AS 5009.1:2003 pdf – Determination of particle size distribution by centrifugal liquid sedimentation methods Part 1: General principles and guidelines.
This correction es usually applied in the equipment software. The value of E is unity for large particles, nsing to 2 as the particles pass through the Mie scattering region (about 0.5 lam). and rapidly reducwig as the particle size decreases into the Rayleigh scattenng region (see reference [1J in the bibliography).
It E Is assumed oonstanl. then the method may be used as a comparison method only as there may be substantial errors in the I tm size range. The value of F may be evaluated from theory or determined experimentally. Note that the value of the extinction coefficient depends on the wavelength of the light and can also depend on the detector configuration. Consequently, exact delem,inations of the extinction coefficient are not readily determined. Foi practical purposes. a value of unity might be assumed and should be stated If an assumed correction Is made by the manufacturer of the apparatus, then this should be stated.
The requirement for extinction coefficient corrections also applies to photosedirnentation used in the homogeneous method.
4.3.3 Homogeneous suspension
The homogeneous technique is limited because of approximations which need to be made lithe calculation of the
results (see reference 131 Ii the bibliography).
43.3.1 Photosedlmentatlon method
In the photosedimentatlon method, th€ extinction coefficient corrections detailed In are also required In the homogeneous method. Centrifugal X-ray method
In the centrifugal X-ray method, the attenuation of the X-ray beam is directly proportional to the mass of sample present in the beam and the result is presented automatically. The centnfugal X-ray method is used only in the homogeneous mode due to the requirement for a higher sample concentration than would be feasible using a line- start method. RadIal dilution correction
If two monosized spheres are released twenty radii apart at the surface of a gravity sedimentation apparatus, they will remain evenly spaced as they settle. If the same spheres are placed a similar distance apart on the inner surface of a centrifugal sedimentation apparatus, they will become more widely separated as they settle, since they sediment along radial lines This effect leads to a dilution of the parlides as they settle outwards in the centrifuge. The dilution is of concern in the homogeneous mode, since particles of the same diameter commence the analysis at different originating radii; it is of negligible significance Ii a line-start, since all particle sizes have effectively the same originating radius.
Particles originating at radius S, in an annular element of thickness S will ocoupy an annulus of thickness .M at a
fixed measurement radius M. The relationship between l, (the mass fraction of particles smaller than diameter i)
and the measured concentration, C, at radius 14 is:
4.3.4 Homogeneous suspension wIth scanning
When scanning is employed with a homogeneous suspension, then .4! changes continuously during the course of
the scan (see reference [1] in the bibliography). Radial dilution correction requirements still apply.
5 Particle size, shape and porosity limitations
5.1 Upper size limit
The upper size limit ‘stu. by centrifugal sedwnentation is largely dependent on the reliabdity of measurements taken in the first minute of the analysis. As in gravitational settling, the restflction on the Reynolds number limits the upper size. An additional complication is that the particles are no longer settling at a constant terminal velocity but are continually accelerating. For a reliable analysis, the accelerating force must be small in comparison to the centrifugal force, as this restricts the upper particle size which can be measured. Centrifugal sedimentation is typically used for sizing particles up to about 5 pm, but this can be extended by the use of a more viscous bquid and/or slower centrifuge speeds
5.2 Lower size limit
The lower limit of size to which the centrifugal sedimentation methods can be applied Is controlled by temperature variation, causing circulatory currents in the suspension, by flocculation of particles during the progress of sedimentation and by diffusion or Brownian motion of the very small particles. The random colisions of the molecules making up the liquid with a particle cause differences in the pressure on the particle from one part of the surface to another such that the particle Is displaced (Brownian motion), see 5.2.1. Density currents are much more bable to occur Wi a centrifugal device than in a gravity system. All these factors render It advisable to shorten the sedimentation process as much as possible.
Note that charged particles in weak electrolytes have an electrical double layer associated with them When these particles settle, the double layer is distorted with the result that an electrical field is set up which opposes motion, These electro-viscous effects can be reduced by the use of non-ionic liquids where possible.