Shenyang Pharmaceutical University
LAB 1: SUSPENSIONS
PHARMACEUTICS I
LABORATORY 1: Preparation of suspensions and
selection of stabilizers
1. LABORATORY OBJECTIVES
a) To learn the preparation methods of suspensions.
b) To know the definition of sedimentation volume and familiarize with the measurement
method.
c) To learn how to select stabilizers based on the properties of the drug substances.
2. INTRODUCTION
Suspensions are defined as preparations containing finely divided drug particles (the
suspensoid) dispersed uniformly in a liquid medium in which the drug exhibits limited
solubility. In most stable pharmaceutical suspensions, the particle diameter is about 1 to 50
µm. There are many factors to be considered when developing and preparing a
pharmaceutically elegant suspension. In addition to the therapeutic efficacy, chemical
stability of the formulation components, and esthetic appearance of the preparation, a
properly prepared pharmaceutical suspension should settle slowly and be readily
resuspended upon gentle shaking without the formation of a solid cake on standing.
Furthermore, the particle size of the suspensoid should remain unchanged in the vehicle
throughout the product shelf-life and an accurate dose should be readily withdrawn from
the container either by pouring or by means of a syringe.
The various factors governing the rate of settling of a particle in a suspension are
described by the Stokes’ equation which is presented as:
2 r 2 ( 1   2 ) g
V
9
where V is the rate of settling, r is the particle radius, ρ1 is the density of the particle, ρ2 is
the density of the medium, g is the gravitational constant, and η is the viscosity of the
medium.
A number of factors can be adjusted to enhance the physical stability of a suspension,
including the diameter of the particles, the density, and the viscosity of the medium. From
the Stokes’ equation, it is apparent that the velocity of sedimentation of a particle is faster
for larger particles than smaller particles when all other factors remain constant. Reducing
the particle size of the dispersed phase can result in a slower rate of settling of the particles.
Also, the greater the density of the particles, the faster is the rate of sedimentation when
the density of the vehicle is kept constant. The rate of sedimentation may be significantly
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Shenyang Pharmaceutical University
LAB 1: SUSPENSIONS
PHARMACEUTICS I
reduced by increasing the viscosity of the dispersion medium and this approach has shown
some success in formulating suspension products. In commercial suspension formulations,
suspending agents such as carboxymethylcellulose, methylcellulose, tragacanth, and
xanthan gum are often added to enhance the viscosity of the dispersion medium.
The wetting of the suspensoid is a primary concern in the preparation of suspensions.
Hydrophilic substances are readily wetted by water or other polar liquids because of low
interfacial tension and the resultant small contact angles. A hydrophobic substance is often
difficult to disperse and the powder may just float on the surface of the liquid despite its
higher density. To improve the wetting characteristics of a hydrophobic drug powder,
anionic or nonionic surfactants are often used.
Fine particles have a tendency to form a compact cake upon settling to the bottom of
the container. The cake is very difficult to breakup upon shaking. To avoid the formation of
a cake, it is necessary to prevent aggregation of particles giving rise to non-dispersable
large aggregates. One common method of preventing aggregation of particles of a
suspension is via the formation of a less rigid or loose agglomerates of particles held
together by comparatively weak inter-particle forces. Such an agglomeration process of
particles is termed the formation of a floc or a floccule, within which particles are loosely
linked through van der Waals forces. Although flocs settle more rapidly than small
individual particles, the presence of dispersion medium within the settling flocs makes
them less prone to compaction as compared to deflocculated particles. The flocs settle to
form a higher sediment volume than the deflocculated particles. The loosely linked
structures of the flocculated particles allow the easy break up and redispersion of the
particles upon gentle agitation. The zeta potential is a measurable parameter which
indicates the electrical charges associated with particles in the dispersion medium. When
the zeta potential is relatively high, the repulsive forces between particles usually exceed
the inter-particle attractive forces. When the particles are individually dispersed, they are
said to be in the deflocculated state. Thus, deflocculated particles settle separately and the
rate of sedimentation is governed by the Stokes’ equation. The supernatant remains cloudy
during the sedimentation process and the resultant sediment is very difficult to resuspend.
There are several methods for the preparation of flocculated suspensions. Electrolytes can
act as a flocculating agent which reduces the electrostatic repulsion between the particles
and facilitates the linking between particles via attractive forces. The addition of an
appropriate concentration of nonionic or ionic surface-active agents can also induce
flocculation of particles in a suspension leading to a higher sedimentation volume.
Suspensions can be prepared by the dispersing method or the coaggregation method.
