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Boric acid is a weak monobasic acid that contains three hydrogen molecules, one boron molecule and three oxygen molecules. The chemical formula of the compound is H3BO3 .
- Boric acid has other names, such as hydrogen borate, boracic acid and orthoboric acid.
- It is a Lewis acid that does not act as a proton donor.
- You can use it as an antiseptic, insecticide, flame retardant and neutron absorber.
- Boric acid was first discovered by Wilhelm Homberg from the element borax and mineral acids like hydrochloric acid.
- The compound was given the name sal sedativum Hombergi, which means "sedative salt of Homberg".
- It is a colourless crystal or a white powder that has antifungal and antiviral properties.
- In this article, we will learn about the chemical properties, structure, reaction, uses, physical properties of boric acid.
Key Terms: Boric Acid, Orthoboric Acid, Proton, Lewis Acid, Structure of Boric Acid, Hydogen Bonding, Borax, Boron Trifluoride, Boron, Oxygen
What is Boric Acid?
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Boric acid is an odourless acid that is soluble in water and found in the nature as the mineral sassolite. The compound is found in many naturally occurring minerals such as borax, boracite, boronatrocalcite and colemanite.
- The compound is also popular with the name acidum boricum.
- The term boric acid is used for type of oxoacid of boron, such as metaboric acid (HBO2) and tetraboric acid (H2B4O7).
- It is most commonly used in cosmetics and pharmaceutical products.
- When the compound reacts with alcohol, it forms the compound borate ester.
- You will be surprised to know that Greeks use boric acid for preserving food and cleaning purposes.
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| Chapter Related Concepts | ||
|---|---|---|
| Sodium chloride | Elimination Reaction | Sulfates |
| Stereochemistry | Sodium peroxide. | Carbonates |
Structure of Boric Acid
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Boric acid forms a two-dimensional layered structure in which three oxygen atoms form a trigonal planar structure with the boron. It is a form of weak acid that releases borate anions linked by hydrogen bonding.
- The compound is an exception because it does not release H+ ions from itself, but it releases H+ ions from water.
- In this structure, boric acid will act as a Lewis acid, and water molecules will act as a Lewis base.
- Hence, the oxygen donates its two electrons or a single lone pair to the boron’s vacant orbital, and they form a coordinate bond.
- When oxygen donates its electrons to boron, it gains a positive charge.
- We know that oxygen is a highly electronegative element, so it would be reluctant to carry a positive charge.
- So, it takes the electrons from hydrogen, and the bond between hydrogen and oxygen is broken, and as a result of this, H+ is released.
- Hence, the basicity of boric acid will be ‘one’.
Boric Acid Structure
Properties of Boric Acid
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The physical and chemical properties of boric acid are as follows:
Physical Properties of Boric Acid
The physical properties of compound are as follows:
- Colour: Boric acid is either white or colourless powder.
- Odour: It is an odourless compound.
- Taste: The compound is acidic in nature and is bitter in taste.
- Texture: It is a crystalline powder or has hexagonal crystals
- Solubility: Boric acid is soluble in water, alcohol and glycerine.
- Density: Its density is 1.435 grams per cubic cm.
- Melting and Boiling Point: The melting and boiling point of boric acid is 170.9 °C (339.6 °F; 444.0 K) and 300 °C (572 °F; 573 K) respectively.
The physical properties are tabulated below:
| Overview | |
|---|---|
| Chemical formula | H3BO3 |
| Molar mass | 61.83 g·mol−1 |
| Density | 1.435 g/cm3 |
| Appearance | White crystalline solid |
| Melting point | 170.9 °C |
| Boiling point | 300 °C |
| Solubility in water | 2.52 g/100 mL (0 °C) |
| Molecular shape | Trigonal planar |
| Dipole moment | Zero |
Chemical Properties of Boric Acid
The chemical properties are as follows:
- Boric acid is a weak monobasic acid that accepts OH- ions rather than donating H+.
