Infrared Quantitative Analysis of Mixtures Containing Isopropenyl Stearate, Stearic Acid, Stearic Anhydride, and Mixed Stearic—Acetic Anhydride

Abstract

A method for the determination of individual components of complex mixtures is presented. The technique used is based on ir-absorbance measurements for stearic acid at 1700 and at 935 cm−1 when the concentration range exceeds 0.50% wt/vol; for isopropenyl stearate at 1145 and at 865 cm−1; for stearic anhydride at 1030 cm−1 and mixed stearic-acetic anhydride at 1000 cm−1. The baseline method was used in all absorbance measurements. Absorbance-concentration relationships obeyed Beer's law from 0 to 2.0% wt/vol for most compounds; the exception being stearic acid (at 1700 cm−1), where linearity was limited to a maximum 0.50% wt/vol. Due to spectral interference between the two anhydrides at low concentration ratios, an empirical percent transmission ratio method was used to estimate the relative concentration of each. Binary and ternary systems were studied and the standard deviations of the differences between theoretical and calculated values indicate that this method is reliable.