Chapter 5th:- Oxyacids of Boron, Phosphorus, and Sulphur 1st year Book
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Oxyacids of Boron

In oxyacids of boron, Borons several oxyacids which are commonly known as boric acids. Though all the acids may be regarded as aquated boric oxides. It involves a different number of water molecules e.g.

Orthoboric acid:              H3BO3      or            B2O3.3H2O

Metaboric acid:               HBO2        or            B2O3.H2O

Pyroboric acid:                H6B4O9     or            2B2O3.3H2O

Tetraboric acid:               H2B4O7     or            2B2O3.H2O

Phase studies of the system B2O3H2O. It shows that orthoboric acid and metaboric acid are stable oxyacids.

  1. Orthoboric acid, H3BO3 or B(OH)3 :


  1. From borax or by treatment of borax with mineral acid (commercial method): Borax can be converted into boric acid by treatment with hydrochloric acid or sulphuric acid.

Boric Acid

(i) By hydrolysis of boron compounds; BX3 where X = H, halogen, OR, NR2:

By the action of super-heated water on BN and B2S3: H3BO3 can be prepared by the action of super-heated water on boron nitride and boron sulfide as:

Boron Nitrate

From colemanite: Most of the world’s supply of boric acid comes from the source. Crystals of boric acid are obtained when SO2 gas is passed into a suspension of the finely powdered mineral in hot water.

Colemanite, boric acid

Physical Properties:

Boric acid is a white flaky crystalline compound that is greasy to the touch.

It is moderately soluble in cold water and the solubility increases with rising temperature. The dissolution is accompanied by the absorption of heat. It volatilizes in steam.

Its boiling point is 170.9 0C and its density is 1.48g cm-3.

Chemical Properties:

The action of heat: when heated, boric acid is first converted by loss of water Meta boric acid and then to glossy, water containing boron trioxide.

Boric Acid

Acidic property: H3BO3 is a very weak monobasic acid that acts as an OH acceptor rather than a proton, H+ It reacts with water to form the tetrarchy corroborate ion; B(OH)4– or H3O+[B(OH)4].


Its acidity is approximately the same as that of hydrogen cyanide (pKa = 9.25). Therefore, the salts of boric acid MH2 BO3.H2O is equal to M[B(OH)4], which may extensively hydrolyze as:

Boric Acid

By the addition of polyhydroxy alcohols e.g. mannitol: H3BO3 can be converted into complex acids which are approximately as strong as CH3. This complex acid so formed gives H+ which makes H3BO3 behave as an acid.BSCBOOKS

The action of Alkali: H3BO3 reacts with the aqueous solution of NaOH or KOH to give a series of polymeric metal borates. As shown In the following equations.

Polymeric Meta Borates

The action of soda ash Na2CO3: When H3BO3 Is neutralized by Na2CO3 solution, borax is formed.

Action of Soda Ash

The action of HF: When H3BO3 Is dissolved in hydrofluoric acid, (HF) fluoro boric acid is formed.

Action of Hydro Fluoric Acid

The action of alcohol in presence of conc. H2SO4: When boric acid or its salts are heated with alcohol in presence of conc. H2SO4, esters are formed.

Action of Alcohols

Action with CaF2 and cone H2SO4: H3BO3 when heated with CaF2 in presence of cons. H2SO4 volatile boron trifluoride, BF3 is formed which burns with green flame.

Action of Volatile Boron Trifluoride

This reaction can also be used for the Identification of borate (BO3)3- radical in inorganic analysis.


Orthoboric acid has a layered structure. It consists of two-dimensional layers 3.181 Ǻ apart. Since boric acid is polar, intermolecular hydrogen bonding occurs in This results in H3BO3 existing in the most stable polymeric [B(OH)3]x and has a ring structure. In which the O—H bond distance is 0.88 Ǻ, the H𝛿+…….O𝛿- distance is 1.84 Ǻ. The B—O distance is 1.361 Ǻ which is intermediate between the 1.47 Ǻ and 1.27 Ǻ calculated for single and double bonds respectively.Oxyacids of Boron


Boric acid is used as a mild antiseptic and as an eye wash (eye-lotion) in medicines. “The disinfectant effect is probably due to the formation of a biologically inactive complex between boric acid and various nutrients essential for bacteria. As a result, the bacteria are unable to assimilate the nutrients and eventually die”.

In the tanning industry and in the candle industry to stiffen wicks.

In borax-phenolphthalein test for glycerol.

Metaboric Acid (HBO2):


(i) From orthoboric acid: Metaboric acid is obtained by heating H3BO3 to 1000C.

Meta Boric Acid


(i) It is a white solid, on heating to 160 °C it gives tetraboric acid H2B4O7 which on further heating gives boric oxide B2O3.

Tetra Boric Acid

(ii) It absorbs a molecule of water forming orthoboric acid.

Ortho Boric Acid


Oxyacids of BoronPyroboric acid; H6B4O9.

Preparation: Pyroboric acid is obtained by heating H3BO3.

Pyroboric Acid

Properties: When dissolved in water H2B4O7 gives H3BO3

Dissolved Water

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