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105
Unit Operations
is tested for complete exhaustion of the drug by various
tests, the marc is then pressed and expressed liquid is
added to the already collected percolate which should be
about 80–90% of the final volume. More of menstruum is
added to produce the required volume. The liquid is then
allowed to stand to settle the suspended particles, decanted
or clarified by filtration.
Tests to check complete exhaustion of the drug:
y Take a few ml of the last percolate and evaporate to dry-
ness, if no residue remains this shows that the drug is
completely exhausted.
y Measure the specific gravity of last few ml of percolate
if it is equal to the specific gravity of the menstruum, it
shows exhaustion of the drug.
y Specific tests may be performed on the percolate for the
drugs containing alkaloids, glycosides, tannins, resins
or bitter constituents.
Reserved Percolation
It is a percolation process in which first portion (about 3/4th
of final product) of the percolate which contains the maximum
amount of active constituents is reserved and subsequent
percolation is completed as usual until the drug is exhausted
but the last part, about l/4th of final volume is collected
separately. The second dilute part is then evaporated to get
a syrupy consistency which is then mixed with the reserved
portion of the percolate and the final volume is adjusted by
adding more of menstruum.
Reserved percolation process is used for the preparation
of liquid extracts as they are more concentrated preparations
as compared to tinctures prepared by simple percolation
process. Generally alcohol is used as menstruum for reserved
percolation process and first portion of percolate which contains
the bulk of dissolved active constituents is reserved and only
the last portion which is dilute is subjected to evaporation, the
concentrated product of which is mixed with the reserved part.
Liquid extract of liquorice is prepared by this process.
Introduction to Pharmaceutics
106
Advantages
The reserved part of percolate which contains the
maximum amount of dissolved principles is not subjected
to heat treatment for evaporation, only the dilute portion of
percolate is evaporated.
This process is economical as whole of the percolate is not
evaporated.
Continuous Hot Percolation Process or
Soxhlet Extraction
Continuous hot percolation process is used for those drugs
where the penetration of the menstruum into the cellular
tissues is very slow and the solute is not readily soluble into
the solvent and the quantity of menstruum is very less. In
such cases Soxhlet extractor is used where small volume of hot
menstruum is passed over the drug time and again to dissolve
out the active constituents until the drug is exhausted. This
process is known as Soxhlation (Fig. 4.19).
Fig. 4.19: Soxhlation
107
Unit Operations
The Soxhlet apparatus required for the continuous hot per-
colation is made from a very high grade of glass and consists of
three parts: (a) A flask in which the menstruum is boiled, (b) an
extraction chamber in which drug is filled, is fitted with a side
tube and a siphon and (c) a condenser.
The drug to be extracted, in suitably comminuted form
is usually packed in a ‘thimble’ made of filter paper which
is then placed into the wider part of che extractor. ‘Thimble’ is
used to prevent choking of the lower part of the extractor by
drug particles. Menstruum is placed in the flask and boiled,
the vapours are allowed to pass through the side tube to the
condenser where they are condensed and fall onto the packed
drug, through which it percolates and extract out the active
constituents. As the volume of menstruum in the extractor
increases, the level of the liquid in the siphon also increases till it
reaches the maximum point from where it is siphoned out into
the flask. On further heating, the menstruum vaporizes while
the dissolved active constituents remain behind in the flask.
The alternate filling and emptying of the body of the extractor
goes on continuously till the drug is exhausted. Thus the same
quantity of menstruum is made to percolate repeatedly, about
14 to 15 times through the drug and the active constituents are
collected in the flask.
This process is not suitable for the drugs containing
thermolabile active constituents.
5
UNIT
Dosage Forms
TABLET DOSAGE FORMS
Tablets may be defined as the solid unit dosage form of
Medicament or medicaments with or without suitable diluents
and prepared either by molding or by compression. They vary
greatly in shape, size and weight which depends upon the
amount of medicament and the mode of administration. Most
commonly the tablets are disk shaped with covex surfaces
but they are also available in special shapes like round, oval,
oblong, cylindrical, square, triangular, etc. They vary greatly
in weight. The tablets for oral administration may weigh from
0.2 to 0.8 gm including the diluents but the tablets meant for
administration other than oral route may be lighter or heavier.
Tablets are the most widely used solid dosage form of
medica ment because they offer a number of advantages to
the patient, prescriber, manufacturer and the manufacturing
pharmacist. Because of these advantages their popularity is
continuously increasing day by day.
