The hydroxyl value is an indicator for the stages of esterification reaction. Instead of the wet chemistry titration method, the near-infrared (NIR) spectroscopic technique was utilized to monitor the hydroxyl values during the reaction. Because of the various raw materials and reaction conditions, the shifting of the -OH absorption band in the NIR spectra of esters was observed. The molecular structure, the reaction environment, and the state of the -OH groups are the most likely parameters responsible for the shifting of the -OH band.
Wei G. Hansen, "Shifting of -OH Absorption Bands on NIR Spectra of Esters," Appl. Spectrosc. 47, 1623-1625 (1993)