The antioxidant activity of Vit C is also determined by FRAP and ABTS assays - actually Vit C is used so that it can be adjudjed whether the compound in question has a better antioxidant capacity than Vit C or not - this is not as if one is setting a blank in a spectophotometer. Both Vit C and tested compound would give antioxidant values in FRAP and ABTS assay. 

Thanks Dr. Chakraborty, but is there any values for vitamin C to be compared with plant extracts.

Dear Faud

Please refer the attachment.

We used Ascorbic acid as standard in ABTS assay and its IC50 value was 6.5 microgram/ml

Yes, there are values for Vit C - as I said earlier the FRAP and ABTS test itself gives those values - you may also find it in the many papers that use FRAP ABTS and Vit C - much like those given by Neeraj. However, there might be slight differences in values depending on indivisual test method variations (even for slight modification that every research tends to do), chemicals used - their grades - solvents etc. which is why every researcher does not use values quoted by others but do a additional test. 

Dear Fuad,

I agree with Dipjyoti, As the IC50 of Vit C is not absolute value and it depends on all the conditions used. Therefore, one is requested to repeat it every time.

Mostly reference standard are used for comparing the test sample. Most of time we get various value for reference standard, due to variance in experimental condition. Thats why we get various value of IC50 value for the Vit C. 

You must be varefully wotk vitamin C, om se conditions ot might be pro oxidant