When the concentration of hydronium ions is equal to the concentration of .
It is at this point, in , that the of hydronium ion is 10-7. Since pH is the () of the hydronium ion concentration, neutral pH is equal to 7 (= -log(10-7)).
A that is not at neutral pH is either or, instead, is .
Note that pH is often defined, though less rigorously, in terms of hydrogen ion concentration. Hydrogen ion, though, are equivalent to hydronium ions, which is to say that in a water solution hydrogen ions are present, more or less, as hydrodium ions. In any case the units of concentration used are those of .