Chemistry is one of the oldest all of the sciences. Our ancestors explored the chemistry of pigments in order to draw cave paintings 35,000 years ago, the modification of clay pots making them more waterproof and methods for tanning animal hides. Chemistry has been integral to the development of civilization. At the same time, it is a young and evolving science. There is much we don’t know and new discoveries are made on a daily basis.
The birth of modern chemistry can be attributed to the John Dalton and the concept of the atom but perhaps a better starting point would be the work in the early 20thcentury, which really saw the discovery of the atom.
Ernst Rutherford was awarded a Nobel Prize in Chemistry for his research on the sub-atomic structure of the atom (although he thought of himself as a physicist). His post-doctoral fellow Niels Bohr won his Nobel Prize for the development of a theoretical explanation of the atom leading to quantum mechanics. Henry Moseley used X-ray emissions to determine the underlying basis of the periodic table was the number of protons in an atom’s nucleus – its atomic number. And Frederick Soddy introduced the term isotopes as a possible explanation for why some elements could exist with different masses.
But in 1914, there was little evidence to back up a theory of isotopes. Soddy had been working on the radioactive decay of elements – for which he would win a Nobel Prize – and had suggested the lead formed from the radioactive decay of uranium would have a different atomic mass to lead that wasn’t formed in this fashion. This was something that caught the interest of other scientists.
In particular, Otto Honigschmid, working at the Radium Institute in Vienna, gave Stephanie Horovitz the arduous task of testing whether Soddy’s suggestion was indeed correct. Horovitz was blessed by having a huge quantity of pitchblende (a uranium ore which also contains radium and lead) readily available as it was mined locally for its radium content. After the radium was removed, Horovitz was provided with what remained.
Her work was two-fold. It first involved bulk separation of the lead from the ore and then very precise measurements to work out its atomic weight. It was tedious work. A synopsis of the experimental procedure from Honigschmid and Horovitz’s 1914 paper describes the process as: grinding and dissolving the ore in acid to extract lead chloride which was then washed, filtered, dissolved as sulfate, precipitated, re-dissolved, filtered, precipitated, and washed some more, then dissolved into nitric acid forming the nitrate which was recrystallized five times, converted back to a chloride, then recrystallized a further five times, and then dissolved and recrystallized a further nine times to get a lead chloride worthy of analysis.
Horovitz then performed precise gravimetric analysis and titration experiments that allowed her to finally calculate the atom weight of the pure lead chloride. And what she found was a crucial result – the lead from uranium had an atomic weight of 206.736 compared to 207.190 for lead from other sources. This was the first direct evidence to support the existence of isotopes.
In 1916, Horovitz published results on another element which had been postulated – ionium. She was able to show the spectroscopic and chemical properties of ionium were actually those of thorium but with a different atomic weight. She both disproved the existence of ionium and demonstrated thorium occurred in multiple isotopes.
Unfortunately, Horovitz’s contributions to chemistry have faded with time. Indeed, according to Geoffrey and Marelene Rayner-Canham of Memorial University in Newfoundland and Labrador, she was one of the many women whose scientific work has been overlooked in modern accounts. According to Geoffrey, “Women could not get academic position in those days. It was unthinkable that a woman could be a professor.” Typically, women would work as unpaid research assistants and the resulting praise for discoveries would accrue to their male counterparts. This is an unfortunate reality across pretty much every scientific discipline.
According to the Rayner-Canhams, it appears Honigschmid tried to maintain her status within the scientific community. He always quoted Horovitz and their two influential papers were co-authored. But after his untimely death, other researchers simply dropped Horovitz’s name. She was just another woman scientist being written out of history, according to the Rayner-Canhams.
As to Stephanie Horovitz’s fate, this is revealed in a letter from Kasimir Fajans to Elizabeth Rona: “You probably have not received any information from Vienna about the fate of Dr. Stephanie Horovitz. I learned about it from a mutual relative in Warsaw. Stephanie moved there after World War I and after her parents had died in Vienna to join her married sister. She was not active in chemistry and both were liquidated by the Nazis in 1940.”
A tragic end to one of the most important chemists of the 20thcentury.