Development of a mild method for the extraction of anthraquinones from their aluminum complexes in madder lakes prior to HPLC analysis

A mild extraction approach using HF, a weak acid but a strong aluminum-complexing agent, was investigated as an alternative to usual methods such as treatment by strong acid solutions (HCl or H2SO4) at high temperature, or methylation by BF3 in MeOH, for the extraction of alizarin and other colorants from their aluminum complexes in madder lakes. HF and HCl extractions were tested on a series of home-made lake powders. HF allowed the recovery of components such as pseudopurpurin, munjistin and glycoside precursors, all of which were partially or totally hydrolyzed when HCl method is used. Furthermore the amount of alizarin recovered was often higher with HF. These improvements are mainly due to the complexation of aluminum by fluoride ions, which allows the extraction of the colorants from the lakes (conveniently accompanied by a visible color change), at pH > 1 and at room temperature. Applied to a micro-sample of 19th century lake powder, HF extraction revealed the presence of many colorant glycosidic precursors, which would have been undetectable by using the strong acid methods.