What Is Orthoboric Acid?
Orthoboric acid, more commonly known as just boric acid, is a weak, monobasic Lewis acid of boron. Orthoboric acid is an odourless and naturally occurring powder in its original form. This chemical substance is quite popular and used widely as a safe alternative to chemical insecticides for pest control in many parts of the world. This acid is found mainly in its free state in some volcanic areas.
This chemical substance is also found as a constituent of many naturally occurring minerals such as boracite, borax, colemanite and ulexite (boronatrocalcite). Alternatively, orthoboric acid and its salts are found in seawater. It is also present in different types of fruits.
The first synthesis of orthoboric acid was performed by Wilhelm Homberg. He used borax, and with the action of mineral acids, he synthesised the compound. Interestingly, if you look at history, the use of borates and boric acid by the ancient Greeks can be seen. It was used mainly for preserving food and cleaning.
| Overview | |
| IUPAC names | Boric acid
Trihydroxidoboron |
| Chemical formula | BH3O3 |
| Molar mass | 61.83 g·mol−1 |
| Appearance | White crystalline solid |
| Density | 1.435 g/cm3 |
| Melting point | 170.9 °C (339.6 °F; 444.0 K) |
| Boiling point | 300 °C (572 °F; 573 K) |
| Solubility in water | 2.52 g/100 mL (0 °C)
4.72 g/100 mL (20 °C) 5.7 g/100 mL (25 °C) 19.10 g/100 mL (80 °C) 27.53 g/100 mL (100 °C) |
| Solubility in other solvents | Soluble in lower alcohols
Moderately soluble in pyridine Slightly soluble in acetone |
| Molecular shape | Trigonal planar |
| Dipole moment | Zero |
Orthoboric Acid Properties
- Orthoboric acid is a white crystalline solid usually found in powder form.
- It has a feeling of soft soapy touch.
- It is sparingly soluble in cold water but highly soluble in hot water.
- Orthoboric acid tends to behave as a very weak acid.
- It dehydrates when heated above 170 °C and goes on to form metaboric acid.
Structure of Orthoboric Acid
It has a trigonal structure. If we look at the chemical formula of orthoboric acid, it is BH3O3. Here, the three oxygen atoms form a trigonal planar geometry around the boron. The B-O bond length is 136 pm, and the O-H is 97 pm. The molecular point group is C3h.
Crystalline boric acid is made up of layers of B(OH)3 molecules that are held together by hydrogen bonds of length 272 pm. The distance between two adjacent layers is 318 pm.
Preparation of Orthoboric Acid
There are two common methods for the preparation of orthoboric acid.
- Using borax: Orthoboric acid can be obtained by treating a hot concentrated solution of borax with either sulphuric acid or hydrochloric acid. As a result of the solution on concentration, crystals of boric acid are formed.
Na2B4O7·10H2O + 2 HCl → 4 B(OH)3 [or H3BO3] + 2 NaCl + 5 H2O
- Hydrolysis of boron compounds: Another method that can be used is hydrolysis. It can be performed on boron compounds such as hydrides, halides and nitrides.
B2H6 + 6 H2O → 2 B(OH)3 + 6 H2
BX3 + 3 H2O → B(OH)3 + 3 HX (X = Cl, Br, I)
Uses of Orthoboric Acid
Some of the common uses of boric acid are as follows:
- Orthoboric acid is a very important tool in pest control and has been used extensively for a long period of time.
- It is used in the manufacture of monofilament fibreglass or heat-resistant boron silicate glasses. This acid is used in the production of glass in LCD flat panel displays.
- It is used as a preservative for milk and other food items.
- Boric acid is used in the medical field as an antiseptic.
- It is used in the manufacture of enamels and glazes in pottery.
- It is also used in some nuclear power plants as a neutron poison.
- In the treatment or prevention of boron deficiencies in plants.
- Boron is commonly used in pyrotechnics for preventing amide-forming reactions between aluminium and nitrates. Orthoboric acid in small quantities is added to the mixture to neutralise alkaline amides that can react with aluminium. It is also used as a colourant to make fire green.
- External wood can be treated with this acid to prevent fungal and insect attacks.
- Orthoboric acid is used in electroplating as part of some proprietary formulas.
Toxicity of Orthoboric Acid
The common question is: Is orthoboric acid safe? Orthoboric acid powder is a very safe and naturally occurring pesticide. However, it can be toxic in certain doses and has the potential to pose a danger to humans and animals. Orthoboric acid can be poisonous if taken internally or inhaled in large quantities. In any case, it should be handled with precaution, and the use of gloves, a protective mask, protective eyewear and long sleeve clothing is recommended.
Orthoboric Acid
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