pH is defined as the negative log of the [[concentration]] of [[hydrogen]] ions in a solution. (Technically, it is not based on concentration, but ''[[activity]]'', however the activity is very close to the concentration for water solutions). | pH is defined as the negative log of the [[concentration]] of [[hydrogen]] ions in a solution. (Technically, it is not based on concentration, but ''[[activity]]'', however the activity is very close to the concentration for water solutions). |