Polarizing microscopy of crystalline drugs based on the crystal habit determination for the purpose of a rapid estimation of crystal habits, particle sizes and specific surface areas of small crystals

Abstract

In 1939 the author reported the results of measured refractive indices of about a hundred crystalline drugs listed in [JP V] at the Takeda Research Laboratory using a Leitz PM polarizing microscope and newly developed immersion oils. When the author had reopened the study of crystalline drugs using a polarizing microscope at the Kobe-Gakuin University starting from 1975 one of the main purposes was to clarify the relation between crystal habits and refractive indices. It had been found that in most cases of crystal habits refractive indices were uniquely measured from a predominant pair of faces forming superior the habit, and they were called as "key refractive indices". The author and his co-workers tried to investigate the possibility of measuring the key refractive indices widely from all the obtainable crystalline drugs listed in the [JP X] or [JP XI], co-operating with the Pharmacy of Kobe University Hospital. Thus, more than 170 kinds of crystalline drugs were tested for their key refractive indices and found that they were measured from about 60-70% of tested drugs. It was also clarified that the difference of 2 key refractive indices, (n2 - n1), the birefringence of the section, was also an unique invariable number for the habit, and it played an important role not only for the graphic representation of log(n2 - n1), abscissa, against (n1, n2), ordinate, for the sake of an analytical purpose but also to measure a thickness of a section (habit) using a retardation color. Then, it had been cleared that the similarity of crystal habits in the microscopic field was based on the facts of measuring the same key refractive indices, and the author had developed a chart for measuring key refractive indices as well as producing a 3 dimensional orthographic projection of a crystal habit simultaneously applying a thickness measuring method using a birefringence. Finally 3 dimensional parameter a, b, c of a crystal habit and "habit coefficients" T: square root of ab/c and L: b/c were determined from the orthographic projection. In conclusion using the similarity in crystal habits the distributions of particle sizes and specific surface areas of all the crystals in the microscopic field had been calculated by a personal computer putting in necessary habit coefficients and obtained data of parameter b. The relation between 2 dispersions of particle sizes in log (V) and specific surface areas in log (SSA) were shown under the rectangular coordinates log (V) on the abscissa and log (SSA) on the ordinate, where the loci of log (SSA) formed simple striped pattern composed of parallel straight lines depending on habit coefficients. It would be possible to estimate the value of a specific surface area of any crystalline substance by plotting the value of log (V) on the straight line of a locus of log (SSA) having the same habit coefficients.