May 2006

Introduction

Magnesium chloride, MgCl2, dissolved in seawater, is an abundant, natural source of the very useful metal magnesium. Magnesium is a common component of alloys. Alloys are produced by combining a pure metal with one or more other elements to form a new substance with desirable properties. (Steel, for example, is a common alloy of iron and carbon.) Magnesium alloys are found in aircraft, automobiles, rockets, luggage frames, portable power tools, cameras, laptop computer casings and mobile phones.

Magnesium lends lightness and strength to alloys--a real plus for the owners of fuel-consuming vehicles, or travelers carting luggage. And magnesium alloy construction helps dampen the noise and vibration associated with the use of hand-held power tools, such as electric drills and nail-shooting guns.

Teaming up with Aluminum

As the element of atomic number 12, magnesium is a Periodic Table neighbor of aluminum, atomic number 13. The strength and hardness of aluminum increase when it is alloyed with magnesium. One-third less dense than aluminum, magnesium makes the alloy lighter, stronger and easier to work. According to the International Magnesium Association, 40 to 45 percent of the magnesium produced is alloyed with aluminum, making that the single greatest use of magnesium metal.

The typical beverage can is an alloy consisting mostly of aluminum and a few percent magnesium, giving the can added strength and "formability." Since more than 60 percent of beverage cans in the U.S. are recycled, magnesium is also being recycled in these operations.

From the Oceans

Just as chlorine is produced from the sodium chloride of ancient oceans, much of the magnesium chloride used today also had its origins in seawater. Salts, such as sodium chloride and magnesium chloride, are transported to the oceans by Earth's rivers as they drain the weathering continents. In that way, over millions of years, the oceans have become huge storage reservoirs of salts. Magnesium can be derived either from seawater or from magnesium-rich minerals deposited during the slow evaporation of ancient seas.

Isolating Magnesium

Although it forms many natural compounds, and is the eighth most abundant chemical element in the Earth's crust, magnesium does not appear in nature as a pure element. Pure magnesium is commonly made by separating it from seawater.

Salts exist as ions in water. Seawater contains many ions, including Mg2+. To separate out magnesium, first, calcium hydroxide, Ca(OH)2, is added to salty seawater. The chemical reaction produces a magnesium hydroxide precipitate, or solid:

The solid magnesium hydroxide from the initial reaction is then reacted with hydrochloric acid to produce magnesium chloride:

The water is evaporated, leaving solid magnesium chloride. Magnesium chloride is then heated, or fused, to produce liquefied magnesium chloride, which, when subjected to an electrical current, produces magnesium metal and chlorine gas:

Magnesium ingots (photo
from New World Alloys,
www.newworldalloys.com)

This process is known as electrolysis. The liquid magnesium formed is cooled into convenient blocks of metal known as ingots. The chlorine gas is recycled to form hydrochloric acid for the production of more magnesium chloride.

In Our Diet

According to the National Institutes of Health, magnesium is the fourth most abundant mineral in the human body and is essential to good health. In fact, magnesium is needed for more than 300 biochemical reactions in the human body. One way to meet your magnesium requirement is by eating green vegetables, such as spinach; magnesium is present in green plants at the center of the chlorophyll molecule.

Chlorophyll is the chemical pigment that gives plants their green color. Solar energy is absorbed by chlorophyll, enabling the process of photosynthesis in which carbon dioxide and water react to form plant carbohydrates and the atmospheric oxygen which we breathe.

In Japan, where much of the diet comes from the sea, a magnesium chloride product from seawater--called Nigari--is used to help make tofu. Tofu is a soy milk-based food. Nigari, added to soymilk, coagulates it, turning it into solid pieces, or curds, which when pressed, becomes tofu.

Follow-up Activities:

  1. Draw a diagram of the "life cycle" of magnesium that is used in an aluminum alloy beverage can, starting with seawater.

  2. Silver (Ag+1) chloride (Cl-1), the compound AgCl, is not soluble in water. Write a chemical reaction for the precipitation of silver chloride beginning with silver nitrate (AgNO3) and potassium chloride (KCl). (Silver nitrate and potassium chloride are both soluble in water.)

  3. What are the compositions of these alloys?
    • Bronze
    • Brass
    • 14 Karat Gold

For a list of previous "Chlorine Compound of the Month" features, click here.

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