Boric acid is a monobasic and weak Lewis acid of Boron. It is an inorganic compound with the chemical formula H3BO3. It is known by multiple names such as boracic acid, hydrogen borate, and orthoboric acid.
It can exist as colorless crystals or as a powder, which is soluble in water. It is known for its antiviral, antifungal, and antiseptic properties. Boric acid is also found naturally in vegetables, grains, fruits and nuts. It is not toxic in small amounts that occur naturally.
Chemical Structure of Boric Acid
Boric acid is made of oxygen, boron and hydrogen atoms (H3BO3). Each molecule of boric acid has boron-oxygen single bonds. The boron is located at the center and is bonded to three hydroxide groups. Its molecular geometry is the trigonal planar.
Boric acid crystals were made for the first time by Wilhelm Homberg in 1702. He prepared it by mixing borax and minerals acids with water. The water gets evaporated and crystals of boric acid are left behind, which were called "Homberg's salt'.
Physical Properties of Boric Acid
- In crystal form, its crystals are white and odorless.
- In powdered form, it appears as baby powder, whereas, in granular form, it looks like fine table salt.
- Its solubility increases with an increase in temperature, for example, at 25 degrees Celsius, its solubility is 57 gm/liter and at 100 degrees Celsius, it is 275 gm/liter.
- Its conjugate base compound is borate anion.
- Its boiling point is 300 degrees Celsius.
- Its melting point is 170 degrees Celsius.
- Its density is 1.435 gm per cubic cm.
- Its molar mass is 61.83 g/mol.
Chemical Properties of Boric Acid
- Its molar mass is 61.83 gm/mole.
- Its molecule has boron-oxygen single bonds.
When boric acid is heated or at a temperature of 170 degrees Celsius, it produces metaboric acid as shown below;
H3BO3 → HBO2 + H2O
However, at 300 degrees Celsius, the product formed above gives tetraboric acid. The reaction occurs as follows;
4HBO2 → H2B4O7 + H2O
When the product formed above is heated above 330 degrees Celsius, it produces boron trioxide, as shown below;
H2B4O7 → 2B2O3 + H2O
It gives borate esters on reacting with alcohols as shown below;
B(OH)3 + 3ROH → B(OR)3 + 3H2O
It reacts with anhydrous sulphuric acid as shown below;
B (OH)3 + 6 H2SO4 → B(HSO4)4 + 2HSO4 + 3H3O+
Uses of Boric Acid
- It acts as an effective pesticide. It can kill cockroaches, rats, flies very easily.
- It is used in swimming pools to maintain the pH of water, prevent algae formation and clear off fungi in the water.
- It can be mixed with regular detergent to remove tough stains on clothes.
- It can also be used to remove dirt, stain and odor from bathrooms, kitchens, etc.
- It has flame-retardant properties owing to this it is widely used in the manufacturing of furniture, mattresses, and insulation. It is also effective in protecting timbers from fungal and insect attacks.
- It is also used in the manufacture of fiberglass, domestic glass products, LCD glass, etc.
- It is used in the leather manufacturing and jewelry industry in combination with denatured alcohol.
- It can be used as a lubricant after mixing with petroleum or vegetable oil. It can also be used to smoothen the surface of carom boards.
- It is also used to fulfill boron deficiency in plants.
Medical Uses of Boric Acid
Boric acid is a natural antiseptic and anti-fungal agent. A solution of boric acid and distilled water can be used for wounds, minor cuts, rashes, acne, etc. It also reduces the itch caused by insect bites.
Its solution in water can treat sore eyes, eye discharge and irritation.
It can also treat vaginal yeast infection or candidiasis.
It also helps cure athlete's foot, ear infections caused by fungi present in the swimming pool.
It is also an ingredient of homeopathic medicine that is used to treat vaginal discharge and itching. The ingredients are vitamin E, and friendly bacteria.
It also helps prevent foul food odor. A mixture of boric acid and talc can be applied to the inner side of footwear to treat smelly feet.
It is also present in the urine sample bottles as a preservative. This is because it helps maintain the quality of the sample while it is shifted to the lab. Its addition reduces the chances of false results.
Preparation of Boric Acid
The simple way of preparing boric acid is by reacting borax with a mineral acid such as hydrochloric acid as shown below;
Na2B4O7.10H2O + 2HCl → 4H3BO3 + 5H2O + 2NaCl
By hydrolysis of diborane:
It is also prepared by the hydrolysis of diborane as shown below;
B2H6 + 6H2O → 2B (OH) 3 + 6H2
By Hydrolysis of Trihalide:
It can also be prepared by the hydrolysis of boron trihalides as shown below;
BX3 + 3H2O → B(OH)3 + 3HX (X=Cl, Br, I)