Hi Jonson:

First you should plot the absorbance of your standard (gallic acid) against its concentrations in ppm or microgram/ml to get the standard curve

but be careful to correct the absorbance readings by subtracting the total abs. from the abs. of yr solvent whether it is water or methanol

Second: through  excel you can get the curve equation ( Y=no.*X - no.) 

Third : through that equation you should get gallic acid concentration in microgram/ml. knowing that Y will be your corrected abs. and you will get X as a result for your extract corrected abs.

Finally: to get total phenolic contents as mg gallic acid equivalent /100g of the extract you should follow this formula:

(1000 * X / extract concentration) * (no. of dilution times if you made any dilution to extr. in the plate wells) * 100 

X is the results from point Third.

Thanks Said. Three issues I need you to help out with:

1. At what point on the Corrected Y (absorbance) axis will i read the gallic acid concentration using the curve equation?

2. Can this formula also hold for DPPH assay using ascorbic acid?

3. Can I have citation for the formula for proper referencing?

Regards,

Jay.

Thanks Wirginia. The resources are helpful. I appreciate.

Hi 

can anyone explain how precisely get calibration curve of Gallic acid. 

i prepared series of gallic acid dissloved in Distill water. is it ok just get absorbance value of these series. What is the blank. other chemical to be added???

If the phenolic content calculation with exact calibration curve is more than 1000 mg. Is it possible? because standard's absorbance is in 0.3 00 and sample solution is 1.100? 1 g of extract contains 1000 mg i.e 100 % polyphenol?