Orthoboric Acid: Definition, Properties, Formula and Structure

Orthoboric Acid

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:28 PM IST

Orthoboric acid, H₃BO₃, is a weak acid derived from boron—an element naturally available in nature. It usually occurs as a white, crystalline solid or as a powder and easily dissolves in water. This compound has antiseptic, antifungal, and insecticidal properties and is added to various medicines and household products. This further contains one boron atom, three oxygen atoms, and three hydrogen atoms—a triangular planar structure promoted chemically. Even though this is a simple compound, the chemical behavior of orthoboric acid can be considered quite interesting. Orthoboric acid acts as a Lewis acid by accepting electron pairs.

This Story also Contains

Forms and Types of Orthoboric Acids
Applications and Relevance
Some Solved Examples
Summary

Orthoboric Acid

Orthoboric acid, H₃BO₃ is a white crystalline solid, with soapy touch. It is sparingly soluble in water but highly soluble in hot water. It can be prepared by acidifying an aqueous solution of borax.

Na₂ B₄O₇+2HCl+5H₂O→2NaCl+ 4B(OH)₃

It is also formed by the hydrolysis (reaction with water or dilute acid) of most boron compounds (halides, hydrides, etc.). It has a layer structure in which planar BO₃ units are joined by hydrogen bonds as shown in the figure given below.
Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion:

BOH₃+2HOH→[B(OH)]⁻+H₃O⁺

On heating, orthoboric acid above 370K forms metaboric acid, HBO₂which on further heating yields boric oxide, B₂O₃

H₃BO₃→ΔHBO₂→ΔB₂O₃

Orthoboric acid

Forms and Types of Orthoboric Acids

The different types or forms of orthoboric acids have characteristic typical features and applications. The hydrated orthoboric acid type is the most commonly used in domestic and pharmaceutical applications. Because of the high reactivity of boric acid in anhydrous conditions, it finds applications in several industrial processes, including the manufacture of glass and metallurgy applications. The fact that boric acid could exist in complex compounds as borates with other elements led to their formation—these have applications in ceramics, and detergents as well as preservatives. These forms, thus underpin the complex behavior of orthoboric acid and its applications, which cut across a wide spectrum.

Applications and Relevance

Orthoboric acid finds very broad and significant practical applications. In everyday life, it is used in antiseptics and eyewashes, and it is also a mild pesticide. It is also effective against fungal diseases, as it is greatly effective against such problems as athlete's foot and yeast infections. For industrial purposes, the strength and heat resistance of orthoboric acid in specialized glass products increase quality, such as fiberglass and ceramics, are greatly improved by its use in their production. It finds an application in chemistry as a forerunner for the synthesis of other boron compounds and as a catalyst for some organic reactions.

Some Solved Examples

Example 1

Question: Boric acid is polymeric due to:

Its acidic nature
The presence of hydrogen bond
Its monobasic nature
Its geometry

Solution: As we have learned, boric acid has a layer structure in which planar H₃BO₃ units are linked by hydrogen bonds. Therefore, the correct answer is option (2).

orthoboric acid

Example 2

Question: When orthoboric acid is heated to red heat, the residue is:

Boron
Boric oxide
Metaboric acid
Polyboric acid

NEET Highest Scoring Chapters & Topics

This ebook serves as a valuable study guide for NEET exams, specifically designed to assist students in light of recent changes and the removal of certain topics from the NEET exam.

Download EBook

Solution: Orthoboric acid, H₃BO₃, when heated above 370K, forms metaboric acid (HBO₂). On further heating, it yields boric oxide (B₂O₃). Therefore, the correct answer is option (2).

Example 3

Question: Orthoboric acid is used as:

It is used in making optical and hard glass
It is used as an antiseptic
It is used as a food preservative
All of the above

Solution: Orthoboric acid, H₃BO₃, is used in making optical and hard glass, as an antiseptic, and as a food preservative. Therefore, the correct answer is option (4).

Summary

Orthoboric acid is a versatile compound that finds wide applications both in everyday life and specialized industries. From the simplest and most fundamental properties and forms to practical and academic relevance, orthoboric acid presents a multifaceted compound and is important. It has a stringent place among medical treatments, household products, and industrial processes—the epitome of utility and adaptability. The characteristics and applications of orthoboric acid highlight information on the integral place this compound holds within modern science and technology.

Frequently Asked Questions (FAQs)

1. What is orthoboric acid?

Orthoboric acid is also boric acid and holds the chemical name H₃BO₃. It is a weak acid derived from boron. At room temperature, it appears like a more or less white odorless crystalline solid or powder. It bears antiseptic, antifungal, and insecticidal properties.

