Decomplexing madder lakes using oxalic acid: A novel method coupled with microwave or ultrasound processes.

The aim of the present study was to establish a mild extraction and universal method to characterize madder lakes of Rubia tinctorum by liquid chromatography coupled with a photodiode array detector. To analyze the lakes, anthraquinone molecules must be decomplexed of metal links. To this end, two processes, one ultrasound and the other microwave, were improved in association with two solutions, an oxalic acid solution (0.5 M in MeOH/H 2 O 50/50) and an acetic acid buffer solution (1 M). First, the decomplexation of an alizarin experimental lake was optimized in comparison with a reference method using a strong acid. The microwave process used oxalic acid and increased the decomplexation yield of alizarin (71%) compared with the reference method (31%). Second, different madder experimental lakes, which were prepared using different metal salts, were decomplexed. The obtained results suggest that the use of microwave associated with oxalic acid solution is the most universal method providing a decomplexation of anthraquinones from lakes without hydrolysis of glycosidic compounds occurring. • Synthesis of madder lakes using five different metallic cations. • Decomplexation of metal-complex dyes by two separate processes: ultrasound and microwave. • Development of a universal, efficient, and nondenaturing method for the study of madder lakes. [ABSTRACT FROM AUTHOR]