The 2012 chemistry textbook, General, Organic and Biological Chemistry, by H. Stephen Stokes; a PBS site, and chemistry.about.com all claim that, after the volatile lachrymator propanethial-S-oxide is produced by a cut onion’s enzymes, it reacts with water in the eyes to produce the irritant sulfuric acid. But Scientific American, Molecule of the Month and the analytical services of onionlabs.com attribute the lacrimatory effects directly to the molecule’s action on receptors. Sulfuric acid is not part of the explanation. From the latter:
LF is the chemical compound which directly causes the eye to tear (often called the onion lachrymator) and the chemical sensation of heat or mouth burn when an onion is eaten. It is measured using HPGC equipment and is reported in µmoles of LF/ml of onion juice. . ..
And then the details from a researcher writing in Scientific American:
The cornea is densely populated with sensory fibers of the ciliary nerve, a branch of the massive trigeminal nerve that brings touch, temperature and pain sensations from the face and front of the head. The cornea also receives a smaller number of autonomic motor fibers that activate the lachrymal (tear) glands. Free nerve endings detect syn-propanethial-S-oxide on the cornea and drive activity in the ciliary nerve–which the central nervous system interprets as a burning sensation–in proportion to the compound’s concentration. This nerve activity reflexively activates the autonomic fibers, which then carry a signal back to the eye ordering the lachrymal glands to wash the irritant away.
I started to get dubious about the alleged formation of sulfuric acid for the following reason. If, in the presence of water in the eye, the lachrymator degrades so quickly, wouldn’t most of the propanethial-S-oxide break down in the onion’s aqueous environment? Let’s assume for a moment that some still escapes unscathed due to its relatively high vapor pressure(v.p.) of 41.2 ± 0.2 mm Hg at 25°C. To put that in perspective, water’s v.p. is only 23.8 mm Hg at that temperature, so it’s about as volatile as ethyl alcohol whose vapor pressure is about 44 mm Hg.
To look for any pH-changes, I taped only the ends of a wet piece of litmus paper in a sealed plastic bag. I chopped an onion and quickly placed the pieces into a ziploc, making sure that the acidic juices of the onion would not be in contact with the litmus. If the lachrymator indeed broke down into compounds that included sulfuric acid, then the exposed wet litmus should turn red between 30 seconds and couple of minutes, the time that it takes for the compound to form and reach the eyes. But there was no color change, consistent with the fact that the lachrymator itself has no pKa, a measure of acidic strength.
One could argue that the amount of acid produced was too small to affect the litmus. This research paper mentions that each milliliter of onion juice contains 1−22 μmol of the lachrymator. Let’s assume a density of 1.0 g/ml and an average mass of 120 g per onion, which is about 90% water, and that about only 30% of the lachrymator reached half a ml of water on the litmus. If it had produced H2SO4 in a 1:1 ratio*, then we could easily calculate that we’d end up with a pH between 3 and 4, enough to turn the litmus red. (*Only one source (the textbook) had included a chemical equation, and it was not even balanced.)
I also mixed a small amount of vinegar with the onions, and within a few minutes, the same wet litmus did indeed turn red, revealing that the simple setup could detect acidity.
And one final detail, most literature on the topic including this blog so far, discuss the lachrymator as if it’s a single compound. But NMR analysis reveals it to be a 19 to 1 mixture of (Z)- and (E)-propanethial S-oxide.
These are not earth-shattering revelations. But what was humbling is how the inaccuracies from a field that I know and enjoy almost got past me. Imagine what happens when I read news about economics and terrorism, stories that I often do not dig into to uncover the truth.