Of all known metals, mercury is probably one of the most famous, if only for its lustrous, liquid form at room temperature. Over the centuries, it has been commonly used in a wide variety of applications, including industrial chemical processes, in cosmetics, for telescope mirrors, thermometers, fluorescent lamps, dental fillings, bearings, batteries, switches and most recently in atomic clocks.
Though hardly free from the controversy often surrounding a toxic heavy metal, it’s hard to argue the myriad ways in which mercury has played a positive role in humanity’s technological progress and scientific discoveries. This article will focus both on its historical, current, and possible future uses, as well as the darker side of this fascinating metal.
Shiny and Useful
Mercury has been highly prized for its use in art and decorations. It’s historically known to have been used in mercury fountains — exactly as it sounds, these were artistic fountains using mercury rather than water — with the most recent example being Alexander Calder’s 1937 Mercury Fountain. Yet for thousands of years, from the Mayans (Temple of the Feathered Serpent in Teotihuacan) to the Egyptians and the Chinese (Emperor’s Qin tomb), mercury was held in high esteem, with many considering it to hold special properties in addition to its remarkable physical properties.
Unfortunately, this led to it being used in medicine, especially in China and Tibet, where it was thought to prolong life, heal injuries and generally improve one’s health. It is rumored that a mixture of mercury and crushed jade given as an immortality mixture was what killed Emperor Qin. Alchemists considered mercury to be a Prima Materia (First Matter) from which other metals are derived.
Clearly more practical was the discovery around 500 BC of amalgams (from medieval Latin amalgama, “alloy of mercury”), the mixing of mercury with other metals which led to its use for dental fillings in China before 1000 AD and in Europe around 1528 AD. Much like the Chinese amalgams back then, dental amalgams today consist of mercury and a metal alloy of silver, tin, and copper.
While polymer resins are being used more commonly instead of amalgam in dentistry, amalgam remains superior in terms of longevity and durability, except for situations where the restored area would be directly visible (polymer resins being white), or the hole in the tooth is fairly small. Here polymer resins are the preferred material.
Despite the scares about mercury poisoning from the elemental mercury in amalgam dental fillings, studies have shown that the amounts of mercury released is low enough that it should pose no health risks. Regardless, dental offices in the EU are required to treat amalgam waste as hazardous waste. US dental offices are facing similar measures, but flushing the amalgam waste down the drain is still common practice.
The Distinction Between Useful and Hazardous
Even outside of dental amalgam, mercury manages to provoke fierce debates about its uses and perceived dangers. One of these involves the many organic compounds that contain mercury, the so-called organomercury compounds. This group includes methylmercury (commonly found in fish like tuna and salmon), ethylmercury , dimethylmercury, diethylmercury, and merbromin.
Commonly used as a preservative agent due to its antiseptic and antifungal properties, thiomersal is regularly used in everything from vaccines to ophthalmic (e.g. eyedrops) and nasal products as well as things like tattoo inks and mascara, where long-time sterility is essential. In the body, thiomersal is broken down into ethylmercury, which is significantly less dangerous to the body than methylmercury. While refrigeration is an alternative to thiomersal, it requires an uninterrupted cooling chain, which can be problematic in some areas, leading to the use of contaminated vaccines.
In the US, fears about the ‘mercury’ in vaccines (related to conspiracy theories involving autism caused by vaccines) led to thiomersal being removed from most vaccines despite a lack of scientific evidence for doing so. Due to a lack of data on ethylmercury’s effect on the body in the 1990s, the data for methylmercury was used instead. Later research showed this to be a wrong equivalence, instead showing just how much more harmful methylmercury is.
Incidentally, the same conspiracy theories that led to the removal of thiomersal from most vaccines is linked into a more grand conspiracy theory about autism being caused by environmental toxins, including lead, mercury and other heavy metals. Chelation therapy is supposed to remove these toxins. This is however strongly recommended against as it is not an effective treatment and can lead to kidney and other potentially fatal damage.
