With time, various chemical notations for the hydroperoxyl (perhydroxyl) radical coexist in the literature. Haber, Wilstätter and Weiss simply wrote HO or OH, but sometimes HO or OH can also be found to stress the radical character of the species.
The hydroperoxyl radical is a weak acid and gives rise to the superoxide radical (O) when it loses a proton:
:HO → H + O
:sometimes also written as:
:HO → H + O
A first pK value of 4.88 for the dissociation of the hydroperoxyl radical was determined in 1970. The presently accepted value is 4.7. This pK value is close to that of acetic acid. Below a pH of 4.7, the protonated hydroperoxyl radical will dominate in solution while at pH above 4.7 the superoxide radical anion will be the main species.