DPPH method is frequently used to measure the antioxidant potentials of plant extracts  by calculating the  IC50 value using a formula.First prepare a known concentration of the plant extract using methanol    as the solvent and make a dilution series of the extract  with methanol .Following the DPPH standard procedure (by adding 0,5ml extract and 2.5ml of DPPH solution) calculate the% of inhibition  for each concentration by doing triplicate of each concentration.Control contains 0.5ml methanol instead of sample.

% of inhibition =(Absorbance of control - Ab of sample)/Ab of control x 100

Draw a graph using concentrations of sample Vs % of inhibition of DPPH   radicals  and find the sample concentration required to inhibit the 50% of DPPH radicals  using the equation of the graph, or can be used Minitab version too.  IC50 value is inversely proportional to the sample concentration. Same procedure can be used for the standard. 

Thanks....Chibuike sir...i have calculated value of IC/EC50 value of Ascorbic acid ..it is 164ug/ml whereas my extract shows 140.605 and 178.24 ug/ml..that means my one extract has more antioxidant activity than standard...

i have taken diiferent concentration of ascorbic acid-

25,50 75,100,200ug/ml ...and get % inhibition -8.20,17.88.27.03,34.98,45.28................and y=0.2788x+4.1424....

then use ur formula to calculate IC50..IT COMES to be 164ug/ml...is it correct

Hi Vikas. It is not impossible to have extracts more effective than individual compounds. This might happen mostly due to synergistic effects.

However, to calculate EC50, you are not supposed to use a calibration curve. The best option is using an interpolation:

(y-y1)/(x-x1)=(y2-y1)/(x2-x1)

where y1 is the activity percentage below 50%, y2 is the activity percentage below 50%, x1 is the extract concentration providing y1, x2 is the extract concentration providing y2 and y is the target activity (50%). In this way, you should determine the x value, which corresponds to EC50.

I'm a little confused with I'm a little confused with João equation I'm a little confused with I'm a little confused with João equation . y1. and y2 means same thing.  Please explain further at least with a reference article. I'll be grateful. Or is there a software one can use? 

Hi Vikas 

See the attached papers. I am sure you will find an answer to your question. In case of any problems, feel free to ask.

regards

You can use a software EZ fit kinetics, just download it....then meet Nepken she will put you through. Please feel free to ask any questions? cheers

78.7%±0.03 i am very confusing in antioxidant data .percentage was easily find but how to calculate this value ±0.03