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المرحلة 2
أستاذ المادة ظافر قحطان سعيد الامين الماشطة
5/27/2011 2:15:35 PM
Densities of particles
Because particles may be hard and smooth in one case, and rough and spongy in another, one must express densities with great care. Density is universally defined as weight per unit Volume; the difficulty arises when one attempts to determine the volume of particles containing microscopic cracks, internal pores, and capillary spaces.
For convenience, three types of densities can be defined: (a) the true density of the material itself, exclusive of the voids and intraparticle pores larger than molecular or atomic dimensions in the crystal lattices, (b) the granule density as determined by the displacement of mercury, which does not penetrate at ordinary pressures into pores smaller than about 10 mm, and (c) the bulk density as determined from the bulk volume and the weight of a dry powder in a graduated cylinder.
When a solid is nonporous, true and granule density are identical, and both can be obtained by the displacement of helium or a liquid such as mercury, benzene, or water. When the material is porous, having an internal surface, the true density is best approximated by the displacement of helium, which penetrates into the smallest pores and is not adsorbed by the material. The density obtained by liquid displacement is considered as approximately equal to true density but may differ from it somewhat when the liquid does not penetrate well into the pores.
The methods for determining the various densities are now discussed. True density, r, is the density of the actual solid material. Methods for determining the density of nonporous solids by displacement in liquids in which they are insoluble are found in general pharmacy books. If the material is porous, as is the case with most powders, the true density can be determined by use of a helium densitometer.
The density of solids usually listed in handbooks is often determined by liquid displacement. It is the weight of the body divided by the weight of the liquid it displaces, in other words, the loss of weight of the body when suspended in a suitable liquid. Because helium penetrates into the smallest pores a crevices (Figure 19-17), it is generally conceded that the helium method gives the closest approximation to true density. Liquids such as water and alcohol are denied entrance into the smallest spaces, and liquid displacement accordingly gives a density somewhat smaller than the true value.
Granule density, rg , can be determined by a method similar to the liquid displacement method. Mercury is used because it fills the void spaces but fails to penetrate internal pores of the particles. The volume of the particles together with their intraparticle spaces then gives the granule volume, and from a knowledge of the powder weight, one finds the granule density.
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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