Selenium: the story on HearLore

Selenium

In 1817, a red solid precipitate emerged from the lead chambers of a sulfuric acid plant near Gripsholm, Sweden, defying the expectations of two chemists who believed it to be an arsenic compound. Jöns Jacob Berzelius and Johan Gottlieb Gahn had been processing pyrite samples from the Falun Mine, expecting to produce sulfuric acid via the lead chamber process, but the ore contained an unknown substance that turned the acid red. When they burned this red powder, it released an odor resembling horseradish, a scent that did not match arsenic but closely mirrored the smell of tellurium compounds. Berzelius initially wrote to Alexander Marcet claiming the substance was tellurium, yet the absence of tellurium in the Falun Mine minerals forced a reanalysis. By 1818, Berzelius confirmed the discovery of a new element, which he named selenium after the Greek goddess Selene, the Moon, to reflect its relationship to tellurium, the Earth. This naming choice highlighted the element's position in the periodic table, sandwiched between sulfur and tellurium, and marked the beginning of a century-long journey to understand its dual nature as both a vital nutrient and a potent poison.

The Light That Conducts

The electrical conductivity of grey selenium changed dramatically when exposed to light, a phenomenon discovered by Willoughby Smith in 1873 that would revolutionize the early electronics industry. Smith found that the resistance of grey selenium dropped significantly under illumination, a property that allowed the element to function as a light-sensitive cell. This discovery led to the development of the first commercial selenium cells by Werner Siemens in the mid-1870s, which were soon employed in Alexander Graham Bell's photophone of 1879. The photophone transmitted sound via light beams, using selenium cells to convert the light back into electrical signals at the receiver. Selenium's ability to transmit an electric current proportional to the amount of light falling on its surface made it indispensable for light meters and early photoelectric devices. By the 1930s, selenium rectifiers began replacing copper oxide rectifiers in commercial applications due to their superior efficiency, and they remained in use until the 1970s when silicon devices took over. Despite being largely replaced in electronics, selenium's photoconductive properties continue to be used in flat-panel X-ray detectors, where amorphous selenium films convert incident X-ray photons directly into electric charge with high sensitivity and minimal trapping by defects.

The Glass That Hides Color

Selenium compounds confer a red color to glass, a property that accounts for about 50% of global selenium consumption and serves to cancel out the green or yellow tints arising from iron impurities in most glass. This application, which involves adding various selenite and selenate salts to molten glass, allows manufacturers to produce clear or colored glass without the unwanted hues that naturally occur in silica-based materials. In some cases, mixtures of cadmium selenide and cadmium sulfide are used to produce specific red shades, while other applications utilize selenium to create deep red glass for decorative purposes. The largest commercial use of selenium remains in glass production, where it is added to neutralize the natural coloration of iron impurities, resulting in the transparent glass used in windows, bottles, and optical lenses. This process, which has been refined over decades, ensures that the final product is free from the greenish or yellowish tints that would otherwise detract from its clarity and aesthetic appeal.

Common questions

Who discovered selenium and when was it discovered?

Jöns Jacob Berzelius and Johan Gottlieb Gahn discovered selenium in 1817 while processing pyrite samples from the Falun Mine near Gripsholm, Sweden. Berzelius confirmed the discovery of the new element by 1818 and named it after the Greek goddess Selene to reflect its relationship to tellurium.

What is the significance of selenium in the history of electronics?

Willoughby Smith discovered the photoconductive properties of grey selenium in 1873, which allowed the element to function as a light-sensitive cell. This discovery led to the development of commercial selenium cells by Werner Siemens in the mid-1870s and enabled the transmission of sound via light beams in Alexander Graham Bell's photophone of 1879.

How is selenium used in the glass industry to improve clarity?

Selenium compounds confer a red color to glass and account for about 50% of global selenium consumption to cancel out green or yellow tints from iron impurities. Manufacturers add selenite and selenate salts to molten glass to produce clear or colored glass without the unwanted hues that naturally occur in silica-based materials.

What is the recommended daily intake of selenium for adults and teenagers?

The US Recommended Dietary Allowance for teenagers and adults is set at 55 micrograms per day. The human body contains between 13 and 20 milligrams of selenium, which functions as a cofactor for antioxidant enzymes such as glutathione peroxidase and thioredoxin reductase.

What caused the fish kills at Belews Lake and Kesterson National Wildlife Refuge?

Selenium pollution caused catastrophic fish kills and reproductive failures in aquatic systems like Belews Lake in North Carolina and the Kesterson National Wildlife Refuge in California. Wastewater from a Duke Energy coal-fired power plant discharged selenium concentrations of 150 to 200 micrograms per liter into Belews Lake between 1974 and 1986, while agricultural irrigation drainage concentrated natural soluble selenium compounds in wetlands at Kesterson.

What are the physical properties of grey selenium compared to other allotropes?

Grey selenium is the most stable and dense form with a chiral hexagonal crystal lattice consisting of helical polymeric chains where the selenium-selenium distance is 237.3 picometers. This form is a semiconductor showing appreciable photoconductivity and is insoluble in carbon disulfide, unlike other allotropes such as black or red selenium.