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:

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

See also

• Law of mass action

literature

• Entry to ion product. In: Römpp Online . Georg Thieme Verlag, accessed on January 2, 2015.
• Peter W. Atkins, Julio de Paula: Physical chemistry. 4th edition, Wiley-VCH, Weinheim 2006, ISBN 978-3-527-31546-8 , p. 264.