- Reacts with NaOH to form sodium metaborate and tetraborate.
- The molecular geometry of boric acid is trigonal planar.
- The boron atom occupies the central position in the compound and is then linked to three hydroxide groups.
Preparation of Boric Acid
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There are two ways to prepare boric acid, which are as follows:
Using Borax
Boric acid is prepared by using a hot concentrated solution of borax (Na2B4O7), which is treated with H2SO4 or HCl. After decomposition, a hot liquid is filtered out and crystals of boric acid are obtained.
Na2B4O7 + H2SO4 + 5H2O ----→ 4H3BO3 + Na2SO4
Na2B4O7 + 2HCl + 5H2O ----→ 4H3BO3 + 2NaCl
Hydrolysis
The process involves hydrolysis of boron compounds such as halides, hydrides, and nitrides. The reactions are as follows:
B2H6 + 6 H2O → 2 B(OH)3 + 6 H2
BX3 + 3 H2O → B(OH)3 + 3 HX (X = Cl, Br, I)
Uses of Boric acid
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Various orthoboric acid uses are as follows:
- Boric acid is a topical agent which is used as an antimicrobial agent.
- It can be used as an antiseptic for minor cuts or burns.
- The compound acts as a weak bacteriostatic agent, so it is used as a local anti-infective.
- 2.5 – 4.5% solution of the compound is used in the formation of eyewash and antiseptic ointment for diaper rashes.
- Boric acid is used to prepare bar glycerin glycerite.
- It is used as welding flux and helps in the process of electroplating.
- Boric acid is used in creams that are meant for the application to mucus membranes, skin or eye.
- The compound is used in the production of monofilament fibreglass and glass in LCD flat panel displays.
- Its usage can be seen in board games like carrom, which is used as a dry lubricant.
- Boric acid is also used in swimming pools to help resist changes in the pH of the water.
Boric Acid Side Effects
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The various effects of boric acid are as follows:
- Boric acid may cause a burning sensation on the skin.
- The compound is toxic to the fetus of the pregnant women.
- It will causes redness in the vaginal area.
- Excess use of the compound may cause diarrhoea and vomiting.
- It can also create a risk to the kidney of an individual, and if left untreated, then it may cause failure of the kidney.
- If someone swallows boric acid, then they might damage the oesophagus, and stomach pain persists for several weeks.
| Chemistry Related Concepts | ||
|---|---|---|
| Ammonia | Silicates | S- Block Elements |
| Oxidizing Agent | Silicon dioxide | The Alkali Metals |
Things to remember
- Boric acid is a weak, acidic hydrate that does not completely ionize in water or any liquid.
- It is either a colourless or white crystalline solid at room temperature.
- The weight of the boric acid molecule is 61.83gm/mole.
- It is used in the preparation of jewellery.
- Boric acid is used for the preservation of grains like rice and wheat.
Sample questions for practice
Ques.Is boric acid a protic acid? Explain? (2 marks)
Ans. Protic acids are the acids ionizes in water to give proton. Boric acid (B[OH3]), in water does not behave as a protic acid as it does not ionize to give proton.
B(OH)3 + 2H2O H2BO2- + H3O+
Rather it behaves as a Lewis acid and a weak monobasic acid.
Ques. Answer: (A) Why boric acid is considered as a weak acid?
(B) What is the result of heating boric acid at 150° C? (2 marks)
Ans. (A) Boric acid is a weak acid because it does not ionize completely in water and it is able to release H+ ions only when it accepts OH- ions from the water molecule.
(B) The result of heating boric acid at 150° is 4H3BO3 ----→ H2B4O7 + 5H2O
Ques. Boric acid can be successfully titrated against sodium hydroxide in the presence of -: (1 mark)
a) Dil. HCl
b) Glycerol
c) Glycine
d) Borax
Ans. Boric acid is a weak acid and sodium hydroxide is a strong base.
Explanation: In presence of glycerol, boric acid acts as a strong acid. Hence, boric acid can be successfully titrated against sodium hydroxide in the presence of glycerol.
