Acid-base equilibria: not exactly like you remember in chemistry class

Acid-base equilibria: not exactly like you remember in chemistry class

Work published in the Royal Society of Chemistry with the support of the Helmholtz Association through the Center for Free-Electron Laser Science at DESY. MAX IV Laboratory, Lund University, Sweden,  European Research Council (ERC) under the European Union’s Horizon 2020 and the Academy of Finland.


Remember doing titrations in chemistry class? Adding acid drop-by-drop to the beaker and the moment you took your eye off it the solution completely changed colour.

We learned in chemistry that by doing this titration, we were actually affecting an important equilibrium in the beaker between acids and bases. This equilibrium was first described at the turn of the 20th century by American biochemist Lawrence Henderson and modified by Karl Hasselbalch giving us the Henderson-Hasselbalch equation. The discovery and subsequent study of acids and bases using this equation has led to the discovery of many important phenomena in the natural world from as how cells function to how materials are formed.

However, after years of study, an idea arose that questioned the validity of the Henderson-Hasselbalch equation, what happens at the surface? If you have a beaker filled with a dilute acid, what happens at the very top atomic layer? The top layer of a liquid in a beaker is special for many reasons, but if you’re a dissolved molecule, it means that you’re no longer surrounded by water on all sides. For hydrophobic molecules, this means that it is favourable to be at the surface. With this in mind, the scientists took another look at the Henderson-Hasselbalch equilibrium equation and thought that it couldn’t work at the surface. Many studies have measured indicator chemical species, and determined that the Henderson-Hasselbalch equation does not seem to apply at the surface, and concluded that the concentration of hydronium or hydroxide ions, which determines the acidity/basicity, is different at the air-liquid interface than in the bulk.

However, a recently published study in the journal Physical Chemistry Chemical Physics shows that maybe the Henderson-Hasselbalch equation can be used to describe the situation at the surface after all. A team of scientists from Sweden and Finland used a technique called XPS (X-ray photoelectron spectroscopy) at MAX IV’s predecessor facility, MAX-Lab. They used X-rays to knock out electrons from a thin jet of liquid. Once the electrons were collected and measured, their energy could tell the researchers which chemical species they came from. XPS is especially useful for studying surface interactions because electrons can’t travel very far without interacting with other atoms. This means that electrons from the bulk of the liquid can’t escape and only electrons at the very surface with reach the detector.

As in earlier studies, the team, including scientists from Uppsala University, Swedish Agricultural University, University of Oulu, and MAX IV, found that the equilibrium between the acid and base appeared to be shifted at the surface. However, they also found that the apparent change in the equilibrium can largely be explained by taking into account the greater tendency for the neutral form of the acid/conjugated base pair to spontaneously go to the surface. This finding suggests a new model for acid-base equilibria at the surface that doesn’t rely on a changed concentration of hydronium or hydroxide molecules. What is more, it means that a slightly modified version of the Henderson-Hasselbalch equation can be used to explain what can be seen at the air-liquid interface of acids and bases.

The XPS measurements were carried out at I411 on the old MAX-II storage ring. However, this type of research is going to be very popular at MAX IV with FlexPES, HIPPIE, VERITAS SPECIES and FinEstBeams having the ability to measure these kinds of reactions both in bulk and at the surface.


Shifted equilibria of organic acids and bases in the aqueous surface region
Josephina Werner, Ingmar Persson, Olle Björneholm, Delphine Kawecki, Clara-Magdalena Saak, Marie-Madeleine Walz, Victor Ekholm, Isaak Unger, Corina Valtl, Carl Caleman, Gunnar Öhrwall and Nønne L. Prisle

Phys. Chem. Chem. Phys., 2018,20, 23281-23293