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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.

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    $\begingroup$ 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. $\endgroup$ – user60221 Mar 15 '18 at 18:04
  • $\begingroup$ chemistry.stackexchange.com/questions/5047/… $\endgroup$ – Mithoron Mar 15 '18 at 20:29
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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.

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  • $\begingroup$ What is this pKa? $\endgroup$ – wav scientist Mar 15 '18 at 19:17
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    $\begingroup$ The pKa is the negative log of the acid constant, the equilibrium constant for an Arrhenius acid to give up its proton. The pKa of water is 14, the pKa of vinegar is ~4.8, HCl is -7 (a very strong acid). $\endgroup$ – reve_etrange Mar 15 '18 at 19:24

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