Amalgam (chemistry)

An amalgam is a specific type of alloy created by combining mercury with another metal. These mixtures can exist in several different states, such as a liquid, a soft paste, or a solid. The physical state of the amalgam depends on the proportion of mercury used in the mixture.

These alloys are held together through a process called metallic bonding. In this process, electrostatic attractive forces use conduction electrons to bind positively charged metal ions into a crystal lattice structure. While many metals can form amalgams, some notable exceptions exist. Metals such as iron, platinum, tungsten, and tantalum do not readily form these alloys with mercury.

The formation of an amalgam often occurs through a chemical reaction when metals contact mercury at room temperature. This process is typically exothermic, meaning the reaction releases heat. The ability of a metal to dissolve into mercury is known as solubility. This solubility generally increases as the temperature rises. Some elements, such as indium, thallium, cadmium, and cesium, are highly soluble in mercury. Conversely, heavier metals are generally less likely to form amalgams than lower mass metals. Because mercury does not dissolve iron, iron flasks were used historically to transport the liquid metal.

Different types of amalgams can be categorized by the metals involved. Alkali metal amalgams, such as those made with potassium, are very sensitive to air and water. These reactions are exothermic and can produce distinct chemical forms like KHg or KHg2. Sodium amalgam (NaHg2) is another specific type that features a unique structure. In this version, mercury atoms form hexagonal layers while sodium atoms form a linear chain. Sodium amalgam is produced as a byproduct of the mercury-cell chloralkali process. It serves as an important reducing agent in both organic and inorganic chemistry.

Post-transition metals also form various amalgams with unique properties. For example, tin amalgam was famously used to create reflective mirror coatings. During the 16th century, glassmakers on the island of Murano developed this by sliding glass over tin flooded with mercury. This specific coating consisted of 75% tin and 25% mercury. Aluminum can also form an amalgam, which is used as a reagent to reduce certain compounds. However, mercury can also cause serious corrosion in aluminum by destroying its protective oxide layer. Because of this risk, mercury is generally not allowed aboard aircraft near exposed aluminum parts.

Amalgams have a long history of use in human industry and art. Amalgam gilding was a common technique used to coat metal surfaces before the invention of electroplating. In China, this method was used to gild silver as early as the third to first centuries BC. While often less satisfactory than gold fire gilding, the technique is still practiced today at the Morimoto workshop in Kyoto, Japan. In the mining industry, mercury has been vital for extracting precious metals. The patio process, developed in Mexico in 1554, used amalgamation to process silver ores.

In gold mining, mercury is used to capture tiny particles of gold from sand or gravel. In placer mining, crushed ore is often washed over copper sheets coated in mercury. The fine gold particles stick to the mercury to form a heavy, dull gray solid mass. To recover the gold, the amalgam is heated in a distillation retort. This process evaporates the mercury, which can then be recovered by a condenser for reuse.

This method is effective for extracting "flour gold," which is too fine for other mechanical methods. However, the release of mercury vapors during this process can cause significant health issues.

Dental amalgams are another widely recognized application of these alloys. Dentists use mixtures of mercury with metals like silver, copper, indium, tin, and zinc to fill cavities. These materials are valued because they are inexpensive and easy to manipulate. The mixture remains soft for a short time, allowing it to be packed into irregular spaces before hardening.

While dental amalgams are generally considered safe, there are modern restrictions due to environmental concerns. In 2018, the EU prohibited dental amalgam for children under 15 and for pregnant or breastfeeding women. Similarly, the US FDA recommends against its use in certain high-risk groups to prevent mercury toxicity and pollution.