Advancedin situtechniques based on
electrons
and X-rays are increasingly used to gain insights into fundamental
processes in liquids. However, probing liquid samples with ionizing
radiation changes the solution chemistry under observation. In this
work, we show that a radiation-induced decrease in pH does not necessarily
correlate to an increase in acidity of aqueous solutions. Thus, pH
does not capture the acidity under irradiation. Using kinetic modeling
of radiation chemistry, we introduce alternative measures of acidity
(radiolytic acidity π* and radiolytic ion productKW*), that account for radiation-induced alterations of
both H+and OH–concentration. Moreover,
we demonstrate that adding pH-neutral solutes such as LiCl, LiBr,
or LiNO3can trigger a significant change in π*.
This provides a huge parameter space to tailor the acidity forin situexperiments involving ionizing radiation, as present
in synchrotron facilities or during liquid-phase electron microscopy.