Dear Sir. Concerning your issue about the concentration (mg/mL) of gallic acid equivalents for calibration curve. Ten mg of gallic acid was dissolved in 100 ml of 50% methanol (this will obtain100 μg/ml) and then further diluted to 6.25, 12.5, 25 or 50 μg/ml. One ml aliquot of each dilution was taken in a test tube and diluted with 10 ml of distilled water. Then 1.5 ml Folin Ciocalteu's reagent was added and allowed to incubate at room temperature for 5 min. Four ml of 20% (w/w) Na2CO3 was added in each test tube, adjusted with distilled water up to the mark of 25 ml, agitated and left to stand for 30 min at room temperature. Absorbance of the standard was measured at 765 nm using UV/VIS spectrophotometer (Schimadzu, Japan) against blank, i.e., distilled water. For more details, I think the following below links may help you in your analysis:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3480753/

http://waterhouse.ucdavis.edu/faqs/folin-ciocalteau-micro-method-for-total-phenol-in-wine

http://file.scirp.org/pdf/AJPS20110600005_45314202.pdf

http://www.sphinxsai.com/Oct_dec_2010_vol2_no.4/PharmTech_vol2_no.4_1_pdf/PT=40%20(2403-2406).pdf

Thanks

I don't know if I understood well your question, but I think that all what you need is to perform a standard curve/scale by:

-doing a serial of increasing concentrations of gallic acid,

-measuring their OD (Absorbances),

-getting the curve equation [Abs=f(c)],

-then, you can calculate your sample concentration, and the value obtained is expressed as mg/µg of GA Eq/mL

And that will mean, that the absorbance obtained for the sample has the same absorbance of the expressed amount of gallic acid (mg or µg of GA equivalent).

I made this test in during my master, it's so easy! But I published all steps in portuguese! If you don't understood any step cited by Isam, I can help you, with calculations too, no problem! My e-mail: [email protected].