Estimating the refractive index of pharmaceutical solids using predictive methods

Refractive index is a basic physical property of pharmaceutical solids. In this paper, the refractive index values of 424 pharmaceutical solids from the literature were surveyed. It was found that the refractive index values exhibit a normal distribution with an overall mean value of 1.603. The Eisenlohr and Vogel methods developed for organic liquids were employed to estimate the refractive index for pharmaceutical solids. The estimated results were compared with experimentally measured values determined by polarized light microscopy. Both Eisenlohr and Vogel RD methods agreed very well with the measured mean refractive index values from the literature with an average absolute percent error of 1.22% and 1.25%, respectively. The evaluation for in-house measurements for Pfizer active pharmaceutical ingredients showed larger differences between the calculated and measured values. The results indicate that the Eisenlohr and Vogel RD methods can provide fast and accurate results for predicting the refractive index of pharmaceutical solids.