You may know that there is a growing interest in the ginkgo tree because herbal supplements using extracts of ginkgo are believed to aid in fighting memory loss. A recent paper [Lang, G.; Wai, C. M. Anal. Chem. 1999, 71, 2929-2933] describes a method for extracting ginkgolides and bilobalides (the pharmacologically active species) from ginkgo leaves.
To accomplish the separation a 1-g portion of powdered ginkgo leaves is placed in 15 mL of distilled water and boiled for 2 minutes. The solid residue is removed by filtration, placed in 10 mL of 0.1% w/v Na2HPO4 and boiled for 15 minutes. After filtering, the filtrate is adjusted to a pH of 4.5-5.0 using H3PO4 and NaOH, and diluted to a final volume of 50 mL. A 5-mL portion of the diluted extract is transferred to a 50–mL separatory funnel and 5 mL of CH2Cl2 added. After shaking and allowing the two phases to separate the amount of ginkgolides and bilobalides in the CH2Cl2 are determined using GC-MS.
(a) The Ka value for ginkgolide B, which we can represent as HG, is 4.82 x 10-8. Explain why a pH of 4.5-5.0 is used for the extraction.
(b) Why are the combined extracts diluted to 50 mL?
(c) The authors report that a single extraction, as described above, results in an 80-86% extraction of ginkgolide B. What is the minimum number of extractions needed to ensure that at least 98% of ginkgolide B is removed?