In the dispersing method, if the drug particles tend to clump together or float on the surface
of the vehicle, the powder must first be wetted to facilitate the dispersion of the particles in
the liquid medium. Alcohol, glycerin, propylene glycol, and other polar liquids can be
employed as the wetting agent when the dispersion medium is an aqueous vehicle. Once
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Shenyang Pharmaceutical University
LAB 1: SUSPENSIONS
PHARMACEUTICS I
the powder is wetted properly, the dispersion medium is added in portions to the powder
and the mixture is thoroughly mixed before the additional portions of vehicle are added.
The last portion of the vehicle is used to rinse the mixing equipment and to bring the
suspension to the final volume. The final product is then passed through a colloid mill or
other dispersion or mixing equipment to ensure even dispersion of the finished product. In
the coaggregation methods, drug substance existing in the ionic or molecular state are
converted into microparticles by using physical or chemical method. The microparticles
are subsequently dispersed in the liquid medium to form a uniform suspension.
Suspensions are generally evaluated with respect to their particle-size distribution,
electrokinetic properties (zeta potential), and rheological characteristics. A number of
evaluating methods can be applied to a suspension product such as sedimentation volume,
resuspendability, and specific gravity measurements. The sedimentation volume (F) of a
suspension can be determined using a graduate cylinder. It is defined as the ratio of the
equilibrium volume of the sediment to the total volume of the suspension. The greater the
value of F, the more stable is the product.
On the label of a suspension product, “Shake Before Use” is stated to ensure uniform
dispersion of the solid particles prior to the withdrawal of the desired dose. For the dosing
accuracy concerns, the suspension dosage form is not recommended for drugs with a very
narrow therapeutic window.
3. METHODS
3.1 Preparation of suspensions of hydrophilic drugs and measurement of
sedimentation volume
3.1.1 Formulations
Table 1 Formulations of zinc oxide suspensions
No.
1
2
3
4
Zinc oxide (g)
0.5
0.5
0.5
0.5
50% glycerin (mL)
—
6.0
—
—
Methylcellulose (g)
—
—
0.1
—
Tragacanth (g)
—
—
—
0.1
Distilled water to (mL)
10
10
10
10
3.1.2 Procedures
(a) Preparation of formulation 1 and 2
Weigh the designated amount of zinc oxide and transfer it to a mortar. Add 0.3 mL of
distilled water or glycerin and levigate the mixture to form a paste. Add the remaining
glycerin and continue mixing. Further dilute the suspension with 80% of the remaining
distilled water, transfer the suspension to a 10 mL graduate cylinder, and add distilled
water to volume (10 mL).
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Shenyang Pharmaceutical University
LAB 1: SUSPENSIONS
PHARMACEUTICS I
(b) Preparation of formulation 3
Weigh 0.1 g of methylcellulose and transfer to a mortar. Add distilled water and mix
until a solution is formed. To this solution, add zinc oxide and levigate to form a paste. Add
80% of the remaining distilled water and continue mixing until a uniform suspension is
formed. Transfer the suspension to a 10 mL graduate cylinder and add distilled water to
volume (10 mL).
(c) Preparation of formulation 4
Weigh 0.1 g of tragacanth, transfer to a mortar, and then add several drops of alcohol as
the wetting agent. Add a small amount of distilled water to the mixture and levigate to
form a paste. Add zinc oxide and follow the procedures as described in (b).
(d) Measurement of sedimentation volume
Agitate the four suspensions contained in the stoppered graduate cylinders by turning it
upside down for five times and let the samples stand. Measure the height of the sediments
(mL) at 1, 5, 10, 30, 60, 90 and 120 min after shaking and calculate the sedimentation
volume. Record the results in Table 3 and plot the sedimentation volume vs. different
formulations.
3.1.3 Experimental notes
a) When preparing the suspension, keep the volume of the solution added, time
of mixing, and intensity of levigation the same for all formulations evaluated in this
experiment.
b) Use stoppered graduate cylinders with the same diameter for the measurement
of sedimentation volume.
c) The mortar should be rinsed with a small amount of distilled water to achieve
complete transfer of the suspension into the graduate cylinder.
3.2 Influence of flocculating agents on the redispersion of suspensions
3.2.1 Formulations
(1)
1.0 g
Bismuth subnitrate (碱式硝酸铋)
Distilled water, q.s.
10 mL
(2)
Bismuth subnitrate
1% sodium citrate solution
Distilled water q.s.
3.2.2 Procedures
(a) Preparation of suspensions
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1.0 g
1.0 mL
10 mL
Shenyang Pharmaceutical University
LAB 1: SUSPENSIONS
PHARMACEUTICS I
Weigh 2.0 g of Bismuch subnitrate and transfer to a mortar. Add 0.5 mL of distilled
water and levigate to form a paste. Add the remaining distilled water in small portions and
mix to give a uniform suspension. Tranfer the suspension into a 10 mL graduate cylinder.