Advantages
Tablets have following advantages:
They are easy to carry.
They are easy to swallow.
They are attractive in appearance.
Unpleasant taste can be masked by sugar coating.
They do no require any measurement of dose. The strip or
blister packing has further facilitated the process of taking
109
Dosage Forms
the dose by the patient. Moreover, it provides a sealed
covering which protects the tablets from atmospheric
conditions like air, moisture and light, etc.
Some of the tablets are divided into halves and quarters by
drawing lines during manufacturing to facilitate breakage
whenever a fractional dose is required.
An accurate amount of medicament even if very small can
be incorporated.
Tablets provide prolonged stabilily to medicament.
The incompatibilities of meclicaments and their deterio-
ration due to environmental factors are less in tablet forms.
Since they are generally produced on a large scale therefore
their cost of production is relatively low, hence economical.
ESSENTIAL QUALITIES OF A GOOD TABLET
They should be accurate and uniform in weight.
The drugs should be uniformly distributed throughout the
tablets.
The size and shape should be reasonable for easy
administration.
The tablets should not be too hard that it may not
disintegrate in the stomach.
There should not be any incompatibilities.
They should be chemically and physically stable during
storage.
They should not break during transportation or crumble in
the hands of the patient.
They should be attractive in appearance.
There should not be any manufacturing defects like
cracking, chipping or discoloration.
After administration it should disintegrate readily.
They should be easy and economical in production.
TYPES OF TABLETS
Mainly the tablets are classified into two classes: (1) Compressed
tablets; (2) molded tablets. The compressed tablets are usually
prepared on large scale production methods whereas the
molded tablets are prepared extemporaneously on a small
Introduction to Pharmaceutics
110
scale. These two main types of tablets are further classified
as follows:
Compressed Tablets
Oral tablets
Chewable tablets
Buccal/sublingual tablets
Lozenge tablets
Soluble tablets
Effervescent tablets
Implants
Vaginal tablets (Inserts)
Enteric coated tablets
Sustained action tablets
Sugar coated tablets
Film coated tablets
Layered tablets
Press coated tablets
Molded Tablets
Hypodermic tablets
Dispensing tablets
Lozenges
Tablet triturates
MOLDED TABLETS
Tablet Triturates “TT”
Molded tablets are small disk shaped tablets which are
prepared by forcing the soft mass into the cavities of the mold.
Generally potent medicaments and highly toxic drugs in small
doses are used for preparing the molded tablets. The potent
medicament is diluted with a diluent like lactose, dextrose,
sucrose or a mixture of lactose and sucrose. The mixed powders
are moistened with a suitable dilution of alcohol (generally 50%
alcohol is used) and mixed thoroughly so as to get a soft mass.
The soft mass so prepared is pressed into the perforations of
the mold with a spatula. The excess of the mass is removed
111
Dosage Forms
by applying pressure over the spatula. This perforated plate
of the mold filled with the mass is placed over another plate
having exactly the same number of projecting pegs as that of
perforations and these projecting pegs completely fit into the
holes. A little pressure is applied over top plate which will
force the plate move downward, leaving the molded tablets
on the projecting pegs. The ejected tablets are spread in single
layers on clean surface and dried either by keeping in a warm
place or hot air oven.
Nowadays tablet triturates may also be prepared on
automatic tablet triturate machines including tablet making
machines (Fig. 5.1). A tablet triturate machine can prepare
2500 tablet triturates per minute.
Hypodermic Tablets
Hypodermic tablets are soft readily soluble tablets which are
made in a tablet triturate mould. They are used for preparing
solutions to be injected, therefore, in selecting the materials
used for preparing the hypodermic tablets care must be
taken that they should be completely and readily soluble
and no insoluble particle should be present. They should be
free from bacterial contamination and proper precautions
should be taken during molding regarding contamination and
cleanliness.
Fig. 5.1: Tablet triturate mold.
Introduction to Pharmaceutics
112
Since the solutions prepared from hypodermic tablets are
rarely sterile and a number of sterile parenteral solutions are
now available therefore the use of hypodermic tablets for
preparing solution for injections is being discouraged.
Dispensing Tablets
These tablets are prepared for providing an accurate and
convenient quantity of a drug that can be incorporated readily
in compounding other dosage forms, e.g. liquids, powders,
or capsules thus eliminating the necessity of weighing small
quantities of potent substances. These tablets are solely
designed to provide a convenient quantity for administration
as a dosage form because sometimes they contain very potent
drugs which may prove fatal.