2. How does orthoboric acid find an application in household products?

It is contained in a number of household materials: antiseptics, eyewash solutions, and insecticides. It contains antifungal and antimicrobial activities; hence, it finds the same applications in minor cautery for wounds and eye irritation and killing unwanted pests like ants and cockroaches.

3. Industrial application of orthoboric acid

This chemical is quite important industrially in the manufacture of fiberglass, ceramics, and glass products as it improves their strength and promotes resistance to thermal shock. This chemical is further used as a precursor for other boron compounds and as a catalyst in organic chemical reactions.

4. Can orthoboric acid be harmful?

Although generally safe, this acid can be dangerous upon ingestion or handling and therefore should be kept out of children's and pets' reach. As such, the precautions against this element need to be carried out at all times when dealing with it.

5. What makes orthoboric acid rather peculiar in research in the domain of chemistry?

One of the very unique properties of orthoboric acid is its ability to act like a Lewis acid by forming complex compounds. Thus, it has a significant role in chemical research for developing new materials or studying reaction mechanisms and seeking some technological applications.

6. What are the main uses of orthoboric acid?

Orthoboric acid has various applications, including:

7. What is the significance of orthoboric acid in plant nutrition?

Orthoboric acid is an important micronutrient for plants. It's essential for cell wall formation, fruit and seed development, flower formation, and pollen tube growth. Plants absorb boron primarily in the form of undissociated orthoboric acid.

8. How does orthoboric acid act as a buffer solution?

Orthoboric acid can act as a buffer in combination with its conjugate base, the borate ion. The buffer system works in the pH range of 8.5 to 10, maintaining a relatively stable pH when small amounts of acid or base are added to the solution.

9. Why does orthoboric acid form esters more readily than typical acids?

Orthoboric acid forms esters more readily because it acts as a Lewis acid, accepting electron pairs from alcohols. This is different from typical carboxylic acids, which form esters through proton transfer. The boron atom's empty p-orbital facilitates this process.

10. What is the role of orthoboric acid in the boron cycle?

Orthoboric acid plays a crucial role in the boron cycle, which describes the movement of boron through the environment. It's the primary form of boron in most soils and natural waters. Plants absorb it for growth, and it eventually returns to the soil through decomposition.

11. What is orthoboric acid, and why is it called "ortho"?

Orthoboric acid, also known as boric acid, is a weak acid with the chemical formula H3BO3. It's called "ortho" because it's the most common and stable form of boric acid. The prefix "ortho" indicates that all three hydroxyl groups (-OH) are directly attached to the central boron atom.

12. How does the structure of orthoboric acid contribute to its properties?

Orthoboric acid has a planar triangular structure with the boron atom at the center and three hydroxyl groups attached. This structure allows for hydrogen bonding between molecules, which contributes to its relatively low melting point and its ability to act as a Lewis acid.

13. Why is orthoboric acid considered a Lewis acid rather than a Brønsted-Lowry acid?

Orthoboric acid is primarily a Lewis acid because it accepts electron pairs from water molecules to form the borate ion [B(OH)4]−. It doesn't directly donate protons like a typical Brønsted-Lowry acid. Instead, it forms a complex with water, which then releases a proton.

14. How does orthoboric acid behave in water?

In water, orthoboric acid acts as a very weak acid. It doesn't dissociate directly but forms a complex with water molecules. This complex then releases a proton, resulting in a slightly acidic solution. The reaction can be represented as: B(OH)3 + 2H2O ⇌ [B(OH)4]− + H3O+

15. What is the difference between orthoboric acid and metaboric acid?

Orthoboric acid (H3BO3) has three hydroxyl groups attached to the boron atom, while metaboric acid (HBO2) has only one. Metaboric acid is formed when orthoboric acid is heated, causing it to lose water: 2H3BO3 → 2HBO2 + 2H2O

16. What is the environmental impact of orthoboric acid?

While orthoboric acid is naturally occurring and essential for plant life in small amounts, high concentrations can be harmful to ecosystems. It can accumulate in soil and water, potentially affecting plant growth and aquatic life. Proper management of industrial boric acid use is important to minimize environmental impact.

17. How does orthoboric acid affect soil pH?

Orthoboric acid is a weak acid and generally has a minimal direct effect on soil pH. However, long-term application of boric acid or boron-containing fertilizers can gradually lower soil pH. This effect is usually less pronounced than with other common fertilizers like ammonium sulfate.

18. How does orthoboric acid interact with amino acids?

Orthoboric acid can form complexes with certain amino acids, particularly those with hydroxyl groups in their side chains (like serine and threonine). These interactions can affect the properties and behavior of proteins and are sometimes used in protein separation techniques.

