Sedimenting suspension, pipette

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... 

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. 

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. 

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. 

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). 

Eliminate red blood cells (if necessary) by adding 10 ml of red blood cell lysis buffer. Pipette up and down. Leave cell suspension at room temperature for 5 min (debris and clumps of cells with sediment at the bottom of the tube). 

Remove cells in suspension with pipette, leaving sediment. 

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 ... 

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. 

---