Sedimentation methods

Different sizes of particle settle at different terminal velocities, and this property can be used in several ways to determine the size distribution of a sample. In the Andreasen Pipette a sample of the material is dispersed in a liquid and allowed to settle from time zero. At intervals a sample of fluid is extract from a depth H below the surface. These samples are evaporated to dryness and weighed to give concentrations. This produces a set of concentrations Cj and the known initial concentration Cq. At any time t the sample withdrawn will contain particles with terminal velocities equal to or less than H/t, at their original concentration, larger particles will have settled below the level of the pipette tip. The objective of the experiment is to determine the particle size distribution of dolomite powder, and compare 

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The particle size deterrnined by sedimentation techniques is an equivalent spherical diameter, also known as the equivalent settling diameter, defined as the diameter of a sphere of the same density as the irregularly shaped particle that exhibits an identical free-fall velocity. Thus it is an appropriate diameter upon which to base particle behavior in other fluid-flow situations. Variations in the particle size distribution can occur for nonspherical particles (43,44). The upper size limit for sedimentation methods is estabHshed by the value of the particle Reynolds number, given by equation 11 ... 

Clays (qv) are aluminosihcate minerals, some of which have definite chemical compositions. In regard to tar sands, however, clay is only a size classification and is usually deterrnined by a sedimentation method. According to the previous definition of fines, the fines fraction equals the sum of the silt and clay fractions. The clay fraction over a wide range of fines contents is a relatively constant 30% of the fines. 

Gravitational Sedimentation Methods In gravitational sedimentation methods, particle size is determined from settling velocity... 

Stokes diameter is defined as the diameter of a sphere having the same density and the same velocity as the particle in a fluid of the same density and viscosity settling under laminar flow conditions. Correction for deviation from Stokes law may be necessary at the large end of the size range. Sedimentation methods are limited to sizes above a [Lm due to the onset of thermal diffusion (Brownian motion) at smaller sizes. 

Centrifugal Sedimentation Methods These methods extend sedimentation methods into the submicron size range. Sizes are calculated from a modified version of Stokes equation ... 

In general, it appears that the Micromerograph, provided that frequent calibration checks are performed, is a good, reproducible instrument for size measurement. The operator time involved is less than with most other methods, and the calcns are not complicated. As in all sedimentation methods, only when the sample particles are spherical does the Stokes diameter that is measured become a measure of absolute particle size. Microscopic examination should be used to check on particle shape and the effect of deagglomeration... 

Sulcer and Denson (Ref 19) used the gas chromatographic—B .T. procedure for the analysis of Class I A1 powder (45 u max dia) which cannot be tested satisfactorily by sedimentation methods because of the presence of aggregates. A rough statistical evaluation of this procedure was made by running twelve determinations and calculating the standard deviation as shown in Table 14 ... 

US Dept of Commerce (1967) 37) R. Abbott, N. Gelber N. Liszt, Particle Size Analysis of Ammonium Perchlorate by the MSA Sedimentation Method", PATM 1778 (1967) 38) C.E. 

Another sedimentation method used involves the ANDREASEN PIPETTE. A typical design is shown in the following dlcigram ... 

Another sedimentation method used is the so-called MSA-analyzer. If the value of "g" in 5.10.8. is increased (such as the use of a centrifuge) one can analyze the very smaU particles in any given distribution in a short time. The problem of course lies in accurate determination of the weight accumulated at a given time under a specific centripetal force. This problem has been neatly solved by careful design of the sedimentation tube, as shown in the following diagram ... 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

Information Chromatographic column MS detection Quantification Instrumental LOD Method (sediment) Method (fish)... 

Numerous [34, 36, 41-46, 49, 60-65] workers have discussed the application of atomic absorption spectrometry to the determination of organomercury compounds in river and lake sediments. Methods [41-44]... 

The particle size and distribution in soils can be easily determined by the sedimentation method. It is based on the fact that large particles will settle faster than smaller ones. No special equipment is needed. 

The size range of a material is determined by sieving for relatively large particles and by sedimentation methods for particles which are too fine for sieving. 

Sedimentation method based on the fact that at the meniscus and at the bottom of the centrifuge cell there is never a flux of the solute across a plane perpendicular to the radial... 

Viscosity of polyamic acid. measured using sedimentation method in NMP. ins insoluble s soluble ps partially soluble. ... 

Using the ultracentrifuge sedimenting method, Gralen and Svedberg [40] have determined the molecular weight of nitrocellulose prepared from various types of cellulose. The nitration was carried out at 0°C by means of a mixture composed of 40% of nitric acid, 40% of phosphoric acid, and 20% of phosphorus pentoxide. The results are collected in Table 79. 

In continuous technological processes direct measurement of products masses is difficult. The recoveries are usually estimated by determination of the fraction content in separated products. This may be done by analyzing their probes, e.g. by sieving, sedimentation method etc. Using the fraction contents, one can estimate the recoveries of coarse and fine fractions by the following respective expressions rjc = Cc(C0 - Cf)/[C0(Cc - Cf)], m = Ff(F0 - Fc)/[F0(Ff — Ec)]. Their average gives total separation efficiency rjaY... 

Central to any investigation of particle swelling is the use of a reliable method of measuring the size of the particles undergoing expansion. A sedimentation method was described previously (5) which can be used to explore the expansion characteristics of carboxylic emulsion polymers. In the present report, we present a comparison of sedimentation results with those obtained with two... 

Sedimentation. The sedimentation method, described previously 5), is based on the relative sedimentation rates of swollen and unswollen latex particles. Starting with the Stokes expression for centrifugal sedimentation, an equation can be developed for the ratio of the sedimentation coefficient of a particle, S, to the sedimentation coefficient, S, of the same particle having a surface layer ... 

Surface layers (adsorbed, solvated, ionic) are of considerable importance in controlling the stability and rheological properties of colloidal systems. Sedimentation methods have proven effective in the measurement of adsorbed layer thickness using equations similar to Equation 1 when the density of the layer could be estimated ( 7,8). The equation can be considerably simplified if the density... 

Viscometric particle swelling experiments were carried out with a Cannon-Ubbelohde shear-dilution viscometer thermostated at 30 + 0.05°C. The shear rate was approximately 2000 sec-1 with flow times determined to + 0.1 sec. As in the sedimentation method, the model latexes were diluted with distilled water to a concentration of approximately 1 percent. Individual samples were adjusted with sodium hydroxide to various pH values and allowed to equilibrate at least 24 hours before measuring the viscosity. 

A quantitative comparison of particle expansion determined by the three methods is given in Table I. The particle diamete of the standard acrylic latex was determined by PCS to be 1120 A. This value was used in the calculation of the increase in particle radius at maximum expansion in each case. The sedimentation method yielded the largest increase in radius, 302 A, followed by the viscometric value of 240 K. Possibly the shear involved in the latter method resulted in a partial collapse of the surface layer. The value determined by PCS was found to be approximately half that determined by sedimentation. Since the PCS determination is presumed to be free of particle interactions at a concentration of 5 X 10 4%, we must conclude that the other two methods (at 1% solids) exhibit such interactions. As a result, the charged particles settle slower (19) and yield a higher viscosity than in the absence of these (repulsive) interactions. 

Comparison of Sedimentation and Light Scattering. The sedimentation method has been presented elsewhere (8) where it was shown that... 

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