Hi. Mitosox red measures mitochondrial superoxide. I dilute one vial (50 µg) with 13 µl DMSO. This gives you a 5mM solution (1000x). Dilute in media or PBS and incubate cells for 10 min in dark. wash 3 times.
More info here https://tools.thermofisher.com/content/sfs/manuals/mp36008.pdf