Why does tonic water glow?

John Woodland
4 min readJul 9, 2020

A brief history of quinine, fluorescence and the probable origin of the G&T.

Tonic water illuminated by ultraviolet light. (Cathy Scola, Getty Images)

Tonic water glows faintly in sunlight — a phenomenon that is even more apparent when it is illuminated by ultraviolet light. This glow, called fluorescence, is a special characteristic of tonic water’s main ingredient: quinine.

Quinine is what chemists call a natural product — a molecule produced by a living organism, usually a plant, that often has valuable therapeutic properties.

Quinine is extracted from the bark of trees belonging to the genus Cinchona. These trees originated in the high altitudes of South America but are now found all over the world.

The discovery of quinine

According to one account, the Native Peruvians chewed the bark of Cinchona trees to stop shivering while they worked in the cold streams of Spanish-owned mines. Early physicians in Peru suspected that concoctions of the bark might also be able to stop the shivering associated with attacks of febrile illnesses. They tried the bark of the “fever tree” on patients suffering from malaria and found that their feverish symptoms were miraculously relieved.

The term “quinine” is derived from quina-quina, the name that the Native Peruvians gave to the special bark and which means medicine of medicines or bark of barks. From the mid-19th century to the 1940s, quinine became the standard treatment for malaria throughout the world. Consequently, it has been described as “a drug that has probably benefited more people than any other in the combat of infectious disease”.

Prior to the isolation of quinine by two French chemists in 1820, the bark was administered as a suspension in wine or spirits to offset its bitterness. British officials stationed in 19th-century India mixed their medicinal quinine with gin. This recipe probably evolved into the gin and tonic drink we know today.

(Incidentally, if you were hoping to drink enough tonic water to act as a malaria prophylactic, a quick calculation suggests that you would need to drink up to ten litres of tonic water to consume a therapeutic dose of quinine! If you drank that much, malaria would be the least of your problems…)

“An extremely vivid and beautiful celestial blue colour”

The glow exhibited by a solution of quinine in sunlight was first observed by Sir John Herschel (1792–1871), a famous polymath, who wrote:

Though perfectly transparent and colourless when held between the eye and the light … it [the quinine solution] yet exhibits … under certain incidences of light an extremely vivid and beautiful celestial blue colour.

This was the first documented description of the “fluorescence” of quinine — a word that was coined some years later by Sir George Stokes, a physicist and mathematician. Together, these observations by Herschel and Stokes laid the foundation for a century-and-a-half of intense development in the field of fluorescence which has had an enormous impact across many disciplines.

Investigating human bone cells under the microscope using modern fluorescence techniques. (Michael W. Davidson, Florida State University)

A curious electron dance

Fluorescence is the absorption and subsequent release of light from a molecule. Most molecules absorb a small “packet” of light energy, called a photon, and then dissipate that energy in the form of heat. But molecules that fluoresce, such as quinine, can release this energy as light instead of heat.

Once it has absorbed a photon, quinine is able to remain in an “excited state” for a very brief period of time (millionths of a second). This excited state is not stable; eventually, the molecule loses its energy and returns to its unexcited ground state. In the process, a photon of light is released and is visible to our eyes as a glow, or fluorescence.

Some energy is always lost during the absorption and emission of light, with the result that the energy of the emitted photon is always less than the energy of the photon that was absorbed. Because the energy of light is inversely proportional to its wavelength, this means that quinine glows with the “celestial blue colour” described by Herschel when it is illuminated by ultraviolet light. On the electromagnetic spectrum, ultraviolet light (which is usually invisible to our eyes, or may look slightly purple) is higher in energy than blue light. The sun also produces ultraviolet light, which is why the fluorescence of tonic water is visible if you happen to be “daytime drinking” outside in the sun!

An interesting final remark is that quinine fluoresces more strongly in acidic solutions. Hence, adding a squeeze of lemon juice to your G&T will lower the pH of the solution, making it more acidic. This, in turn, makes the fluorescence of quinine under ultraviolet light (or in sunlight) even more pronounced. Cheers!

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John Woodland

Research scientist, musician and broadcaster ​based in Cape Town, South Africa.