Sedimentation pipette

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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. 

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

Gravitational and centrifugal sedimentation using a pipette is attractive due to its versatility and low capital cost, but the analysis requires a skilled operator and is time consuming. 

Carefully sample 50 pi supernatant from each well using a pipette. Transfer each sample together with the pipette tip into the scintillation vial, taking care to avoid disturbing magnetically sedimented complexes. 

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. 

Sediment samples were thawed and mixed to ensure homogeneity. The dry weight and percent water content of the sediments were determined by drying approximately 5 g of the sediment at 105-110°C for several hours until a constant weight was reached. Grain-size distribution was determined by wet sieving and pipette analysis. 

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... 

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