The Scale of Scrutiny
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Whether or not materials are satisfactorily mixed depends on the subsequent operations in which the mixture plays a part.
THE SCALE OF SCRUTINY
Whether
or not materials are satisfactorily mixed depends on the subsequent operations
in which the mixture plays a part. Any mixture, if examined on a small-enough
scale, will show regions of segregation. An acceptable degree of mixing is
related to subsequent operations in a process sequence. Danckwerts introduced
the term “scale of scrutiny” to describe the minimum size of the regions of
segregation in a particular mixture, which would cause it to be regarded as
insufficiently mixed (Dankwerts, 1953). For example, if a tablet is to contain
0.1 g of drug A and 0.1 g of drug B, the powder from which the tablets are to
be made must be sufficiently mixed so that on drawing a sample of 0.2 g from
the mixture, the sample will contain, within narrow limits, the correct amounts
of A and B. The way in which A and B are dispersed within the sample may be of
no importance so long as the tablet is not divided. The scale of scrutiny is
here determined by the weight of the tablet.
In
general, a small scale of scrutiny is applied if the unit size of the product
is small and if too much or too little of one component is very undesirable.
Two
further useful concepts may be introduced here to describe unmix-edness: the
scale of segregation and the intensity of segregation. The scale of segregation
is a measure of the size of the regions of unmixed materials. In the example
given above, the intensity of segregation shows the extent to which A has been
diluted with B and vice versa. These two concepts are usually inter-related. A
high intensity of segregation can be tolerated so long as the scale of
segregation is small. Alternatively, a larger scale of segregation may be
tolerated if the intensity of segregation is reduced.
