The procedures outlined were designed to determine whether as given formula is flocculated. Since there is more than one method of preparing stable suspensions, the following tests were found useful for determining the stability of both flocculated and dispersed systems.
Photo-microscopic techniques:
The microscope can be used the estimate and detect changes in particle size distribution and crystal form. Its usefulness can be enhanced through the use of a Polaroid camera attached to the eyepiece of a monocular microscope to permit the rapid processing of photo-micrographs. This method is helpful to distinguish between flocculated and non- flocculated particles and to determine changes ion the physical stability of such systems conveniently with time.
Particle Coulter Counter:
It is an electronic particle counter and sizer that measure the change of resistance caused by the presence of a particle in an electrolyte. Particles pass through the aperture substantially one at a time. The size range of the counter is 0.2 to 300μm. It is helpful to determine the particle size distribution of hydrophobic particles such as steroids and some antibiotics. As the suspensions must be diluted with electrolytes and surfactants, the stability of the suspensions may be hampered so this method is not used to determine the physical stability and properties of particles aggregates.
Cylindrical Graduates:
Cylindrical Graduates (100-1000ml) are quite useful for determine the physical stability of suspension. It may be used to determine the settling rates of flocculated and non-flocculated suspension, by making periodic measurement of sedimentation height without disturbing the system.
Bookfield Viscometer with Hellpath Attachment:
Bookfield Viscometer is a valuable rheological equipment for measuring the settling behavior and structure of pharmaceutical suspensions. The instrument consists of a slowly rotating T-bar spindle, which while descending slowly into suspensions encounters new, essentially undisturbed material as it rotates. The dial reading of the viscometer measures resistance to flow that the spindle encounters form the structure at various levels in the sediment. Taking rheograms a various time intervals, under standard conditions of sample preparation, gives a description is most useful suspension and its physical stability. It mostly used in viscous suspensions containing high solid particles, that develop sufficient shear stress. It is also excellent for characterizing flocculated systems.
Specific gravity measurement with Hygrometers:
Specific gravity is ratio of the density (mass of a unit volume) of a substance to the density (mass of the same unit volume) of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water. Temperature and pressure must be specified for both the sample and the reference. Specific gravity or density of pharmaceutical suspension is measured to provide the qualitative information on the amount of air entrapped by a suspension during manufacture.
Measurement of specific gravity: A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water. A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity can be read directly.
Aging tests:
This is performed by subjecting suspensions to cyclic temperatures of repeated freezing and thawing or exposed them to elevated temp (>400c) for short periods of storage to test for physical stability. At elevated temp causes significant amounts of drug to go into solution and subsequent cooling indices excess drug in solution to re-precipitate. If suspension is able to withstand exposure to extremes in temp, it is claimed as physically stable suspension but if it fails to meet the test for significance then it must not be considered a bar to father testing.
Zeta potential:
Zeta potential is electric potential in the interfacial double layer (DL) at the location of the slipping plane versus a point in the bulk fluid away from the interface. In other words, zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. It has practical application in the stability of systems containing dispersed particles. If the zeta potential is reduced below a certain value, the attractive forces exceed the repulsive forces and particles come together. This phenomenon is known as flocculation.
The significance of zeta potential is that its value can be related to the stability of colloidal dispersions (e.g. a multivitamin syrup). The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles (the vitamins) in a dispersion. For molecules and particles that are small enough, a high zeta potential will confer stability, i.e. the solution or dispersion will resist aggregation. When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate. So, colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate.
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