April 13, 2012 (Source: The Guardian) — This week’s element is cerium, which has the symbol Ce and the atomic number 58. Like its prototype, lanthanum, cerium is a rare earth element, although it is not very rare at all, also similar to lanthanum. In fact, cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth’s crust by weight.
As you can see from the featured image above, instead of showing you yet another picture of a soft, lustrous silvery-grey metal that is ductile, I am showing you one of the minerals that contains cerium. This mineral is allanite. In this image, allanite is the lustrous grey crystals embedded in the smoky quartz crystals.
Cerium gets its name from the asteroid, ceres, which had been discovered two years prior to this element’s discovery. The asteroid got its name from the Greek god of agriculture, Ceres. (No, don’t ask me what an asteroid far distant from Earth has in common with agriculture, nor do I know what this element has in common with an asteroid. I was hoping you’d tell me!)
Cerium is a strong reducing agent and ignites spontaneously in air between 65 and 80 °C, and so it was difficult to isolate and purify. Although first found in a reddish-brown mineral in 1751 by Swedish chemist Axel Cromstedt, it was assumed to be a strangely heavy lump of tungsten. Many attempts were made to isolate all the elements from this mineral, but no one managed to do it using then-conventional methods. This led chemists to conclude that cerium was new to science. Finally, more than one hundred years later, two American chemists isolated a pure sample of cerium by passing an electric current through molten cerium chloride.
Cerium is extraordinarily useful. For example, cerium(III) oxide is used in catalytic converters to reduce CO emissions, especially from diesel engines, and it is also used in self cleaning ovens. Cerium(III) oxide is also used to make colourless glass that is extremely efficient at absorbing ultraviolet light.
Unlike the white or colourless cerium(III) salts, cerium(IV) salts are orangeish-red or a lovely bright yellow colour [cerium(IV) sulfate pictured, right (creative commons)]. When combined with titanium (IV) oxide, cerium(IV) oxide “stains” glass yellow. Cerium(IV) oxide has a high refractive index and is added to enamel to make it more opaque.
Cerium is also used to increase the photostability of pigments and to prevent clear polymers from darkening in sunlight. Several especially familiar uses for cerium is in TV screens and fluorescent lamps.
Since I am a biologist, I get to tell you a little about this element’s effects upon living things. Cerium is not considered to be particularly toxic to living creatures, depending upon how you define toxicity. However, I did find something indicating that large doses of injected cerium does cause animals to die from heart attacks, although “large doses” and the form of the injected cerium (for example, was it a salt?) are not defined.
The medical establishment flirted with a variety of cerium-containing medications ever since the element was discovered, using it with some success to treat a number of gastrointestinal ailments, particularly vomiting, stomach pains and diarrhea. Perhaps inspired by American actress, playright and sex symbol, Mae West — who once famously said; “If a little is great, and a lot is better, then way too much is just about right!” — doctors and their patients tested this idea, and discovered that ingesting very high doses of cerium-containing medicines often caused the very symptoms they were intended to treat.
So predictably, the medical use of cerium stopped. However it was recently found that a weak solution of cerium nitrate works as a powerful antiseptic against gram-negative bacteria in wounds resulting from third degree burns [PMID: 135364]. So effective is it that cerium nitrate is now part of standard burn management practice.