Describes mechanism of filtration, types of filter papers and other filter media used in laboratory and common aids to filtration.

1. Filter Aids
- Devyani Joshi

2. • Mechanism of filtration basically involves
a two step process
▫ 1. The filter paper itself resists the flow of
solid material while permitting the passage of
liquid.
▫ 2. During the course of filtration the
suspended solid material builds up on the
filter medium and thereby forms a filter bed
which acts as a second and often more
efficient filter medium.

3. • Rate of flow is directly proportional to
(driving force) (cross sectional area)
Resistance

4. Filter Paper
• Filter paper is a semi-
permeable paper barrier
placed perpendicular to a
liquid or air flow. It is used to
separate fine solids from
liquids or air
• The important parameters
are wet strength, porosity,
particle retention, flow rate,
compatibility, efficiency and
capacity.

5. When possible first few
milliliters of filtrate should
be
discarded to eliminate
contamination of
pharmaceutical
product by
fibers associated with most
of the filter papers.

6. Types of laboratory filter
papers
1.Qualitative Filter Papers
• 100% cotton linter cellulose
• pH tolerant: 0 to 12
• Thermostable: up to 120°C
• Ash Content: 0.1%
▫ APPLICATIONS
• Clarify and remove precipitates
• Preparation for qualitative analysis

7. 2.Quantitative/Hardened Filter
Papers
• 100% cotton linter cellulose
• Ash Content: 0.01%
• Acid washed: Double acid washed in
hydrochloric then hydrofluoric acid, then rinsed with
ultrapure water to neutralize.
▫ APPLICATIONS
• Gravimetric analysis
• Environmental monitoring

8. 3.Chromatography Papers
• High quality papers are carefully tested for spot
formation, capillary action, water flow rate and
absorption speed to assure uniformity and
reproducibility
• Better resolution with slower flow rate papers
APPLICATIONS
• Chromatography
• Electrophoresis and blotting
• Separation of heavily loaded solutes

9. Other filtering media

10. Cotton filters
▫ A small pledged of absorbent cotton, loosely
inserted in the neck of funnel, adequately
serves to remove large particles of
extraneous material from clear liquid.
▫ It is sometimes necessary to return the
liquid a number of times to secure perfect
transparency.

11. •Glass-wool filters
▫ When solutions of highly reactive chemicals
such as strong acids, are to be filtered, filter
paper cannot be used. In its place glass
wool may be used. This material is resistant
to ordinary chemical action, and when
properly packed into the neck of funnel it
constitutes very effective filtering medium.

12. • Sintered glass filters
▫ These filters have as a
filtering medium a flat or
convex plate consisting of
particles of jenna glass
powdered and sifted to
produce granules of
uniform size that are
molded together. They
vary in porosity depending
on size of granules used in
the plate. A vacuum
attachment is necessary to
facilitate the passage of
liquid through the filter
plate.

13. •Funnels
▫ They are conical shaped
utensils widely used to
support the filter media.
Buchner funnel is largely
used today. A piece of
round filter paper is laid
on the perforated
porcelain diaphragm and
the filtration is
conducted. This funnel
is specially applicable
for vacuum filtration.

14. Aids to filtration
• Addition of insoluble adsorbent powder to a
liquid prior to filtration greatly increases the
efficiency of the process.
• Purified talc, siliceous earth (kieselgurh), clays,
charcoal, paper pulp, magnesium carbonate,
bentonite, silica gel have been used for this
purpose.

15. • Talc- it is nonadsorbent to materials in solution and is
chemically inert medium for filtering any liquid
provided it is not impalpably fine variety that will pass
through the filter paper.
• Keiselgurh- It is almost pure silica (SiO2).It is as
applicable as talc for general filtration purposes with
no danger of removing active constituents by
adsorption.
•
• Siliceous earths or clays-like fuller’s earth or
kaolin in hydrated form which is produced when they
are brought into contact with aqueous liquids are safe
for general use only in filtering fixed oils.

16. • Charcoal- it possesses adsorptive properties not only
towards colour but for many active constituents such
as alkaloids or glycosides. Thus charcoal should never
be used as filtering medium unless removal of such
materials is desirable.
• Chalk and magnesium carbonate- they react
readily with acids and possess finite solubility in water
and aqueous fluids. When they are added to alkaloidal
preparation prior to filtration will precipitate and
remove all the alkaloidal constituents.