Folin-Ciocalteu spectrophotometric assay of ascorbic acid in pharmaceutical tablets and orange juice with pH adjustment and pre-extraction of lanthanum(III)-flavonoid complexes

BACKGROUND: This study proposes modifications to the conventional Folin-Ciocalteu (FC) spectrophotometric method for individually determining ascorbic acid (AA) in complex matrices in the presence of other phenolics and potential interferents. The conventional FC assay in the aqueous phase, which normally measures total water-soluble phenolics and other antioxidants, has recently been modified by incorporating isobutanol (iso-BuOH) in the solvent mixture for the simultaneous determination of lipophilic and hydrophilic antioxidants in foods. RESULTS: Interference effects of other flavonoids and phenolics to individual AA assays were overcome by preliminary extraction-removal as their La(III) chelates into ethyl acetate (EtAc). The pH of the medium was adjusted to 3.0 in order to prevent conversion of AA into over-oxidation products further beyond the dehydroascorbic acid stage, as encountered in the conventional FC assay carried out at pH 10. This pH does not permit most phenolics to ionise, rendering their oxidation difficult. CONCLUSION: Both methods (conventional and iso-BuOH-modified FC at pH 3, with and without La(III)/EtAc pre-extraction) were applied to the determination of ascorbic acid in pharmaceutical tablets and orange juice, showing good agreement with HPLC results. The proposed spectrophotometric methods with their low cost, simplicity, reliability, versatility and accuracy offer novelty to the determination of AA in complex matrices.