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5/27/2011 2:02:41 PM
Methods for determining particle size
Many methods are available for determining particle size. Only those that are widely used in pharmaceutical practice and are typical of a particular principle are presented. Microscopy, sieving, sedimentation, and the determination of particle volume are discussed in the following section. None of the measurements are truly direct methods.
Although the microscope allows the observer to view the actual particles, the results obtained are probably no more "direct" than those resulting from other methods because only two of the three particle dimensions are ordinarily seen. The sedimentation methods yield a particle size relative to the rate at which particles settle through a suspending medium, a measurement important in the development of emulsions and suspensions. The measurement of particle volume, using an apparatus called the Coulter counter, allows one to calculate an equivalent volume diameter. However, the technique gives no information as to the shape of the particles. Thus, in all these cases, the size may or may not compare with that obtained by the microscope or by other methods.
Optical microscopy
It should be possible to use the ordinary microscope for particle-size measurement in the range of 0.2 to about 100 mm. According to the microscopic method, an emulsion or suspension is mounted on a slide or ruled cell and placed on a mechanical stage. The microscope eyepiece is fitted with a micrometer by which the size of the particles can be estimated. The field can be projected onto a screen where the particles are measured more easily, or a photograph can be taken from which a slide is prepared and projected on a screen for measurement.
Sieving
This method uses a series of standard sieves calibrated by the National Bureau of Standards. Sieves are generally used for grading coarser particles. Sieves produced by photoetching and electroforming techniques are available with apertures from 90 mm to as low as 5 mm. According to the method of the U.S. Pharmacopeia for testing powder fineness, a mass of sample is placed on the proper sieve in a mechanical shaker. The powder is shaken for a definite period of time, and the material that passes through one sieve and is retained on the next finer sieve is collected and weighed.
Sedimentation
The particle size in the subsieve range can be obtained by gravity sedimentation as expressed in Stokes s law.
Several methods based on sedimentation are used. Principal among these are the pipette method, the balance method, and the hydrometer method. Only the first technique combines ease of analysis, accuracy, and economy of equipment.
Particle volume measurement
A popular instrument for measuring the volume of particles is the Coulter counter (Figure 19-9). This instrument operates on the principle that when a particle suspended in a conducting liquid passes through a small orifice on either side of which are electrodes, a change in electric resistance occur.
In practice, a known volume of a dilute suspension is pumped through the orifice. The suspension is sufficiently dilute and the particles pass through essentially one at a time. A constant voltage is applied across the electrodes to produce a current. As the particle travels through the orifice, it displaces its own volume of electrolyte, and this results in an increased resistance between the two electrodes. The change in resistance, which is related to the particle volume, causes a voltage pulse that is amplified and fed to a pulse-height analyzer calibrated in terms of particle size. By systematically varying the threshold settings and counting the number of particles in a constant sample size, it is possible to obtain a particle-size distribution. The instrument is capable of counting particles at the rate of approximately 4000 per second, and so both gross counts and particle size distributions are obtained in a relatively short period of time. The data may be readily converted from a volume distribution to a weight distribution.
The Coulter counter has been used to advantage in the pharmaceutical sciences to study particle growth and dissolution and the effect of antibacterial agents on the growth of microorganisms.
Particle shape and surface area
A knowledge of the shape and the surface area of a particle is desirable. The shape affects the flow and packing properties of a powder, as well as having some influence on the surface area. The surface area per unit weight or volume is an important characteristic of a powder when one is undertaking surface adsorption and dissolution rate studies.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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