Appendix XVII A. Particle Size of Powders

The degree of fineness of a powder may be expressed by reference to sieves that comply with the specifications for non-analytical sieves (2.1.4).

Where the degree of fineness of powders is determined by sieving, it is defined in relation to the sieve number(s) used either by means of the following terms or, where such terms cannot be used, by expressing the fineness of the powder as a percentage m/m passing the sieve(s) used.

The following terms are used in the description of powders:

Coarse powder Not less than 95 per cent by mass passes through a number 1400 sieve and not more than 40 per cent by mass passes through a number 355 sieve.

Moderately fine powder Not less than 95 per cent by mass passes through a number 355 sieve and not more than 40 per cent by mass passes through a number 180 sieve.

Fine powder Not less than 95 per cent by mass passes through a number 180 sieve and not more than 40 per cent by mass passes through a number 125 sieve.

Very fine powder Not less than 95 per cent by mass passes through a number 125 sieve and not more than 40 per cent by mass passes through a number 90 sieve.

If a single sieve number is given, not less than 97 per cent of the powder passes through the sieve of that number, unless otherwise prescribed.

Assemble the sieves and operate in a suitable manner until sifting is practically complete. Weigh the separated fractions of the powder.

Within the monographs of the British Pharmacopoeia, the above terms may be used to specify the degree of coarseness or fineness of a medicinal or pharmaceutical substance in powder form that is to be incorporated into a formulated preparation. The following terms may also be used for such purposes.

When the use of sieves is inappropriate, the definition is expressed in terms of the particle size as determined by suitable microscopical examination.

Moderately coarse powder Not less than 95% by weight passes through a number 710 sieve and not more than 40% by weight passes through a number 250 sieve.

Microfine powder Not less than 90% by weight passes through a number 45 sieve.

Superfine powder Not less than 90% by number of the particles are less than 10 µm in size.

Particle-size distribution is estimated by analytical sieving (2.9.38) or by application of other suitable methods where appropriate. A simple descriptive classification of powder fineness is provided in this chapter. For practical reasons, sieves are commonly used to measure powder fineness. Sieving is most suitable where a majority of the particles are larger than about 75 µm, although it can be used for some powders having smaller particle sizes where the method can be validated. Light diffraction is also a widely used technique for measuring the size of a wide range of particles.

Where the cumulative distribution has been determined by analytical sieving or by application of other methods, particle size may be characterised in the following manner:

It is recognised that the symbol d is also widely used to designate these values. Therefore, the symbols d₉₀, d₅₀, d₁₀ may be used.

The following parameters may be defined based on the cumulative distribution.

Q_r(x) = cumulative distribution of particles with a dimension less than or equal to x where the subscript r reflects the distribution type.

Q_r(x) = 0.90 when x = x₉₀

Q_r(x) = 0.50 when x = x₅₀

Q_r(x) = 0.10 when x = x₁₀

An alternative but less informative method of classifying powder fineness is by use of the descriptive terms in Table 2.9.35.-1.