Tailoring the Acidity of Liquid Media with Ionizing Radiation: Rethinking the Acid-Base Correlation beyond pH

Advanced in situ techniques based onelectronsand X-rays are increasingly used to gain insights into fundamentalprocesses in liquids. However, probing liquid samples with ionizingradiation changes the solution chemistry under observation. In thiswork, we show that a radiation-induced decrease in pH does not necessarilycorrelate to an increase in acidity of aqueous solutions. Thus, pHdoes not capture the acidity under irradiation. Using kinetic modelingof radiation chemistry, we introduce alternative measures of acidity(radiolytic acidity pi* and radiolytic ion product K (W)*), that account for radiation-induced alterations ofboth H+ and OH- concentration. Moreover,we demonstrate that adding pH-neutral solutes such as LiCl, LiBr,or LiNO3 can trigger a significant change in pi*.This provides a huge parameter space to tailor the acidity for in situ experiments involving ionizing radiation, as presentin synchrotron facilities or during liquid-phase electron microscopy.

Automated summary

Advanced in situ techniques based on electrons and X-rays are increasingly used for studying fundamental processes in liquids, but ionizing radiation can alter the solution chemistry. In this study, we show that a decrease in pH induced by radiation does not necessarily correspond to an increase in acidity. We introduce alternative measures of acidity (radiolytic acidity pi* and radiolytic ion product K (W)*) that consider the radiation-induced changes in both H+ and OH- concentration.