After shaking, divide the suspension into two equal parts. One part is diluted with distilled
water to 10 mL as formulation (1). To the second part, 4 mL of distilled water and 1.0 mL
of 1% sodium citrate solution are added to give formulation (2). Shake both suspensions in
the graduate cylinders and let the samples stand for 2 hours prior to evaluations.
(b) Examine the appearance of the sediment
Visually examine the suspension with respect to the appearance of the supernatant and
sediment. Turn the graduate cylinder upside down and observe the ease of redispersion of
the sediment. Record the state of redispersion of the sediment as a function of the number
of upside down turns of the graduate cylinder.
(c) Experimental notes
When shaking the graduate cylinders upside down, use the same intensity and avoid
vigorous shaking since this many obscure the subtle different in dispersion state of the
suspension.
3.3. Preparation of suspensions with hydrophobic drugs and comparison of different
wetting agents.
3.3. 1 Formulations
The composition of different sulfur suspensions are listed in Table 2.
3.3.2 Procedures
Weigh 0.2 g of refined sulfur and transfer in a mortar. Add the appropriate amount of
distilled water, alcohol, glycerin, soft soap or Tween 80 (with small amount of distilled
water) and levigate. Add the remaining amounts of distilled water by small portions and
mixed until a uniform suspension is formed. After shaking, note the dispersion state of the
sulfur particles and record the observation.
Table 2. Composition of different sulfur suspensions
No
1
2
3
Refined sulfur (g)
0.2
0.2
0.2
4
0.2
Alcohol (mL)
50% glycerin (mL)
Soft soap (mL)
—
—
—
2.0
2.0
—
—
—
1
—
—
—
Tween-80(g)
Distilled water to (mL)
—
10
—
10
—
10
0.03
10
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Shenyang Pharmaceutical University
LAB 1: SUSPENSIONS
PHARMACEUTICS I
3.3.3 Experimental notes
(a) Weigh the same amount of the sulfur for all formulations in order to ensure
meaningful measurement for comparison purposes.
(b) The suspension prepared in the mortar should be transferred to the graduate
cylinders without any loss.
3.4 Preparation of the sulfur suspension by coaggregation method
Add 5 mL of 4%（W/V）hydrochloric acid and 5 mL of 20%（W/V）hyposulphite in
a 10 mL graduated cylinder. After shaking, observe the dispersion state of the sulfur and
record your observation. .
4. RESULTS AND DISCUSSION
4.1 Fill in the sedimentation volume results in Table 3.
Table 3. The change in sedimentation volume as a function of time
Time
Formulation No.
(min)
1
2
3
4
Hu
Hu/H0
Hu
Hu/H0
Hu
Hu/H0
Hu
Hu/H0
5
10
30
60
90
120
Note：H0 is the height of suspension；Hu is the height of the sediment.
4.2 Based on the data in Table 3, plot the sedimentation curve using Hu/H0 as the Y-axis
and time as the X axis and compare the suspending capacity of different suspending
agents.
4.3 Record the appearance of the supernatant and sediment of the Bismuth subnitrate
suspensions 2 hours after preparation and the numbers of upside down turn required to
resuspend the sediment.
4.4 Record the appearance of the supernatant and sediment of different sulfur suspensions
and discuss the stabilizing effect of different wetting agents.
4.5 Record the appearance of the supernatant and sediment of the sulfur suspensions
prepared by the dispersing method and that prepared by the coaggregation method.
Discuss the impact of preparation methods on the dispersion state and physical
stability of the suspensions.
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Shenyang Pharmaceutical University
LAB 1: SUSPENSIONS
PHARMACEUTICS I
5. QUESTIONS
a) Explain the differences between zinc oxide suspension and sulfur suspension with
respect to their formulation and method of preparation.
b) To improve the stability of the following formulation, what stabilizers should you
select? Please describe the function of each ingredient in the formulation and the
method of preparation.
Formulation
Zinc oxide
6.0 g
Liquid phenol
1.0 mL
Glycerin
2.0 mL
Limewater
q.s.
Total
100 mL
c) What is the difference between a suspension prepared by the dispersing method and that
prepared by the coaggregation method in terms of product appearance and physical
stability ?
References
[1] H. C. Ansel, N. G. Popovich and L. V. Allen, Jr. Pharmaceutical Dosage Forms and
Drug Delivery Systems, 8th ed., Williams and Wilkins, Baltimore, 2005.
[2] M.E. Aulton. Pharmaceutics: The science of dosage form design. 1988.
(Shirui MAO)
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