Orthoboric Acid, also known as boric acid, is a mild Lewis acid of Boron. Orthoboric Acid is found in volcanic areas and various minerals. It appears as a white crystalline solid and has a trigonal planar structure. Orthoboric Acid serves diverse purposes, from being a pesticide and food preservative to having applications in medicine and the manufacturing industry.
This article provides an overview of its properties, molecular structure, preparation methods, reactions and performance as compared to borax. It also delves into the uses of orthoboric acid, emphasizing its role as a generally safe pesticide while highlighting precautions due to its potential toxicity.
What is Orthoboric Acid?
Orthoboric acid, popularly known as boric acid is a mild, odorless monobasic Lewis acid of Boron. It naturally occurs as a powder and is found mainly in its free state in some volcanic areas.
Orthoboric Acid is also unearthed as a constituent of many naturally occurring minerals like borax, boracite, sassolite, and ulexite. Orthoboric acid and its salts are also found in seawater and many different types of fruits. It is used as a safe alternative to chemical insecticides for pest control. Orthoboric acid yields borate anions, and salts, and can react with alcohols to form borate esters.
Molecular Formula of Orthoboric Acid
Molecular formula of orthoboric acid is BH3O3 also written as B(OH)3.
Structure of Orthoboric Acid
The chemical formula of orthoboric acid is BH3O3. It has a trigonal structure. In this configuration, three oxygen atoms adopt a trigonal planar arrangement around the boron. The B-O bond length measures 136 pm, while the O-H bond is 97 pm.
Two crystalline forms of orthoboric acid exist: triclinic and hexagonal. Crystalline boric acid comprises layers of B(OH)3 molecules, interconnected by hydrogen bonds with a length of 272 pm. The separation between two consecutive layers measures 318 pm. The hexagonal form is thermodynamically more stable and can be obtained with a special preparation method.
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Triclinic and Hexagonal Forms of Orthoboric Acid.
Properties of Orthoboric Acid
The properties of Orthoboric Acid is mentioned below
- It has a white crystalline form.
- It has a feeling of soft soapy touch.
- It is sparingly soluble in cold water but highly soluble in hot water.
- It tends to behave as a very weak acid.
- It dehydrates when heated above 170°C and forms metaboric acid.
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Trihydroxidoboron
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Boric Acid
Hydrogen Orthoborate
Boracic Acid
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BH3O3
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61.83 g/mol
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1.435 g/cm3
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White Crystalline Solid
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Trigonal Planar
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Zero
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170.9 °C (339.6 °F; 444.0 K)
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300 °C (572 °F; 573 K)
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Soluble in water, lower alcohols, acetone and pyridine
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Preparation of Orthoboric Acid
Wilhelm Homberg was the first one to extract and synthesize Orthoboric Acid. Utilizing borax and mineral acids, he synthesized the compound. Interestingly, historical records reveal that ancient Greeks employed borates and boric acid, primarily for food preservation and cleaning purposes.
There are two methods for preparing orthoboric acid:
Using Borax: A hot concentrated solution of borax is treated with sulphuric acid or hydrochloric acid. Crystals of boric acid are formed as a result of the reaction:
Na2B4O7.10H2O + 2HCL → 4B(0H)3 + 2NaCl + 5H2O
Hydrolysis of Boron Compounds: Hydrolysis is the process of reacting hydrides, halides and nitrides with water:
B2H6 + 6H20 → 2B(OH)3 + 6H2
BX3 + 3H2O → B(OH)3 + 3HX (X = Cl, Br, I)
Orthoboric Acid Reactions
Orthoboric Acid undergoes following Recations
- Pyrolysis
- Reaction with water
- Esterification
These reactions are discussed below:
Pyrolysis
Orthoboric acid undergoes three step dehydration when heated. The pyrolysis of Orthoboric acid at different temperature is given below:
Orthoboric Acid at 140°C
When orthoboric acid is heated above 140°C, it undergoes the first step of dehydration, resulting in the formation of metaboric acid (HBO2) and the release of one water molecule:
B(OH)3 → HBO2 + H2O
The reaction shows that one molecule of orthoboric acid transforms into one molecule of metaboric acid and one molecule of water.
Metaboric Acid at 180°C
Heating metaboric acid above 180°C triggers the second dehydration step, leading to the formation of tetraboric acid, also known as pyroboric acid (H2B4O7). Additionally, one water molecule is released in this step:
4 HBO2 → H2B4O7 + H2O
This reaction indicates that four molecules of metaboric acid yield one molecule of tetraboric acid and one molecule of water.
Tetraboric Acid at 530°C
Further heating of tetraboric acid to about 530°C results in the third and final step of dehydration, leading to the formation of boron trioxide (B2O3) and another water molecule:
H2B4O7 → 2 B2O3 + H2O
The reaction shows that one molecule of tetraboric acid transforms into two molecules of boron trioxide and one molecule of water.
Acqueous Solution
Orthoboric Acid when dissolved in water undegergoes three different types of reactions which are mentioned below:
Partial Dissociation of Orthoboric Acid
In this reaction, orthoboric acid partially dissociates to yield metaboric acid and water. The equilibrium sign (⇌) indicates that the reaction can proceed in both directions, with the forward and reverse reactions occurring simultaneously.
B(OH)3 ⇌ HBO2 + H2O
Ionization of Orthoboric Acid in Water
The reaction shows that orthoboric acid reacts with water to produce hydrated borate ions [BO(OH)2]– and hydronium ions H3O–. This contributes to the mild acidity of the solution.
