# What is so special about positive hydrogen ions that they are used in pH calculation?

I just learned about pH. If I understand it correctly, all it really tells is the concentration of positive hydrogen ions (protons) there are in a given liquid.

Why is this pH scale entirely dedicated to positive hydrogen ions? What is so special about protons/hydrogen ions? I would expect the scale to would count all positive ions, even if they are of atoms different from hydrogen. I don't understand why is the proton ion a "superstar", being so special that people made a scale dedicated just to it.

• There's also the $pOH$ scale as well, which works in the same way, but for $OH^{-}$ ions instead... The 'p' denotes $log_{10}$. They're interchangeable; at $25^{°}C$, $pH + pOH = 14$, just like $K_{w} = [H^{+}][OH^{-}]$. They're just logarithmic scales to make it easier to represent small numbers. – user60221 Mar 15 '18 at 18:04
• chemistry.stackexchange.com/questions/5047/… – Mithoron Mar 15 '18 at 20:29

What we're really talking about with the acidity of a solution is the proportion of water which is dissociated into hydroxide $OH^-$ or hydronium ions $H_3O^+$ (hydronium is usually a more accurate representation than a free proton).
Other ions do affect the acidity of a solution, but by inducing water to dissociate. Check out the example on the wiki page. Pure water alone has a $pH$, because of spontaneous dissociation of some water molecules into ions. The $pH$ of pure water is 7, because the $pK_a$ of water as a weak acid is 14.
Basically, water is the special thing, and its constituent ions happen to be hydroxide and hydrogen / hydronium. Those ions are what we measure with a $pH$ electrode, so the scales are named after them. Water has a concentration around 55 M, vastly predominating in aqueous solutions.