Metaboric acid is the name for a family of

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

s with the same

empirical formula In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is th ...

HBO₂. that differ in their

molecular structure Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that determ ...

. They are colourless water-soluble solids formed by the

dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...

decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...

boric acid Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolves ...

. Metaboric acid is formally the parent acid of the

metaborate A metaborate is an anion (negative ion) consisting of boron and oxygen, with empirical formula ; or any salt with such anions, such as sodium metaborate, or calcium metaborate . It is one of the boron oxoanions or borates In aqueous solution ...

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

Structure

The main forms of metaboric acid are: * Modification III, or trimeric, with the molecular formula . The molecule has C_3h symmetry, with a six-member ring of alternating boron and oxygen atoms at the core, with groups attached to the borons. The

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

with a sheet-like structure, similar to that of boric acid itself. It is obtained by heating orthoboric acid at 80-100 °C, with loss of water: ::3 → + 3 * Modification II. A polymer with structure similar to modification III, except that the rings are connected and 1/3 of the boron centres are tetrahedral. The molecular formula is therefore The crystal structure is

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

. This form has a higher melting point (201 °C) and density (2.045 g/cm³) It is obtained by heating the trimeric form at 130-140 °C in a sealed ampoule (to prevent dehydration), orthorhombic metaboric acid converts to the monoclinic form (II):

* Cubic form. It is a white solid and is only slightly soluble in water that melts at about 236 °C. It is obtained by heating either modification II or III above 140 °C.

Reactions

When heated above about 170 °C, metaboric acid dehydrates, forming

tetraboric acid Tetraboric acid or pyroboric acid is a chemical compound with empirical formula . It is a colourless water-soluble solid formed by the dehydration or polymerization of boric acid. Tetraboric acid is formally the parent acid of the tetraborate an ...

, also called pyroboric acid (H₂B₄O₇): : 4 HBO₂ → H₂B₄O₇ +

Metaborates

Metaborates are derivatives of BO₂⁻. Like metaboric acid, the metaborates exist with disparate structures. Examples are sodium and potassium metaborates, salts formed by deprotonation of orthorhombic metaboric acid containing the cyclic B₃O₆³⁻ ion and calcium metaborate, Ca(BO₂)₂, which contains the chain polymeric ion (BO₂⁻)_n.

References

Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. . H. J. Becher "Metaboric Acid" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 791. W. H. Zachariasen "The crystal structure of monoclinic metaboric acid" Acta Crystallogr. 1963, vol. 16, pp. 385-389. Gurwinder Kaur, Shagun Kainth, Rohit Kumar, Piyush Sharma and O. P. Pandey (2021): "Reaction kinetics during non-isothermal solid-state synthesis of boron trioxide via boric acid dehydration." ''Reaction Kinetics, Mechanisms and Catalysis'', volume 134, pages 347–359. Siavash Aghili, Masoud Panjepour, and Mahmood Meratian (2018): "Kinetic analysis of formation of boron trioxide from thermal decomposition of boric acid under non-isothermal conditions." ''Journal of Thermal Analysis and Calorimetry'', volume 131, pages 2443–2455. {{DEFAULTSORT:Oxoborinic acid Borates Inorganic polymers