thin, flexible disc made of materials like cellulose acetate, cellulose nitrate, or polycarbonate.
• The membrane is supported by a metal or plastic holder.
• A vacuum pump or pressure source is attached to speed up filtration.

Working

1. The liquid to be filtered is poured onto the membrane.
2. Suction or pressure is applied to draw the liquid through the pores.
3. Suspended particles and microorganisms are trapped on the membrane surface.
4. The clear filtrate is collected in a receiver flask.

Advantages

• Produces sterile filtrate without heating.
• Removes particles and microorganisms effectively.
• Simple, fast, and inexpensive.
• Used for heat-sensitive materials (e.g., antibiotics, vaccines).

Disadvantages

• Membrane may get clogged easily.
• Limited volume can be filtered at once.
• Fragile and must be handled carefully.

Applications

• Sterilization of heat-sensitive injections and ophthalmic solutions.
• Clarification of culture media and biological fluids.
• Used in laboratories for microbial analysis and water testing.

Sintered Glass Filter

Principle

The sintered glass filter works on the principle of mechanical straining and adsorption.
The liquid passes through a porous glass disc, and the solid particles are trapped in the tiny pores of the glass matrix.

Construction

• Made of borosilicate glass fused together to create uniform pores.
• Available in different grades (G1 to G5) based on pore size.
• Mounted in a glass funnel connected to a suction flask for vacuum filtration.

Working

1. The mixture is poured into the sintered glass funnel.
2. Vacuum is applied to draw liquid through the porous glass disc.
3. The solids are retained on or within the pores.
4. Clear filtrate collects in the receiver flask.

Advantages

• Chemically resistant and reusable.
• Provides uniform filtration.
• Withstands high temperatures.
• Easy to sterilize by heating.

Disadvantages

• Fragile and expensive.
• Difficult to clean if pores are clogged.

Applications

• Used for fine filtration in laboratories.
• Suitable for chemical and biological solutions.
• Used in sterile filtration under vacuum conditions.

Applications of Filtration in Pharmaceutics

• Clarification and sterilization of liquids.
• Filtration of air and gases in aseptic areas.
• Recovery of solids from suspensions.
• Used in tablet coating, solution preparation, and vaccine production.