I am currently evaluating the antioxidant activity of apple cider vinegar using the dpph test, so I don't know how to convert the volume of vinegar added into weight for expressing the result in mg/ml ?
firstly you must start with a rather large concentration, by mixing 1mL of the sample to be tested with 4mL of DPPH, if the percentage of inhibition is greater you must re-prepare dilutions from the sea solution otherwise you must increase the concentration because the concentration changes from one sample to another depending on the antioxidant power of the sample. you can consult my article for more details of the operating mode (http://dx.doi.org/10.13171/mjc8219041101ao). Good luck.