Andreasen pipette technique

Gravity sedimentation is normally carried out by using the "Andreasen" pipette technique. In its simplest form, a suspension of approximately 0.1% by volume of particles is placed in a measuring cylinder, and a pipette used to withdraw 10 mL from a fixed depth below the surface. Such an apparatus is shown in Figure 2.31. The concentration of samples are measured and compared with the initial one. Since all the particles will settle at their terminal velocities, the ratio of the two concentrations is the fractional weight of parficles to have reached a depth of H cm. Excluded will be the particles with a diameter smaller than (Table 1.2) as given in the relationship ... 

A simple sedimentation technique, which readily lends itself to the determination of crystal size distribution in the range 1-50 pm, is the Andreasen pipette method. Although it is generally better to prepare a fresh suspension of the crystals under test in a suitable inert liquid, it is possible to classify crystals suspended in their own mother liquor. If the difference in density between the particles and suspending liquid is <0.5gcm special care must be taken to avoid convection currents. The method, briefly, is as follows (BS 3406/2, 1986). 

Instrumental techniques for measurement of particle size distribution of powders have had a tremendous advancement in recent times. Numerous methods and procedures have been developed at a steady pace over the years, and there is the possibility of covering the wide spectrum from nanosystems, to ultrafine powders, and to coarse particulate assemblies. Many instruments offer nowadays quick, reliable results for a wide variety of powders and particulate systems, and for a number of applications. There is still, however, the need to understand the basic principles under which sophisticated instruments operate, as well as to resource to direct measurements under some circumstances. Some of the most modern instrumental techniques are based on an indirect measurement and carry out transformations among the different ways of expressing particles size distributions, that is, by number, surface, or mass. Sometimes it is advisable to avoid transformations because instruments assume a constant shape coefficient on such transformation, which is not necessarily the case, and overestimation or underestimations of size of certain particles may arise. Also, in very specific applications, or in cases of basic or applied research, is better to measure directly the most relevant particle size and particle size distribution. For example, if research is carried out in modeling of solid-liquid separations, a direct measurement of the Stokes equivalent diameter would be most appropriate. The aim of the exercise is to measure the particle size distribution of a sample of medium-sized dolomite, and compare the results with those of the Andreasen Pipette method. 

Sedimentation balances and (3 back-scattering techniques have been used for the cumulative measurements while sampling (Andreasen pipette method) or the absorption of radiation (photosedimentation or use of X-rays) are most frequently used for the incremental measurements. It seems that the incremental techniques, because of their advantages of easy operation and evaluation, and relatively simple instrumentation, have somewhat wider application. 

An improvement of the Andreasen pipette method is to use a pan attached to a sensitive balance which records the changes in weight of the pan as an increasing amount of suspending particles settle on it. Later, sedimentation techniques using light extinction by changes in turbidity of the suspension and x-ray were introduced for more sensitive and rapid measurements. 

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