There are various protocols for Gallic acid standard curve.  Make stock solution of gallic acid first, then dilute it from 50 mg/L to 900 mg/L.  And using sodium carbonate and 2 N FC for your reaction, generate standard curve for your experiment. I have attached our published paper for your reference.

I'll take a look at it. Thank you Mr. Vaidya.

I would only consider A1 values. When analysing your samples, you just have to prepare a negative control (with 1 mL of the solvent in which your samples are dissolved) and use the concentration of sample that produces a blue/green solution with an absorbance below 1. Then it is just a question of obtaining a concentration (in gallic acid equivalents) value from the calibration curve. Don't forget to consider the sample concentration that you use in the calculations.

example (consider a sample concentration of 500 mg/mL)

calibration curve Abs = 2.089*conc + 0.2514

conc1 = Abs/2.089 - 2.514

GAE = conc1/0.5*188.1

188.1 is the molar mass of gallic acid. If your standard curve is based in mol, this factor is not, obviously, required.

I understood until conc1 = Abs/2.089 - 2.514. My standard curve is based on mg/ml.

Would you explain me what this GAE = conc1 / 0.5 * 188.1 is about? Thank  you Mr. Joao.

GAE is gallic acid equivalents

conc1 is the value obtained in the previous equation

0.5 is the concentration of the extract solution in g/L (please note that in my previous answer when I write 500 mg/mL, I really meant 500 mg/L)

188.1 is the molar mass of gallic acid, however if your calibration curve is based on mg/mL and not in mol/L, you don't need to consider this factor.

Now I got it. Thank you again Mr. Joao. Regards.

One more question Mr. Joao,

What would it happen if I perform substraction (A2-A1)? Instead only use the absorbances of the standard solutions. I think R value would be the same but values of the equation (ax+b) would be negative and could vary a little (-ax+b) prejudicing the final value of the plant extract. 

Hi Andrés

The question is not purely mathematical. Simply, you don't need to perform the subtraction. However, if you do it, the R value would be the same and the a value would be lower, but not negative. You can not have negative a values, because it would indicate that the absorbance would decrease with increasing concentrations in phenols.

Excuse me Mr. Joao, that is, I must perform the substraction because "a" value shouldn't be negative. I would like you to see this standard curve of ascorbic acid for example. There are two graphs, one is performed with the substraction and the other with absorbance of standard solutions (no substraction). Thank you.

All the experimental work conducting to A2 values is, in my opinion, unnecessary. Your  question was about a gallic acid calibration curve. The reaction end-point is blue (or greenish) and you don't need (shouldn't) to subtract the aborbance of your stock solution. The yellow color of Folin-Ciocalteu reagent is mainly due to molybdenum VI, whic is then reduced, presenting a blue color. Basically, you just need to measure the intensity of color blue. When analysing your samples, if they present any blueish or greenish color, you should make a correction by measuring the absorbance of your testing solution diluted by the same factor as that used in the assay. Simply, because part of the absorbance could be due to the natural color of the sample intstead of reduced molybdenum, which would overestimate your result.

Understood, thanks a lot Mr. Joao.