Compressed Tablets
Oral Tablets
These tablets are swallowed as such. They are placed over
the tongue and swallowed with a drink of water or any other
suitable liquid. Most of them are formulated in such a way that
they disintegrate in the stomach and dissolve in the gastric
fluids thus absorption takes place from there. Some of the
tablets like enteric coated tablets pass the stomach as such and
disintegrate in the intestines from where the absorption takes
place. Among the compressed tablets the oral tablets constitute
the major group because majority of the compressed tablets
are meant for oral administration.
Since the oral tablets are required to be disintegrated in a
very short time therefore either the medicaments itself should
be freely soluble in gastrointestinal fluids or disintegrating
agent must be added in the formulation of tablets.
Chewable Tablets
These are the tablets which are required to be broken and
chewed in-between the teeth before ingestion. These tablets
are given to the children who have difficulty in swallowing
and to the adults who dislike swallowing. A number of antacid
113
Dosage Forms
tablets and multivitamin tablets are prepared as chewable
tablets.
For the preparation of chewable tablets mannitol is used as a
base. It is a white crystalline, chemically inert, nonhygroscopic,
thermostable powder and is as sweet as that of glucose. It does
not have any objectionable effects but since it is expensive so it
cannot be used in tablets of low cost, therefore, other substances
like sorbitol lactose, chocolate powder, dextrose and glycine
can be substituted in place of mannitol.
These tablets should have very acceptable taste and
flavour. They should disintegrate in a short time and produce
cool sweet taste. Chewable tablets can be taken at any place
even if water is not available. These tablets do not require any
disintegrating agent to be present in the formulation. The use
of gums and other substances which produce hard granules
should be avoided. The lubricants used should have agreeable
taste.
Buccal/Sublingual Tablets
These tablets are required to be placed below the tongue (sub-
lingual) or in the side of the cheek (buccal) for the slow release
of the medicament except in the case of nitroglycerin and
mannitol hexanitrate tablets which should dissolve in 1 to 2
minutes to produce immediate effect.
Generally these types of tablets contain those drugs which
are destroyed, inactivated or not absorbed in the GIT but
are directly absorbed through the mucosal tissues of the oral
cavity. These tablets contain large proportions of sweetening
agents like mannitol and/or sucrose to impart sweetness.
Lozenge Tablets
These tablets should not disintegrate in the oral cavity but
should dissolve slowly in the mouth to produce continuous
effect on the mucous membrane of the throat. They can be
prepared by molding as well as by compression method. The
lozenges prepared by compression are known as lozenge
tablets. In the formulation of lozenges no disintegrating agent
is included rather the quantity of binding agent is increased so
Introduction to Pharmaceutics
114
as to produce slow dissolution. The formulation must contain
sweetening and flavoring agents.
Soluble Tablets
These tablets are required to dissolve completely in the liquid
to produce solution of definite concentration. The solutions
prepared by dissolving the soluble tablets may include mouth
washes, gargles, skin lotions, douches, antibiotics and certain
vitamins. The use of soluble tablets, ‘hypodermic tablets’ for
the preparation of injectable solutions is restricted because the
solutions so prepared are not sterile.
For the preparation of soluble tablets all the ingredients
used in the formulation must be completely soluble in solvent.
Effervescent Tablets
These tablets along with the active medicament contain other
ingredients like sodium bicarbonate, citric acid and tartaric
acid which react in the presence of water liberating carbon
dioxide and producing effervescence leading to disintegration
of the tablet, thus hastens solution formation and increases the
palatability. These tablets have the advantage over effervescent
powders of being in tablet dosage form.
Vaginal Tablets (Inserts)
Sometimes vaginal suppositories or pessaries are prepared by
compression which are known as vaginal tablets. Antibiotics
and steroids are formulated in this way. Soluble additives are
used for their preparation. They are generally ovoid or almond
shaped for ease of insertion. Plastic tube inserters can also be
used for placing these tablets in vaginal tract.
Some laxative suppositories are also formulated as com-
pressed tablets. The active medicaments are mixed with such
disintegrating agents which either swell up after absorb-
ing moisture or produce effervescence thus facilitating
disintegration. These rectal tablets are covered with thin layers
of polyethylene glycol which act as protective covering and
also facilitate the insertion of these tablets into the rectum.