Don’t Eat That Fish
Methylmercury is the most common form of organomercury, as it’s formed from inorganic mercury by microbes that live in aquatic systems. The resulting methylmercury is readily consumed by algae, which in turn are consumed by ever larger fish and other aquatic organisms in a process called biomagnification. As a result, the consumption of fish is the largest source of methylmercury and mercury in general for the population.
Mercury poisoning became well known due to the sudden outbreak of the then new Minamata disease in Japan, which turned out to be caused by the release of methylmercury into the environment from chemical factories, ending up in aquatic organisms that the local population would then catch and consume. In Japan this disease would cost 1,784 lives of 2,265 officially identified victims. Other nations experienced their own outbreaks of this disease.
Even without deliberate spills of methylmercury or its precursors, the amount of mercury in the environment is such that for fish species like swordfish, tuna, cod and pike one should not eat more than 170 grams of it per week, to avoid an unhealthy bioaccumulation of mercury in one’s body. Some places like Florida’s Everglades end up acting like scrubbers for mercury that is released in the air, severely raising local mercury levels there, with the advice being to never eat fish caught in those areas.
Mercury Today and Tomorrow
It is hard to think of a world without mercury. Whether it’s in dentistry, industry, laboratories or in astronomy, an essential role is played by mercury in some fashion. In astronomy especially, mercury essentially enables liquid mirror telescopes, which provide a highly effective and low-cost alternative to expensive and fragile glass mirrors. Mercury sees common use as an electrode in chemistry and in X-ray crystallography studies of proteins in structural biology with the multiple isomorphous replacement (MIR) approach, even as its use in more mundane tasks such as diffusion vacuum pumps has diminished over time, it still remains relevant there, as recently covered on Hackaday.
One of the most exciting new applications for mercury in the near future is that Jet Propulsion Lab’s Deep Space Atomic Clock (DSAC). The exciting thing about the DSAC is that it essentially takes the accuracy of a rubidium-based atomic clock (AC) and stuffs it into a package many times smaller. This is all courtesy of the properties of mercury ions which allowed for such a level of miniaturization, allowing it to be used in weight-sensitive applications, such as space probes and satellites.
Mercury in Space
The obvious advantage of the DSAC project is that the high clock stability improves the on-board time-tracking and thus navigation and communication abilities, which would be ideal for deep space missions. This is further detailed in a 2012 JPL paper on the project. It describes the crucial role the onboard timing source has on deep space navigation when it comes to forming multi-way coherent Doppler and range measurements. The essential benefit is that a spacecraft can do more by itself, with higher accuracy and higher useful data rates across the network.
Mercury is similar to rubidium in that it has a hyperfine transition that emits a very precise electromagnetic signal. Much of the miniaturization is enabled by the fact that in a microwave-driven atomic clock (like in current rubidium ACs and the DSAC) the frequency that is required to drive the clock also determines the dimensions of the oscillator which drives the clock. Whereas a rubidium clock uses a paltry ~6.834 GHz, mercury-199 uses 40.5 GHz.
At those higher frequencies, the required circuitry and other components can be made much smaller, resulting in an atomic clock (current version) that’s a mere 29 by 26 by 23 cm at 17.5 kg, yet show no more drift than about 1 microsecond in 10 years of operation.
A DSAC prototype was launched on June 25th 2019 from Kennedy Space Center on a SpaceX Falcon Heavy rocket, as part of the Orbital Test Bed (OTB) satellite, which hosts four additional payloads in addition to the DSAC. NASA activated the DSAC prototype on August 23rd, with the entire mission expected to take about a year.
Being the Right Atom, in the Right Place
Throughout history, mercury has been a bit of a celebrity metal. In addition to its highly unusual liquid state at room temperature, it has enabled many areas of science to progress in ways that would have been difficult without mercury. Whether one looks at diffusion pumps and mercury thermometers, its myriad roles in chemistry and industry, the preservation of vaccines and similar substances, it’s hard to think of a material which has impacted human civilization more in ways that are subtle but ever-present.
Now it appears that mercury will be with us on our journey to the final frontier as well, keeping our space probes and possibly crewed space ships safe as they travel to Mars, Venus, and beyond. Here’s to a long, healthy relationship with a really special metal.