Accurate monitoring of acidity in industrial lubricants is essential for the predictive maintenance of machinery used in different industries. The total acid number (TAN) is commonly determined using colorimetric titration methods. However, these methods often fail when analyzing dark, degraded or turbid oil samples, where the color change at the endpoint is difficult to observe clearly. The purpose of this study is to overcome this limitation by developing a fluorescent organic indicator for highly sensitive TAN determination.
A fluorescent organic indicator 1 was prepared from the reaction of N,N′-dicyclohexylcarbodiimide and picric acid. 1 exhibits a bright yellow fluorescence at 572 nm when excited at 405 nm. 1 was used to determine the TAN of various lubricating oils used in thermal power plants, based on a reversible protonation–deprotonation mechanism.
Upon interaction with acidic species, the molecule undergoes reversible protonation, leading to a complete fluorescence “switch-off” and conversion to a colorless, non-emissive state. This sharp fluorescence change enables highly sensitive detection of acid content. Compared to the conventional bromocresol green colorimetric method, the fluorescence-based approach provides a much sharper and more reliable endpoint, especially for dark and highly degraded oil samples where visual color detection is extremely challenging.
This work introduces a novel fluorescence-based method that offers a robust and practical alternative for accurate quality control and monitoring of oil degradation. It specifically addresses a critical analytical challenge in industrial applications, providing significant value for predictive maintenance protocols.
