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Sodium sulfate

physical science Maturity 7-9 Next article →

Sodium sulfate is an inorganic compound with the chemical formula Na2SO4. It exists as a white solid that is highly soluble in water. This substance is considered a major commodity chemical because of its massive global production. Every year, about 6 million tonnes of sodium sulfate are produced. It is used in many industries, ranging from making laundry detergent to manufacturing paper and glass. Because it is so versatile, it is a fundamental building block in many industrial processes.

The chemical properties of sodium sulfate are defined by its ionic bonding. It is a typical electrostatically bonded ionic sulfate. In a liquid solution, free sulfate ions are present. You can prove this by adding barium or lead salts to the solution. These salts react to form insoluble sulfates. Sodium sulfate is generally unreactive toward most oxidizing or reducing agents. However, at very high temperatures, it can undergo carbothermal reduction. This process uses heat and charcoal to convert the sulfate into sodium sulfide. This specific reaction was once a key part of the Leblanc process.

Sodium sulfate exists in several distinct forms, mostly defined by how many water molecules are attached to it. The anhydrous form, which contains no water, is known as the mineral thénardite. This form is often used as a drying agent in organic synthesis. Another form is the heptahydrate, which is considered very rare. The most common industrial form is the decahydrate, which contains ten water molecules. This form is known as the mineral mirabilite or Glauber's salt. The crystals of this decahydrate have a unique structure where water molecules are bound to sodium ions in an octahedral geometry.

Na2SO4 solubility
Na2SO4 solubility

The history of this compound is tied to the chemist Johann Rudolf Glauber. He was a Dutch-German apothecary who lived from 1604 to 1670. In 1625, he discovered the decahydrate form in Austrian spring water. He named it "miraculous salt" because of its medicinal uses. For a long time, the crystals were used as a general-purpose laxative. This medicinal use continued until more sophisticated alternatives were developed in the 1900s. By the 18th century, the salt became an industrial raw material. It was reacted with potash to produce soda ash, which is sodium carbonate.

The solubility of sodium sulfate in water is quite unusual. Its solubility rises more than tenfold between 0 °C and 32.384 °C. At that specific temperature, it reaches a maximum solubility of 49.7 g/100 mL. After this point, the solubility becomes almost independent of temperature. This specific temperature is actually used as a reference for calibrating thermometers.

Na2SO4 solubility
Na2SO4 solubility

This phase change also makes the salt useful for heat storage. The decahydrate has a high heat of fusion of 82 kJ/mol. This allows it to store solar heat for use in space heating applications.

Production of this chemical comes from both natural and synthetic sources. About two-thirds of the world's decahydrate comes from the natural mineral mirabilite. Large deposits are found in lake beds, such as those in southern Saskatchewan. In 1990, Mexico and Spain were the leading natural producers, each providing 500,000 tonnes. The United States, Russia, and Canada also produced significant amounts. About one-third of production is a by-product of other chemical processes. For example, it is produced during the manufacture of hydrochloric acid through the Mannheim or Hargreaves processes.

Today, sodium sulfate serves many specialized roles in modern technology. In the textile industry, particularly in Japan, it helps dyes penetrate fibers evenly. It increases the ionic strength of the solution to help with a process called "levelling." In the glass industry, it acts as a fining agent. This helps remove small air bubbles from molten glass to ensure clarity. It is also used in the food industry as a diluent for food colors, labeled as additive E514.

Even in laboratories, it can be used to dry organic liquids by absorbing water.

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Na2SO4 solubility.svg
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