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Sodium metaborate

physical science Maturity 11-13 Next article →

Sodium metaborate is a chemical compound made of sodium, boron, and oxygen. It is an important substance used in many industrial processes. This compound appears as colorless crystalline solids in various forms. It can exist in an anhydrous state, which means it contains no water. It also exists as several different hydrates. A hydrate is a solid that holds water molecules within its crystal structure. The anhydrous form is hygroscopic, meaning it pulls moisture from the surrounding air.

The structure of sodium metaborate changes based on how much water it holds. The anhydrous form uses a hexagonal crystal system. Within this system, the atoms form six-membered rings. These rings consist of alternating boron and oxygen atoms. One extra, negatively charged oxygen atom attaches to each boron atom. All nine of these atoms sit on a single flat plane. The density of this anhydrous form is 2.348 grams per cubic centimeter.

When water is added, different types of hydrates form at specific temperatures. A tetrahydrate forms between -6 and 53.6 degrees Celsius. A dihydrate forms between 53.6 and 105 degrees Celsius. A hemihydrate forms from 105 degrees Celsius up to the boiling point. The dihydrate uses a triclinic crystal system. This system is nearly monoclinic because its angles are close to 90 degrees. The dihydrate has a density of 1.905 grams per cubic centimeter.

Scientists use different methods to prepare sodium metaborate in a laboratory. One common method is the fusion of sodium carbonate and boron oxide. Another way is to fuse borax with sodium hydroxide at 700 degrees Celsius. You can also create the anhydrous salt by heating tetraborate to 270 degrees Celsius in a vacuum. When heated to very high temperatures, between 900 and 1400 degrees Celsius, the vapor shows isolated clusters. These clusters follow the formula (BO3)n. Electron diffraction studies by Akishin and Spiridonov helped reveal these linear structures.

This compound can undergo several important chemical reactions. When dissolved in water, the anion combines with two water molecules. This process creates a tetrahydroxyborate anion. Sodium metaborate can also be converted into borax through electrolysis. This happens in a concentrated aqueous solution using an anion exchange membrane. Another useful reaction is the reduction to sodium borohydride. This can be done using reducing agents at high temperatures and pressure. It can also be done by ball milling with magnesium hydride at room temperature.

Sodium metaborate is used for many practical purposes in technology and agriculture. One major use is the manufacture of borosilicate glasses. These glasses are special because they have a small coefficient of thermal expansion. This means they do not expand or contract much when temperature changes. Because of this, they resist breaking from uneven or fast heating. Sodium metaborate is also used in the composition of herbicides to manage weeds. Additionally, it is used to raise the pH of fluids during oil extraction.

Understanding the boiling points of these chemicals helps scientists manage their work. The boiling point of sodium metaborate is 1434 degrees Celsius. This is lower than the boiling point of borax, which is 1575 degrees Celsius. It is also lower than the boiling point of boron oxide, which is 1860 degrees Celsius. When borax boils, it gives off a vapor of sodium metaborate. This vapor contains a small excess of sodium oxide. These specific measurements allow chemists to control how they create and use these materials.

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