DETERMINATION OF SEDIMENTATION VOLUME OF SUSPENSIONS WITH EFFECT OF DIFFERENT CONCENTRATIONS OF A SUSPENDING AGENT

Aim: To determine the sedimentation volume of suspensions using different suspending agents

Principle
Suspensions are defined as preparations containing finely divided drug particles 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 suspended particles should remain unchanged in the vehicle throughout the product shelf-life and an accurate dose should be readily withdrawn from the container either by pouring or by means of a syringe.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:



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 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.

1. 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



3.1.2 Procedures
Preparation of formulations
Weigh 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).

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. RESULTS AND DISCUSSION
4.1 Fill in the sedimentation volume results in Table 2.
Table 2. The change in sedimentation volume as a function of time



Note：V0 is the height of suspension；Vu is the height of the sediment.
4.2 Based on the data in Table 2, plot the sedimentation curve using Vu/V0 as the Y-axis and time as the X axis and compare the suspending capacity of different concentrations of suspending agents.

Report :
The sedimentation volumes were found to be
_________________________ for formulation 1.
_________________________ for formulation 2.
_________________________ for formulation 3.
_________________________ for formulation 4.