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, 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.