Scientists Finally ‘See’ a Proton Move Through Water, and It Only Took 200 Years

For over two centuries, scientists have known that water transports a positive charge through protons. But they had never actually seen it happen—until now.

In a Science paper published September 11, Yale researchers reported that they devised a method to track, measure, and effectively “see” a proton’s journey through water. For the experiment, the team used a 30-foot-long mass spectrometer—an instrument that separates different elements by mass—that took years to customize and refine. The device allowed them to benchmark how quickly protons moved through six charged water molecules.

“We show what happens in a tiny molecular system where there is no place for the protons to hide,” said Mark Johnson, senior author of the study and a chemist at Yale University, in a release.

Solving a seemingly obvious mystery

There’s a surprisingly long list of things in science that we know—or strongly suspect—to be true, but that have either never been directly confirmed or still lack a good explanation.

That hasn’t stopped scientists and engineers from using these yet-to-be-confirmed ideas to achieve some remarkable breakthroughs. Protons in water, for example, play a role in “everything from eyesight to energy storage to rocket fuel,” the researchers explained.

But protons are terribly small and display quantum mechanical properties, which makes them frustratingly difficult to track.

“They aren’t polite enough to stay in one place long enough to let us observe them easily,” Johnson said. “They are thought to conduct the charge through an atomic-scale relay mechanism, in which protons jump from molecule to molecule.”

Trapped in an organic ‘taxi’

To observe such processes in action, Johnson and his team used 4-aminobenzoic acid, an organic molecule capable of taking an extra proton in two different sites. The two locations can be distinguished by the color of light they absorb, said study co-lead author Payten Harville, a postdoctoral student at Yale, in the release.

For the experiment, the team attached the 4-aminobenzoic acid molecules to the six water molecules. Harville explained that in this setup, protons can only “get from one docking site to the other [by hitching] a ride on a water network ‘taxi.’”

When the protons “hitch” onto the taxi, the team’s specialized mass spectrometer “destructively” analyzes each reaction ten times per second with carefully timed lasers, the researchers explained.

To be clear, the experiment still hasn’t caught the intermediate steps of the proton’s path through water. However, it sets the most stringent parameters for the process so far, Johnson said.

“We’re able to provide parameters that will give theorists a well-defined target for their chemical simulations, which are ubiquitous but have been unchallenged by experimental benchmarks,” he added.

Indeed, if this technology could expand beyond Yale’s custom spectrometer, it could give an extra boost to the precision of experiments in fundamental chemistry. Given how it’s taken science 200 years to get to this point, taking a few more to really drive this method home should be a shorter wait.