Cannizzaro, Stanislao (1826-1910), an Italian chemist, advanced the understanding of the composition of chemical compounds and the use of standard chemical formulas. He is remembered for a chemical reaction that he discovered that bears his name.

Cannizzaro, the youngest of 10 children, was born in Palermo, Sicily. His father, Mariano Cannizzaro, was a magistrate and minister of police. His mother, Anna di Benedetto, was descended from Sicilian aristocracy. When he was born in 1826, the Bourbon kings of Naples ruled Sicily. The Bourbons were a French royal family whose members ruled France, Spain, Naples, and Sicily. Cannizzaro's family supported the Naples regime and one of Cannizzaro's sisters was a lady-in-waiting to the queen. However, there were political liberals on Cannizzaro's mother side: three of his mother's brothers fought and died in Giuseppe Garibaldi's fight to unify Italy. In time, Cannizzaro's loyalties shifted, and in 1848, he participated in a revolution to overthrow the monarchy.

Cannizzaro received a classical education in the Palermo schools. In 1841, he enrolled at the University of Palermo to study medicine. Although he was a medical student, he became interested in chemistry, and in 1845, he accepted a job as a laboratory assistant for Raffaele Piria, professor of chemistry at the University of Pisa, and a leading Italian chemist. For the next two years, Cannizzaro studied chemistry and assisted with investigations of salicin and glucosides. While in Pisa, Cannizzaro met Cesare Bertagnini, a pupil of Piria's, and the two students became good friends. Bertagnini died when he was 30 but not before he played a key role, along with Cannizzaro and Piria, in the development of an Italian school of chemistry in the early 1850's.

On break in the summer of 1847, Cannizzaro returned to Palermo with the intention of resuming his studies in the fall. But he changed his mind when he learned about plans for a Sicilian Revolution against the Bourbons. Instead of returning to Pisa, he joined the uprising, serving as an artillery officer. The Bourbons were foreed to leave Naples in 1848, but even so, the rebellion failed, and Cannizzaro was condemned to death. In 1849, he fled to Marseilles but soon moved to Paris. There he worked with Michel Chevreul and Stanislaus Cloëz. While in France, Cannizzaro researched cyanamide and its derivatives and successfully synthesized cyanamide.

A few years after his exile, Cannizzaro was able to return to Italy. In 1851, he accepted a position as professor of physics, chemistry, and mechanics at the Technical Institute in Alessandria. The institute had limited facilities, but Cannizzaro expanded the research laboratory and carried out some of his best organic chemistry work there. In 1853, while investigating aromatic alcohols, he discovered that when benzaldehyde reacts with potassium hydroxide, it goes through an oxidation-reduction process that produces both benzoic acid and benzyl alcohol. This finding, known as “Cannizzaro's reaction,” has proven especially useful in the field of synthetic organic chemistry.

In 1855, Cannizzaro left Alessandria for Genoa, where he became professor of chemistry at the university. Genoa lacked a laboratory, but Cannizzaro, an excellent teacher, devoted much time to developing his chemistry course. While living in Genoa, Cannizzaro wrote a letter outlining the distinction between atoms and molecules. Cannizzaro sent the letter to Sebastiano de Luca, the chair at Pisa, a post that had been previously held by Bertagnini and before him, Piria. In 1858, the letter was published in the Pisa journal Nuovo cimento as “Sunto di un corso di filosofia chimica fatto nella Reale Università di Genova.” A year later, “Sunto” was reprinted in pamphlet form and since then, it has been republished many times and translated into other languages, including English and German.

Cannizzaro based his ideas on a hypothesis proposed by fellow Italian Amedeo Avogadro. Using Avogadro's law, Cannizzaro clarified the distinction between atoms and molecules and by doing so, laid a solid foundation for the formulation of the periodic law. Cannizzaro's revival of Avogadro's law came at a time when chemists were in a state of confusion regarding atomic weight and the terms atom and molecule. In 1811, Avogadro had stated that equal volumes of all gases at the same temperature and pressure contain the same number of chemical units. He distinguished between gases composed of simple units (atoms) and gases made up of complex units (molecules). His theory also allowed him to calculate atomic and molecular weights from gas densities. While a few scientists accepted his hypothesis, the leading chemists of the day did not accept it. They believed it was not necessary to distinguish between atoms and molecules. In 1860, Cannizzaro presented a logical argument supporting Avogadro's law. Cannizzaro had a solid knowledge of the history of chemistry, and he showed how the theory explained inconsistencies over the years. He also demonstrated how the hypothesis could be used to determine molecular weights. Much of what Cannizzaro offered had been stated or implied by Avogadro. But Cannizzaro was able to present the information in such a clear, understandable way that other scientists could grasp it and within a short time, Avogadro's law became the standard.

Although most of Cannizzaro's legacy rests on the principles expressed in “Sunto,” its initial printing did not cause much of a stir. At the 1860 Karlsruhe Congress, the scientific community became aware of the ideas expressed in “Sunto.” In September 1860, an international meeting of chemists was held in Karlsruhe, Germany, over a three-day period. The purpose of the congress was to help scientists establish a systematized approach to their studies by developing more precise definitions of the concepts of “atom, molecule, equivalent, atomicity, alkalinity,” and other concepts. Most of the participants were young, eager scientists who were heavily involved in research. Cannizzaro attended the meeting and joined the discussions, which focused on clarifying the definitions of terms such as “atom,” “physical molecule,” and “chemical molecule.”

On the third day of the congress, participants debated whether they should adopt Jons Jakob Berzelius 's principles for purposes of nomenclature. Cannizzaro spoke before the congress, delivering a lengthy refutation of this proposal, in which he summarized the arguments he had made in the “Sunto.” He then spoke in favor of Avogadro's hypothesis. Cannizzaro's presentation failed to sway the crowd, and it was decided that individual scientists could choose to use either Berzelius's or Avogadro's system. But the matter did not end there. Angelo Pavesi, a professor of chemistry at the University of Pavia and a friend of Cannizzaro's, passed out copies of “Sunto” that Cannizzaro had brought to the congress. One person who read the paper, Julius Lothar Meyer, understood its importance and in 1864, wrote an influential book that incorporated Cannizzaro's ideas. It was the acceptance of Avogadro's hypothesis that led Meyer and Dmitri Ivanovich Mendeleev to devise the periodic law in the late 1860's.

After the congress, the political situation in Sicily changed once again when Garibaldi won his fight to unify Italy. In 1860, Cannizzaro made his way back to Palermo, where he became a member of the council in the new government. The following year, he returned to the University of Palermo. As a professor of inorganic and organic chemistry, Cannizzaro put his efforts into building a top-notch research facility, and Palermo became a leading center for chemical studies. A decade later, Cannizzaro moved to Rome, serving as a senator and then as vice president of the Italian senate. Through all the turbulence in his life, he continued his scientific pursuits and study of natural substances.

During his later years, Cannizzaro received honors and awards from many of the leading scientific societies of Italy and the rest of Europe, including the Copley Medal of the Royal Society in 1891.