# Ion product

The ion product is the product of the molar concentrations (or more precisely the activities ) of all ions dissolved in a medium (e.g. water ) by electrolytic dissociation . If it is a saturated solution, the ion product corresponds to the solubility product . If only ions produced by autoprotolysis are present in a system , the ion product corresponds to the autoprotolysis constant of the system.

## Ion product of water

The ion product of pure water ( K W ) is particularly important . By autoprotolysis, water contains oxonium - (H 3 O + ) and hydroxide ions (OH - ):

${\ displaystyle \ mathrm {H_ {2} O + H_ {2} O \ rightleftharpoons H_ {3} O ^ {+} + OH ^ {-}}}$

For the ion product K W applies (with c ° = 1 ): ${\ displaystyle {\ tfrac {mol} {L}}}$

${\ displaystyle K _ {\ mathrm {W}} = {\ frac {c (\ mathrm {H_ {3} O ^ {+}})} {c ^ {o}}} \ cdot {\ frac {c (\ mathrm {OH ^ {-}})} {c ^ {o}}} \ = a (\ mathrm {H_ {3} O ^ {+}}) \ cdot a (\ mathrm {OH ^ {-}}) }$

The activities of the two types of ions at 25 ° C are each 1.004 · 10 −7 , the ion product is 1.008 · 10 −14 , which roughly corresponds to the numerical product of the concentrations:

${\ displaystyle K _ {\ mathrm {W}} = 10 ^ {- 7} \ cdot 10 ^ {- 7} \ = {10 ^ {- 14}}}$

These variables determine the scale and the neutral value of the pH value , which is the negative decadic logarithm of the oxonium ion concentration:

${\ displaystyle \ mathrm {pH} = - \ lg \ left ({\ frac {c \ mathrm {(H_ {3} O ^ {+})}} {c ^ {o}}} \ right) = \! \ 7}$