Ques. Starting from boric acid, prepare-:
a) Boric anhydride
b) Boron trichloride
c) Boron trifluoride
d) Metaboric acid
e) Ethyl borate (4 marks)
Ans. The reaction are as follows:
- 2H3BO3 red hot B2O3 + 3H2O----→ 2H3BO3 red hot B2O3 + 3H2O
- B2O3 + 3C + 3Cl2 ----→ 2BCl3 + 3CO
- 2H3BO3 + 3CaF2 + 3H2SO4 ----→ 2BF3 (g) + 3CaSO4 + 6H2O
- H3BO3----→ HBO2 + H2O
- B(OH)3 + 3 C2H5OH ----→ (C2H5O)3B + 3 H2O
Ques. How can we neutralize boric acid? (2 marks)
Ans. Boric acid, also known as orthoboric acid, is a weak Lewis acid that is [BO3]3—linked by hydrogen bonding. Neutralization damages the acidic properties of the compound, whether it is an acid or a base. Baking soda and Lime are two inexpensive compounds that can be used to neutralize boric acids.
Ques. How can we use boric acid for skin treatment? (2 marks)
Ans. The only quantity of boric acid that are deemed safe for the skin are those found in moisturizers and skin care products. It is utilized as borax, which is also found in sunscreen. Excessive use of such products can lead to the formation of skin rashes and cause irritation to the applied areas.
Ques. What are the uses of boric acid? (3 marks)
Ans. The uses of boric acid are as follows:
- Household cleaner: Boric acid is a natural disinfectant and can be used to kill cockroaches, ants, and other household pests. It can also be used to remove mold and mildew.
- Industrial applications: The solution is used in a variety of industrial processes, including the manufacture of fiberglass, ceramics, and fire retardants. It is also used as a neutron absorber in nuclear reactors.
- Cosmetics: Boric acid is sometimes used in cosmetics as an astringent or preservative.
Ques. What are the side effects of boric acid? (3 marks)
Ans. Boric acid can be toxic if ingested in large amounts. It is especially dangerous for young children and pets. Symptoms of acid poisoning include nausea, vomiting, diarrhea, and seizures. The compound can also irritate the skin and eyes. It is important to wear gloves and eye protection when handling boric acid. It should not be used as a douche or vaginal suppository. The compound can disrupt the balance of vaginal flora and increase the risk of infection.
Ques. What happen when boric acid is treated with ethyl alcohol? (2 marks)
Ans. When the solution of boric acid is treated with ethyl alcohol, the compound will create a green-coloured flame which results in the formation of ethyl borate. The reaction of boric acid with ethyl alcohol is as follows:
B(OH)3+C2H5OH→B(C2H5O)3+H2O
Ques. What is the difference between boric acid and borax? (3 marks)
Ans. The difference between boric acid and borax are as follows:
| Boric Acid | Borax |
|---|---|
| Boric acid is a form of lewis acid. | Borax is a form of natural mineral. |
| The IUPAC name of the compound is Trihydroxyborane. | The IUPAC name of the compound is sodium tetraborate decahydrate. |
| The molar mass of the boric acid is 61.83gm/mole. | The molar mass of the borax is 381.38 in case the compound is hydrated. |
Ques. What happens when boric acid undergoes pyrolysis? (5 marks)
Ans. When we heat boric acid, it will undergo a three-step dehydration process. When the compound is heated at 140 degrees Celsius, it will yield a compound called metaboric acid (HBO2), which is formed with the loss of one water molecule.
B(OH)3 → HBO2 + H2O
- When we heat metaboric acid above 180 degrees Celsius, it will eliminate another water molecule and form tetraboric acid, which is also called pyroboric acid
4 HBO2 → H2B4O7 + H2O
- Further heating of the solution at a temperature of 530 degrees Celsius which lead to the formation of boron trioxide:
H2B4O7 → 2 B2O3 + H2O
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