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Boric acid

physical science Maturity 5-7 Next article →

Boric acid, also known as orthoboric acid, is a chemical compound composed of boron, oxygen, and hydrogen.

Boric-acid-unit-cell-3D-balls
Boric-acid-unit-cell-3D-balls

It is often found as a white powder or as colorless crystals. In the natural world, it occurs as a mineral called sassolite. This substance is classified as a weak acid. It is highly versatile and serves as a precursor for many other boron-based compounds. Because of its unique chemical properties, it is used in everything from medicine to heavy industry.

The molecular structure of orthoboric acid is quite specific. In its geometry, three oxygen atoms form a trigonal planar shape around the central boron atom. The bond between the boron and oxygen is 136 pm, while the oxygen-hydrogen bond is 97 pm.

Boric-acid-layer-3D-balls
Boric-acid-layer-3D-balls

There are two known crystalline forms. The most common is the triclinic form, which consists of layers of molecules. These layers are held together by hydrogen bonds. The second form is trigonal, which is more thermodynamically stable but requires special preparation methods. In the triclinic phase, the layers are somewhat offset from one another.

When boric acid is heated, it undergoes a process called pyrolysis. This is a three-step dehydration process where the substance loses water molecules. First, when heated above 140 °C, it yields metaboric acid. If the temperature rises above 180 °C, it loses another water molecule to become tetraboric acid. Finally, if it is heated to approximately 530 °C, it transforms into boron trioxide.

Boric-acid-layer-3D-balls
Boric-acid-layer-3D-balls

This step-by-step breakdown shows how temperature directly controls the chemical state of the boron.

Boric acid also shows interesting behavior when dissolved in water. In an aqueous solution, it partially dissociates to create metaboric acid. This makes the solution mildly acidic. The acidity can be influenced by other substances. For example, adding cis-vicinal diols, such as mannitol or glycerol, significantly increases the acidity. When mannitol is added, the solution's pKa drops from 9 to below 4.

Borate pH
Borate pH

This specific reaction is used in analytical chemistry. Scientists use it to determine the borate content in liquids, such as monitoring water in light-water reactors.

The history of boron use stretches back to the ancient Greeks. They used borates for cleaning and food preservation. However, the specific preparation of orthoboric acid is credited to Wilhelm Homberg. Between 1652 and 1715, Homberg prepared the acid from borax using mineral acids. He originally called it the "sedative salt of Homberg." Today, the term "boric acid" is often used generically to describe any oxyacid of boron, including metaboric and tetraboric acids.

Industrial applications for boric acid are vast and varied. It is a primary ingredient in manufacturing monofilament fiberglass, which reinforces plastics for boats and circuit boards. It is also used in the jewelry industry to prevent oxidation during soldering. In the energy sector, it is used in hydraulic fracturing to control fluid viscosity.

Borate buffer
Borate buffer

Even Silly Putty contains boric acid, which is mixed with polyvinyl alcohol or silicone oil. In electrical fuses, it acts as a de-ionization agent. When a plasma arc occurs, the acid decomposes into water vapor to help interrupt the electrical fault.

While useful, boric acid must be handled with care due to its toxicity. It is generally only poisonous if inhaled or ingested in large amounts. For a 70 kg adult, an oral dose of 350 g could be fatal. Research shows that long-term exposure may cause kidney damage. The European Commission has classified it as a substance that may damage fertility or the unborn child.

Borate buffer
Borate buffer

In medical settings, it is used safely in very dilute forms as an antiseptic for minor burns or as an eye wash.

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