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A new successor at the University Institute of Physical Chemistry


The fast diffusion of proton and hydroxyl ions
Manfred Eigen Scientist
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I didn't solve the problem, the mixing problem. I circumvented it, and this proved to be a very useful method. I can perhaps explain it still a little more directly with the reaction we started two years later, when Leo De Maeyer came from Belgium and entered our group. I think he came in 1954. By that time we tried to study the most prominent reaction many people had tried before, namely the neutralisation reaction - proton plus hydroxyl ion to a water molecule. Now to go into equilibrium means into an equilibrium where the two partners, proton and hydroxyl ion, are present at equal concentration. That means we had to produce very, very pure water, as Kohlrausch did in the '90s of the last century. So we had to distil for a week, we had to heat the surface of the apparatus, of the distillation apparatus, so that no impurities could come over, and we reached a conductivity which indicated that we had 10-7 molar protons and hydroxyl ions versus 55 moles of water molecules. And we decided to disturb this equilibrium by an electric wave... an electric wave with a very large electric field strength, something like 100,000 volt per centimetre. Now, such an electric wave can travel with almost vacuum light velocity, less than that, which means it travels over 1 centimetre within 10-10 seconds, that's ten billionths of a second. So with that we could see indeed a shift of the equilibrium and we could measure the relaxation time by which this equilibrium was achieved... a new equilibrium.

You know, in ionic reactions that was found already by Maxwell, long ago, and was theoretically fully explained by Lars Onsager.  An electric field shifts any ionic equilibrium by pulling the ions apart, so to speak. Now we used that and we could measure the reaction rate... such a recombination reaction which in proton and hydroxyl ion of course has a relaxation time which is dependent on the concentration, and since the concentration of these partners is very low, as I say 10-7 molar, the time was in the microsecond region and we could easily follow that. The rate constant we derived from that turned out to be the rate constant, the fastest ever measured rate constant in solution. Later on it turned out it's the combination of a proton with the hydroxyl ion is even faster than the combination of a proton with an electron. One could think that the electron would be more mobile, but that's not true. It's probably more localised and, to give you a feeling of it, if you would have one normal... one molar acid solution, and could mix it fast enough with the one molar base solution, the reaction would be over in about 10-11 seconds. That means a hundredth of a billionth of a second. That's real damned fast indeed.

[Q] Yes, but you can also call it diffusion controlled, instead of damned fast indeed.

Yes, it's diffusion controlled, that's true, where one should note that diffusion of proton and hydroxyl ion is faster than of any other ion because it can jump through hydrogen bonds and have a special anomalous mechanism.


Listeners: Ruthild Winkler-Oswatitch




Tags: neutralisation reaction, pure water, ionic reactions, diffusion control, recombination reaction, Leo de Maeyer, Friedrich Wilhelm Georg Kohlrausch, James Clerk Maxwell, Lars Onsager

Duration: 4 minutes, 59 seconds

Date story recorded: July 1997

Date story went live: 24 January 2008