Autoionization of water: does it really occur?

TL;DR

This paper proposes that high-frequency conductivity measurements reveal proton exchange dynamics in water rather than spontaneous ionization, challenging traditional views on water autoionization.

Contribution

It introduces a new approach to evaluate ion concentrations using high-frequency conductivity, suggesting water ionization is not spontaneous but driven by proton exchange.

Findings

High-frequency conductivity reflects proton exchange, not ionization.

Ion concentration c is temperature-independent.

Water molecule lifetime is less than a nanosecond.

Abstract

The ionization constant of water Kw is currently determined on the proton conductivity sigma1 which is measured at frequencies lower than 10^7 Hz. Here, we develop the idea that the high frequency conductivity sigma2 (~10^11 Hz), rather than sigma1 represents a net proton dynamics in water, to evaluate the actual concentration c of H3O+ and OH- ions from sigma2. We find c to be not dependent on temperature to conclude that i) water electrodynamics is due to a proton exchange between H3O+ (or OH-) ions and neutral H2O molecules rather than spontaneous ionization of H2O molecules, ii) the common Kw (or pH) reflects the thermoactivation of the H3O+ and OH- ions from the potential of their interaction, iii) the lifetime of a target water molecule does not exceed parts of nanosecond.