By the immersion method with a polarizing microscope a principal agent phenytoin was identified from its 10 times trituration containing vehicles such as lactose and starch. A particle size distribution of phenytoin crystals in its trituration was determined by observing one of the habit parameters b from all the detectable particles of phenytoin under a microscope in a suspension of an immersion oil, n
D : 1.555, in which phenytoin crystals having key refractive indexes, n
1 : 1.606, n
2 : 1.631, were clearly detectable from other vehicles. By the similarity of crystal habits in the same lot the particle size distribution of phenytoin crystals in its trituration was calculated from the values of observed parameter b and average values of habit coefficients, T (√(ab/c)) : 2.004, L (b/a) : 2.545, which were obtained from some previously identified phenytoin crystals.
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