There are several methods for the determination of the size of colloidal particles. Some of these are given below;
- By Using Ultrafilters
- From Brownian Movement
- Scattering of Light
1. Using Ultrafilters
An approximate idea about the size of particles in the colloidal system can be obtained using ultrafilters. These are prepared by impregnating filter papers with gelatin, which are then hardened by immersing them in formaldehyde. The pores can be made small enough to retain particles of colloidal dimensions. The pore size depends on the particular filter paper employed and the concentration of colloidal or gelatin solution used to impregnate it.
It is possible to obtain a series of colloidal solutions that may be separated into fractions containing particles of different sizes. An approximate estimate of the size of particles can be obtained from the knowledge of the dimensions of the pores of the ultrafilters. The latter parameter determines the pressure to force air or water through the pores.
The results obtained by this method are only approximate because pore size is by no means the only factor determining whether a given particle will pass through an ultrafilter.
2. Brownian Movement
The French Physicist Jean Perrin (1870-1942) investigated the Brownian motion of colloidal particles to determine Avogadro’s number.
Colloidal particles are suspended in a liquid medium due to Brownian movement. Colloidal particles also settle down due to gravitation. Under this both effects, the particles distribute themselves in a vertical column following the equation,
n1 and n2 are numbers of particles at two depths, h1 and h2 of the vertical column can be counted with the help of an ultramicroscope and densities of the particles and the liquid medium ρ and ρ’ is determined by the usual methods, r is the radius of particles, it can be easily calculated.
3. Scattering of light
To determine the size of the particles of colloidal dimensions, Zsigmondy used the ultramicroscope. As already mentioned, each light spot viewed in an ultramicroscope corresponds to a particle. Therefore, the number of particles in a given volume of a solution can be counted. The observation takes place many times, and the average is taken. An eye-piece micrometre is used to determine the length and breadth of the field vision. The depth is determined by rotating the slit through 90◦. From this dimension, the solution’s exact volume contains the observed number of particles obtained. From this, the number of particles, n, included per unit volume of the solution can be determined.
The known volume of the colloidal solution is evaporated to dryness. The mass of colloidal particles per unit volume can be easily obtained from the residue’s mass. Two assumptions are made. First, the particles are spherical; second, the density ρ of the colloidal particles is the same as that of the material in the bulk state. The volume of the colloidal phase is m/ ρ, and therefore,
or
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