The degree of fineness of a powder may be expressed by reference to sieves. 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 types of pharmaceutical powders are commonly used in pharmaceutical industry:
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.
Note: 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.
Where the cumulative distribution has been determined by analytical sieving or by application of other methods, particle size may be characterized in the following manner:
- d90 = particle size corresponding to 90 per cent of the cumulative undersize distribution;
- d50 = median particle size (i.e. 50 per cent of the particles are smaller and 50 per cent of the particles are larger;
- d10 = particle size corresponding to 10 per cent of the cumulative undersize distribution.
