What Is Boric Acid?

What is boric acid - bottle of boric acid

Boric acid is a weak, naturally occurring acid. As the name suggests, boric acid has the element boron as its anion component. Its other chemical constituents are hydrogen and oxygen.

In purified crystalline form, boric acid is white, odourless, and almost without any taste. Depending on the refinement process, it either resembles fine table salt or baby talcum powder.

Boric acid powder, boric cream and a boric acid drops bottle
Boric acid in powder, cream, and droplet forms

What are the natural sources of boric acid?

Few naturally occurring acids have the wide range of applications of boric acid. In nature, boric acid is in the form of a mineral called sassolite. It can be found in some volcanic areas like the Italian region of Tuscany. When sassolite mixes with steam from volcanic fissures in the ground, it becomes an acid in aqueous solution.

Boric acid can also be found as a component of other minerals like:

  • Borax
  • Boracite
  • Ulexite
  • Colemanite

Borates is the general name given to minerals that contain compounds of boron. Borates are available in dried salt lake beds of deserts. Death Valley in California, for instance, has a high concentration of borates.

Seawater also contains boric acid and its salts. Many species of plants, particularly their fruits, have boric acid as one of the constituents.

What is boric acid used for?

The German natural philosopher and scientist, Wilhelm Homberg, was the first person to synthesise boric acid crystals in 1702. He achieved this by mixing water with borax and mineral acids. Other European researchers discovered that a certain concentration of boric acid is useful as a mild antiseptic and eyewash.

Refined boric acid crystals and solutions have wide range of practical uses in the following areas:

  • Industrial – the primary industrial application of boric acid is in the manufacture of fibreglass. It is also important in the manufacture of LCD glass and is a constituent of proprietary formulas in electroplating.
  • Medical – as early as the 1700s, European researchers discovered the antiseptic properties of boric acid. As such, it is used as part of salves and wound dressings; and as eyewash in diluted form.
  • Insecticidal – boric acid is an effective insecticide that can kill various domestic insects like cockroaches, termites, fire ants, fleas, and silverfish. 
  • Preservation – the efficacy of the acid in preserving wood is related to its insecticidal property. It can be combined with an ethylene glycol carrier to treat and protect wood against fungi and insects.
  • pH buffer –  together with a conjugate base, the acid’s borate is commonly used as a buffer system for swimming pools in the concentration range of 50 to 100 ppm boron equivalents.
  • Lubrication – when used as a colloidal suspension of nanoparticles dissolved in petroleum oil, boric acid can be a very efficient lubricant for ceramic and metal surfaces. 
  • Nuclear power – the boron in the acid is effective in absorbing some thermal neutrons. This reduces the probability of thermal fission in nuclear power plants.
  • Agriculture – boron is an essential inorganic component of plants. The acid helps in preventing boron deficiencies. It is also used in the preservation of grains like wheat and rice. 

Boric acid chemical name

Boric acid is an inorganic acid known by several names. The two standard IUPAC names of this acid are boric acid and trihydrooxidoboron. The second of these names is more descriptive of the chemical composition of the acid. It says that a molecule of the acid has three hydrogen atoms (tri-hydro), three oxygen atoms (oxido), and one boron atom (boron).

Other names for this acid are: 

  • Hydrogen borate
  • Boracic acid
  • Orthoboric acid

Aside from these chemical names, the acid also has identifiers and related commercial brands. 

The chemical structure of boric acid
The chemical structure and formula of boric acid

Boric acid chemical formula

The chemical formula for boric acid is H3BO3, which means that each molecule of this acid has three hydrogen atoms, one boron atom, and three oxygen atoms. The combination of these atoms adds up to the molecular weight of 61.84 g/mol. 

As a very weak acid with a pKa value of only 9.2, it is classified as a monobasic Lewis acid of Boron. This means that it has an empty orbital that is capable of accepting two electrons from a Lewis base. It can also be classified as a tribasic acid in the Bronsted sense because of its chemical behaviours when reacting with some chemicals.

In terms of molecular structure, the three oxygen atoms form a trigonal planar shape around the boron atom. The hydrogen atoms are attached to the oxygen atoms, forming three –OH groups that are spread evenly at 120-degree angles from each other around the boron atom.

What is boric acid made of?

Boric acid is made of hydrogen, boron, and oxygen. These elements have specific proportions and molecular structure that define boric acid. The acid is naturally occurring and can be purified from natural sources but it can also be synthesised through various chemical reactions.

These are the two main ways that boric acid can be synthesised:

  • Borax (sodium tetraborate decahydrate) reacting with a mineral acid

Na2B4O7·10H2O + 2 HCl → 4 B(OH)3 [or H3BO3] + 2 NaCl + 5 H2O

  • Hydrolysis of boron trihalides and diborane

B2H6 + 6 H2O → 2 B(OH)3 + 6 H2

BX3 + 3 H2O → B(OH)3 + 3 HX (where X = Cl, Br, I)

Is boric acid safe for humans? 

Boric acid is used as an insecticide and pesticide. Hence, it should be stored in safe containers. It can kill various species of insects, arachnid, and fungus. It also has antibacterial properties. In small amounts, it is used in the medical treatment and prevention of infections. It is mildly toxic to humans at a certain amount when ingested.

In the form of borax, it may cause serious irritations to the eyes or skin. When ingested or inhaled, it may cause the following symptoms:

When ingested:

  • Nausea
  • Vomiting
  • Stomach ache
  • Diarrhoea
  • Red, “boiled lobster”-like skin rash
  • Skin loss

When particles of borax are inhaled:

  • Dry mouth, nose, and throat
  • Coughing
  • Sore throat
  • Shortness of breath
  • Nose bleed

Does boric acid dissolve in water?

Boric acid is soluble in boiling water but you need to heat it to above 170°C for it to dehydrate. This results in the formation of metaboric acid:

H3BO3 → HBO2 + H2O

Understanding what boric acid is will involve knowing its solubility in water. When on their own, boric acid and borax are only partially soluble in water but when combined they become highly soluble.

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