B(OH)3 + H2O ⇌ [BO(OH)2]− + H3O+
Ionization of Metaboric Acid in Water
Metaboric acid undergoes ionization in water, forming borate ions [BO2]– and hydronium ions in water.
HBO2 + H2O ⇌ [BO2]− + H3O+
Esterification
Orthoboric acid reacts with alcohols to form borate esters, B(OR)3 where R is an alkyl or aryl. The reaction is performed in the presence of a dehydrating agent.
B(OH)3 + 3 ROH → B(OR)3 + 3 H2O
Sulphuric Acid Solution
Orthoboric acid also dissolves in anhydrous sulfuric acid:
B(OH)3 + 6 H2SO4 → [B(SO4H)4]− + 2 [HSO4]− + 3 H3O+
Orthoboric Acid vs Borax
Borax (Na2[B4O5(OH)4]·8H2O) is often confused with boric acid, but they are different formulations of the same compound. Both of them are known as borates. Borax is a mineral found naturally and is used as a cleaning agent whereas is extracted, processed and refined to form a powder that is mostly used in pest control.
Both the products look similar in appearance, but there are many differences as mentioned:
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It is comparatively less toxic.
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Borax is highly corrosive, toxic and corrosive to eyes.
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It’s molecular formula is BH3O3.
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It’s molecular formula is Na2[B4O5(OH)4].8H2O.
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It is also known as orthoboric acid, Hydrogen Orthoborate, boracic acid or Trihydroxidoboron.
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It is also known as sodium borate, sodium tetra-borate or disodium tetra-borate.
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It can be procured from borax by reacting it with hydrochloric acid.
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It is mined out of the ground.
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Due to the procedure involved for its production boric acid is costly.
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Borax is cheaper than boric acid.
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Boric acid has a higher concentration of boron (17.48%). Farmers may chose boric acid over boron if they want to increase boron levels in the soil.
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It has lesser concentration of boron (11.34%).
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It is produced as white crystals and is easier to work with.
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It is pretty rough as it is mined as a rock and has to be broken down to form a powder.
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It doesn’t contain sodium and hence is a better alternative for farmers.
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Borax contains sodium which increases salinity of soil and water which may affect plants and wildlife.
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Boric acid does not become rock hard over time.
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Borax contains 8 molecules of water and overtime it may lose these molecules and dry out and becomes rock hard. It leads to the concentration of other elements increasing.
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Read more about Borax.
Uses of Orthoboric Acid
The uses of orthoboric acid are mentioned below:
- It has been extensively used as a pesticide in pest control.
- It can be used as a preservative of milk and other food items.
- It can be used as an antiseptic in the medical field.
- It is used in the manufacture of enamels and glazes in pottery.
- It is used as a neutron poison in nuclear power plants.
- It can be used to treat boron deficiencies in plants.
- It can be used in the treatment of fungal and insect attacks along with wood.
- It is used in electroplating as part of some proprietary formulae.
- It is used in the manufacture of monofilament fibreglass or heat-resistant boron silicate glasses.
- It is also used in the production of glass in LCD flat panel displays.
Toxic Effect of Orthoboric Acid
Orthoboric acid powder serves as a generally safe and naturally existing pesticide. Nonetheless, it holds the potential for toxicity in specific amounts, posing a threat to both humans and animals. The substance can be harmful when ingested or inhaled in substantial quantities. Therefore, it is advised to handle orthoboric acid with care, and the use of protective gear such as gloves, a protective mask, safety goggles, and long-sleeved clothing is recommended.
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Orthoboric Acid – FAQs
1. Why is Orthoboric Acid weak?
Orthoboric Acid is classified as a weak acid since it cannot produce its own hydrogen ions. The boric acid takes hydroxide ions from a water molecule and releases ions.
2. What is the Difference between Orthoboric Acid and Metaboric Acid?
These acids differ mostly in their molecular formula and structure. Orthoboric acid has a trigonal planar structure and is only present in molecular form, whereas metaboric acid is present in two forms: molecular and chained. On heating orthoboric acid dehydrates to form metaboric acid.
3. Why Orthoboric Acid is not Protonic Acid?
Orthoboric Acid does not donate hydrogen ions (H+) so is considered not a protonic acid but it can accept electrons from so it is a Lewis Acid and form complexes with water molecules.
4. What is the Formula of Orthoboric Acid?
The molecular formula of orthoboric acid is BH3O3 also written as B(OH)3.
5. Does Orthoboric Acid kill Fleas?
Yes, Orthoboric acid can kill fleas.
6. What happens when Orthoboric Acid is heated?
Orthoboric acid undergoes three step hydration when heated, which is known as pyrolysis. When heated above 140°C orthoboric acid yields metaboric acid (HBO2) and water. Heating metaboric acid above 180°C eliminates another water molecule forming tetraboric acid, also called pyroboric acid (H2B4O7). Further heating to about 530°C leads to boron trioxide (B2O3).
7. Does Orthoboric Acid kill Bed Bugs?
As boric acid appears as a powder or bust, bed bugs will not ingest it as they feed on blood. Hence this is not effective against bed bugs.
8. How to Use Orthoboric Acid?
Orthoboric Acid should be applied on a dry surface only. Make a dilute solution of orthoboric acid to use it.
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