Powder sample dispersed in liquid

Fig. 1 Schematic of turbidimeter

The reading of the millivolt recorder for the intensity of the light beam shining through the clear solution is adjusted to 100%, while the concentration of the suspension is adjusted so that the reading for the intensity of the light beam through the suspension before any settling has occurred ranges from 20 to 40% of that through the clear solution. As the suspension settles, the projected area of the particles in the suspension decreases, and the intensity of the light beam increases.

At the beginning of the settling, all particle sizes are uniformly distributed through the volume of the sedimentation cell. As settling proceeds, large particles settle faster than small particles. After a given time (tx), all particles larger in diameter than x have settled below the level of the light beam. The concentration of particles at the light beam level is now equal to the original concentration of particles, minus all particles with diameters equal to or larger than x. The projected surface of the particles at time, tx, is therefore smaller than that of the particles in the original suspension, and the intensity of the transmitted light is greater.

Determination of Particle Size Distribution. To obtain information on particle size distribution from the plot of light intensity versus time, use is made of the relationship between the total weight of n particles of size x, which is proportional to nx3, and the projected surface area of the n particles, which is proportional to nx2. Therefore, the cumulative weight of particles up to a given particle size xlim, which is:

which is the integral of the product of particle size and projected surface area integrated from 0 to size xlim.

Turbidimetric measurements using white light are relatively inexpensive and have proven quite reproducible and useful. They are used in research and in routine analyses for comparing different lots of refractory metal powders. These methods generally are used for comparative work on the same type of material.

When x-rays instead of white light are used to determine particle size distribution of a subsieve particle suspension, the attenuation of the x-ray beam intensity is proportional to the mass of the powder particles rather than their projected area.

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