19. How is orthoboric acid produced industrially?

Orthoboric acid is typically produced by reacting borax (sodium tetraborate) with a strong acid like hydrochloric acid. The reaction can be represented as: Na2B4O7·10H2O + 2HCl → 4H3BO3 + 2NaCl + 5H2O

20. How does the acidity of orthoboric acid compare to other common acids?

Orthoboric acid is a very weak acid compared to common inorganic acids. Its pKa value is about 9.24, which is much higher than strong acids like hydrochloric acid (pKa ≈ -7) or even weak acids like acetic acid (pKa ≈ 4.76). This means it dissociates very little in water.

21. How does orthoboric acid interact with polyols like glycerol?

Orthoboric acid forms complex esters with polyols like glycerol. These reactions involve the boron atom coordinating with multiple hydroxyl groups from the polyol, forming cyclic esters. This property is used in qualitative tests for polyols and in some industrial applications.

22. How does the solubility of orthoboric acid change with temperature?

The solubility of orthoboric acid increases significantly with temperature. At room temperature, its solubility is moderate (about 5.7g per 100mL of water at 20°C), but it increases to about 27.6g per 100mL of water at 100°C. This property is useful in purification processes.

23. Why is orthoboric acid sometimes used as a food preservative?

Orthoboric acid has mild antiseptic and antifungal properties, which make it effective as a food preservative. It inhibits the growth of microorganisms without significantly affecting the taste or appearance of food. However, its use as a food additive is regulated due to potential health concerns.

24. How does orthoboric acid contribute to flame retardancy?

When heated, orthoboric acid decomposes to form metaboric acid and eventually boron oxide. This process absorbs heat and releases water vapor, which helps to suppress flames. The resulting boron oxide also forms a protective glassy layer on surfaces, further inhibiting combustion.

25. What is the relationship between orthoboric acid and borax?

Borax (sodium tetraborate decahydrate, Na2B4O7·10H2O) is a salt that can be converted to orthoboric acid through reaction with a strong acid. Conversely, orthoboric acid can be used to produce borax by reaction with sodium hydroxide. They are both important compounds in the industrial chemistry of boron.

26. How does orthoboric acid behave in non-aqueous solvents?

In non-aqueous solvents, orthoboric acid often behaves differently than in water. For example, in anhydrous acetic acid, it acts more like a traditional Brønsted-Lowry acid, directly donating protons. Its behavior depends on the specific solvent and its ability to accept or donate electron pairs.

27. What is the role of orthoboric acid in nuclear reactors?

In nuclear reactors, orthoboric acid is used as a neutron poison. The boron-10 isotope in boric acid has a high neutron capture cross-section, meaning it readily absorbs neutrons. This property is used to control the rate of nuclear fission in pressurized water reactors.

28. How does orthoboric acid interact with proteins?

Orthoboric acid can form complexes with certain proteins, particularly those with adjacent hydroxyl groups. This interaction can affect enzyme activity and protein structure. In some cases, this property is used in protein purification techniques or in studying protein structure.

29. How does orthoboric acid contribute to glass formation?

In glass production, orthoboric acid (or its anhydride, boron oxide) acts as a glass former and a fluxing agent. It lowers the melting point of the glass mixture, reduces thermal expansion, and increases chemical resistance. These properties make boron-containing glasses useful for laboratory glassware and heat-resistant cookware.

30. What is the role of orthoboric acid in organic synthesis?

In organic synthesis, orthoboric acid and its derivatives are used as catalysts or reagents in various reactions. For example, it's used in the synthesis of boronic acids and esters, which are important intermediates in organic chemistry, particularly in Suzuki coupling reactions.

31. How does the pH of a solution affect the form of boric acid present?

The form of boric acid present in solution depends on the pH:

32. What is the connection between orthoboric acid and boron neutron capture therapy?

Boron neutron capture therapy (BNCT) is an experimental cancer treatment that uses compounds derived from boric acid. These compounds concentrate in tumor cells and are then irradiated with neutrons. The boron-10 in these compounds captures neutrons and undergoes nuclear fission, releasing high-energy particles that destroy the tumor cells.

33. What is the significance of orthoboric acid in the production of boron fibers?

Orthoboric acid is a precursor in the production of boron fibers. These fibers are made by chemical vapor deposition of boron onto a tungsten or carbon substrate. The process often starts with the thermal decomposition of boric acid to produce boron oxide, which is then reduced to elemental boron.

34. How does orthoboric acid interact with carbohydrates?

Orthoboric acid can form complexes with carbohydrates, particularly those with cis-diol groups. This interaction is the basis for some methods of carbohydrate analysis and separation. The complexation can also affect the properties of carbohydrate-containing solutions, such as their viscosity.

