Pipette method, sedimentation

A time chart for mechanical analyses by the pipette method. J. Sediment. Petrol., 5 93-95. 

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). 

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. 

S. Berg, Determination of Particle Size Distribution by Examining Gravitational and Centrifugal Sedimentation according to the Pipette Method and with Divers, ASTM, Special Technical Publication No. 234, 1958, pp. 143-171. 

Berg, S. Determination of particle size distribution by examining gravitational and centrifugal sedimentation to the pipett method and with divers. Symp. PSA, June 1958, Boston, ASTM STP 234 (1959), p. 143 - 171 /4/ Chung, H. S. Hogg, R. The effect of Brownian motion on particle size analysis by sedimentation. Powder Techn. 41 (1985) 3, p. 211 - 216 /5/ Allen, T. Sedimentation techniques of particle size measurement. Conf. PSA Sept. 1985, Bradford, Proceed, p. 24 - 45... 

Due to these disadvantages, especially the long and tedious analysis procediues involved in the pipette methods, sedimentation balance method, and diver method, these widespread devices and methods have been replaced with modem technologies that do not depend on separation. The current versions of sedimentation that are still widely used are primarily centrifugal sedimentation using light or x-ray detection [12]. 

In this technique, a sample is extracted from the sedimenting suspension at appropriate intervals by means of a pipette. These methods are incremental, and the sample is taken in one of two ways (a) at a fixed position in the app, or (b) at a fixed depth below the surface of the suspension. It is assumed in both instances that no disturbance of the suspension takes place by eddies, etc., while the sample is being taken, that the sample is representative of the suspension at the extraction point, and that the sample taken is small. Method (a) must take into account any lowering of the level of the top surface of the suspension... 

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

Information on particle size may be obtained from the sedimentation of particles in dilute suspensions. The use of pipette techniques can be rather tedious and care is required to ensure that measurements are sufficiently precise. Instruments such as X-ray or photo-sedimentometers serve to automate this method in a non-intrusive manner. The attenuation of a narrow collimated beam of radiation passing horizontally through a sample of suspension is related to the mass of solid material in the path of the beam. This attenuation can be monitored at a fixed height in the suspension, or can be monitored as the beam is raised at a known rate. This latter procedure serves to reduce the time required to obtain sufficient data from which the particle size distribution may be calculated. This technique is limited to the analysis of particles whose settling behaviour follows Stokes law, as discussed in Section 3.3.4, and to conditions where any diffusive motion of particles is negligible. 

Schweyer (1942) compared various methods of particle-size measurement (except centrifuging). He found excellent agreement between pipette and hydrometer methods. He considers the former the best method for determining the particle-m/.c distribution of sub-sieve material by sedimentation, and prefers the hydrometer as a rapid control procedure. 

The microscopic method was used in only one set of experiments, viz., for a material whose median effective diameter was 25 /x. Good agreement was found between pipette, hydrometer, and microscopic methods for sizes ranging from 25 to the upper limit of the experiment, 100 ix. Below 25 ix, both sedimentation methods gave similar results, but there was a marked difference with the microscopic method. Thus, at 10 ix, the percent found undersize by the sedimentation methods was 25 percent and that found undersize by the microscopic method was 12 percent. This difference can be attributed in part to difficulties in preparing samples for measurement, but undoubtedly the failure to commute shape factors can be regarded as the chief source of divergencies. 

To obtain these (weight) concentrations, a defined volume V of the suspension is removed at the measuring location by means of a pipette (Figure 35). After evaporation of the sedimentation liquid the respective masses M(t) or Mq are determined by weighing. Ordinarily, the pipette analysis is considered as a very accurate method because potential sources of error can be well defined. Therefore, this technique is often used for calibration of and comparison with other test methods. 

Another sedimentation method involves the ANDREASEN PIPETTE. This glass apparatus is inexpensive and consists of a bottle having an internal sampling tube and calibrated sampling volume (5 ml). One draws a sample... 

A method for the determination of soluble nitro-cellulose in gun-cotton and smokeless powder has been published by K.B. Quinan (Jour. Amer. Chem. Soc. 23 [4], 258). In this method about I grm. of the finely divided dry sample to be analysed is placed in an aluminium cup 1.9 inch in diameter and 4-1/8 inch deep. It is then covered and well stirred with 50 c.c. of alcohol, 100 c.c. of ether are then added, and the mixture is stirred for several minutes. After removing the stirrer, the eup is lightly covered with an almninimn lid, and is then placed in the steel cup of a centrifugal maehine, whieh is gradually got up to a speed of2,000 revolutions per minute, the total centrifugal force at the position oeeupied by the cups (which become horizontal when in rapid rotation) is about 450 lbs. They are rotated at the full speed for ten to twelve minutes, and the maehine is then gradually stopped. By this time the whole of the insoluble matter will be at the bottom of the eup, and the supernatant solution will be clear. It is drawn off to within a quarter of an inch of the bottom (without disturbing the sediment), with the aid of a pipette. 

Gravity and centrifugal sedimentation can be combined for the same sample in order to directly determine Stokes diameter for a wide range of particle sizes. In such a way conversion are avoided and a mass distributions, applicable to processes where gravimetric efficiencies are relevant, can be properly derived. Ortega-Rivas and Svarovsky (1994) determined particle sizes distributions of fines powders using a combined Andreasen Pipette-pipette centrifuge method. They derive relations useful to model hydrocyclone separations, which were later employed to describe apple juice clarification. 

Analyse granulometrique par sedimentation par gravite dans un liquide Methode de la pipette... 

The Andreasen pipette introduced in the 1920s is perhaps the most popular manual apparatus for sampling from a sedimenting suspension. Determination of the change in density of the sampled particle suspension with time enables the calculation of size distribution of the particles. As Stokes law applies only to spherical particles, the nonspherical particles give a mean diameter referred to as Stokes equivalent diameter. The size range measurable by this method is from 2 to 60 pm (8). The upper limit depends on the viscosity of liquid used while the lower limit is due to the failure of very small particles to settle as these particles are kept suspended by Brownian motion. 

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