# How was the dissociation constant of water determined?

I was studying about pH and pOH of different acids and bases and I read about the dissociation constant of water ($\ce{H2O}$).

It turned out to be $1.00 \cdot 10^{-14}\ \mathrm{mol^2\cdot L^{-2}}$ at $25~\mathrm{^\circ C}$.

Could I get some insights to how was this specific value determined?

• – Mithoron Nov 1 '16 at 12:10
• @Mithoron NO....I was asking how was it determined that the concentration of hydroxide and hydronium ions is $10^{-14}$ M and not something else......How was this specific value determined? – user35508 Nov 1 '16 at 13:52
• You can do it using a pehameter and pure water... Are you asking about history or what? BTW you're mistaking concentration with constant in your comment. – Mithoron Nov 1 '16 at 14:07
• Thanks for pointing that out......but could you please tell me what is a pehameter...I googled it but couldn't find anything useful... I know how dissociation constant is calculated...but for that don't we need concentration of ions ...How was this concentration calculated? or is there some other way? – user35508 Nov 1 '16 at 14:18
• I meant pH meter – Mithoron Nov 1 '16 at 14:26

I'd dare to say that it was first determined in experiments on electrical conductivity around 1890 by the physical chemist Friedrich Kohlrausch.

In Die Wasserstoffionenkonzentration. Ihre Bedeutung für die Biologie und die Methoden ihrer Messung (Leonor Michaelis, 1922, ISBN: 978-3-642-88800-7) the author writes

F. Kohlrausch und A. Heydweiller kamen durch wiederholte Destillation des Wassers unter ganz besonderen Kautelen zu einem Grenzwert der Leitfähigkeit des Wassers, der sich durch weitere Reinigungsversuche nicht weiter herabdrücken ließ und daher als die eigene wahre Leitfähigkeit des Wassers gedeutet werden mußte. Sie stellten auf diese Weise die Dissoziation des Wassers zum erstenmal zahlenmaßig fest. Später wurden dann mehrere andere Methoden zur Dissoziation des Wassers gefunden, die wir bald kennenlernen werden, und diese bestätigten die Resultate von Kohlrausch.

This roughly translates to: Upon repeated destillation of water under extreme precautions, F. Kohlrausch and A. Heydweiller eventually reached a limit for the conductivity […]. This way, they were the first to quantitatively determine the dissociation of water[…]

In a footnote to this paragraph a reference is given:

F. Kohlrausch und A. Heydweiller, Zeitschr. f. physikal. Chemie, 14, 317 (1894).

• I have a last question......I understand how water was distilled but still you just couldn't count the concentration of ions...how was concentration of ions determined? – user35508 Nov 2 '16 at 13:21
• I love the old language … ‘the own and true conductivity of water’ =D – Jan Nov 2 '16 at 15:40