35. What is the role of orthoboric acid in the production of boron carbide?

Orthoboric acid is a key starting material in the production of boron carbide (B4C), one of the hardest materials known. The process typically involves the reduction of boric acid with carbon at high temperatures: 4H3BO3 + 7C → B4C + 6CO + 6H2O

36. How does orthoboric acid behave at high temperatures?

When heated, orthoboric acid undergoes a series of dehydration reactions:

37. What is the relationship between orthoboric acid and boron trifluoride?

Boron trifluoride (BF3) can be produced from orthoboric acid through a series of reactions. First, boric acid is converted to boron oxide, which is then reacted with calcium fluoride and sulfuric acid to produce BF3. Both compounds are important in boron chemistry, with BF3 being a stronger Lewis acid.

38. How does orthoboric acid affect the growth of algae?

Orthoboric acid, in appropriate concentrations, is essential for algal growth as boron is a micronutrient. However, excessive levels can be toxic to algae. Some species of algae are more sensitive to boron than others, making boric acid concentration an important factor in aquatic ecosystem management.

39. What is the role of orthoboric acid in the production of sodium borohydride?

Orthoboric acid is a starting material in the production of sodium borohydride (NaBH4), a powerful reducing agent. The process involves the reaction of sodium hydride with trimethyl borate (derived from boric acid) at high temperatures: 4NaH + B(OCH3)3 → NaBH4 + 3NaOCH3

40. How does orthoboric acid contribute to the fireproofing of cellulose materials?

Orthoboric acid is used to fireproof cellulose materials like cotton and wood. When heated, it decomposes to form a glassy layer of boron oxide on the surface of the material. This layer excludes oxygen and prevents the spread of fire. Additionally, the water released during boric acid decomposition helps to cool the material.

41. What is the significance of orthoboric acid in the production of boron nitride?

Orthoboric acid is a precursor in the production of boron nitride (BN). The process typically involves reacting boric acid or its oxide with ammonia or urea at high temperatures. Boron nitride is valuable for its high thermal conductivity and electrical insulation properties.

42. What is the role of orthoboric acid in the production of boranes?

Orthoboric acid is a starting point for the production of various borane compounds. For example, diborane (B2H6) can be produced by reacting boric oxide (derived from boric acid) with magnesium at high temperatures. Boranes are important in organic synthesis and as potential hydrogen storage materials.

43. How does orthoboric acid affect the properties of polymers?

Orthoboric acid and its derivatives can be used as cross-linking agents in some polymers, improving their mechanical and thermal properties. For example, in polyvinyl alcohol, boric acid can form reversible crosslinks, creating a material with interesting rheological properties.

44. What is the significance of orthoboric acid in geochemistry?

In geochemistry, orthoboric acid is important as the primary form of boron in most natural waters and many minerals. Its concentration and isotopic composition in various geological materials can provide information about past environmental conditions, volcanic activity, and tectonic processes.

45. How does orthoboric acid contribute to the production of boron-doped semiconductors?

Orthoboric acid can be used as a boron source in the production of boron-doped semiconductors. When heated, it decomposes to boron oxide, which can then be reduced to elemental boron. This boron is incorporated into the semiconductor crystal structure, altering its electrical properties.

46. What is the role of orthoboric acid in the wine industry?

In the wine industry, orthoboric acid has been used (though it's now restricted in many regions) as a preservative due to its antimicrobial properties. It can inhibit the growth of yeasts and bacteria that cause wine spoilage. However, its use is controversial due to potential health concerns.

47. How does orthoboric acid interact with transition metal complexes?

Orthoboric acid can act as a ligand in some transition metal complexes. It typically coordinates through its oxygen atoms, forming borate esters. These complexes can have interesting catalytic properties and are studied for their potential applications in organic synthesis.

48. What is the significance of orthoboric acid in the production of optical fibers?

Orthoboric acid, or more commonly its anhydride boron oxide, is used as a dopant in the production of optical fibers. It alters the refractive index of the glass, which is crucial for controlling the propagation of light through the fiber. Boron-doped silica is often used in the cladding of optical fibers.

49. How does orthoboric acid affect the properties of cement and concrete?

Small amounts of orthoboric acid can be used to retard the setting time of cement and concrete. It interferes with the hydration reactions of cement minerals, particularly tricalcium silicate. This property is useful when longer working times are needed, especially in hot weather concreting.

50. What is the role of orthoboric acid in the production of rare earth magnets?

Orthoboric acid is used in the production process of some rare earth magnets, particularly neodymium magnets. It's used to form a protective coating on the magnet particles during sintering, preventing oxidation and improving the magnetic properties of the final product.