9 junio, 2024

Bromous acid (HBrO2): physical and chemical properties, and uses

He bromous acid It is an inorganic compound with the formula HBrO2. Said acid is one of the oxacids of bromine where it is in the 3+ oxidation state. The salts of this compound are known as bromites. It is an unstable compound that has not been isolated in the laboratory.

This instability, analogous to iodinous acid, is due to a dismutation (or disproportionation) reaction to form hypobromous acid and bromic acid as follows: 2HBrO2 → HBrO + HBrO3.

Bromous acid can act as an intermediate in different reactions in the oxidation of hypobromites (Ropp, 2013). It can be obtained by chemical or electrochemical means where the hypobromite is oxidized to the bromite ion such as:

HBrO + HClO → HBrO2 + HCl

HBrO + H2O + 2e– → HBrO2 + H2

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Physical and chemical properties

As mentioned above, bromous acid is an unstable compound that has not been isolated, so its physical and chemical properties are obtained, with some exceptions, theoretically through computational calculations (National Center for Biotechnology Information, 2017).

The compound has a molecular weight of 112.91 g/mol, a melting point of 207.30 degrees Celsius, and a boiling point of 522.29 degrees Celsius. Its solubility in water is estimated to be 1 x 106 mg/L (Royal Society of Chemistry, 2015).

There is no record of any type of risk in the handling of this compound, however, it has been found to be a weak acid.

The kinetics of the bromine(III) disproportionation reaction, 2Br(III) → Br(1) + Br(V), was studied in phosphate buffer, in the pH range of 5.9-8.0, monitoring the optical absorbance at 294 nm using stopped flow.

The dependencies of [H+] and [Br (III)] were of order 1 and 2 respectively, where no dependence on [Br-]. The reaction was also studied in acetate buffer, in the pH range of 3.9 – 5.6.

Within experimental error, no evidence was found for a direct reaction between two BrO2- ions. This study provides rate constants 39.1 ± 2.6 M-1 for the reaction:

HBrO2 + BrO2→ HOBr + Br03–

Rate constants of 800 ± 100 M-1 for the reaction:

2HBr02 →HOBr + Br03– + H+

And an equilibrium ratio of 3.7 ± 0.9 X 10-4 for the reaction:

HBr02 ⇌ H++BrO2–

Obtaining an experimental pKa of 3.43 at an ionic strength of 0.06 M and 25.0 °C (RB Faria, 1994).

Applications

alkaline earth compounds

Bromic acid or sodium bromite is used to produce beryllium bromite according to the reaction:

Be(OH)2 + HBrO2 → Be(OH)BrO2 + H2O

Bromites are yellow in color in the solid state or in aqueous solutions. This compound is used industrially as a descaling agent for oxidative starches in textile refining (Egon Wiberg, 2001).

Reducing agent

Bromic acid or bromites can be used to reduce the permanganate ion to manganate as follows:

2MnO4– + BrO2– + 2OH–→ BrO3– + 2MnO42- + H2O

What is convenient for the preparation of manganese (IV) solutions.

Belousov–Zhabotinski reaction

Bromous acid acts as an important intermediate in the Belousov-Zhabotinski reaction (Stanley, 2000), which is an extremely visually striking demonstration.

In this reaction, three solutions mix to form a green color, which turns blue, purple, and red, then back to green and repeats.

The three solutions that are mixed are as follows: a 0.23 M KBrO3 solution, a 0.31 M malonic acid solution with 0.059 M KBr, and a 0.019 M cerium (IV) ammonium nitrate solution and 2.7M H2SO4 .

During presentation, a small amount of the indicator ferroin is introduced into the solution. Manganese ions can be used instead of cerium. The overall BZ reaction is the cerium-catalyzed oxidation of malonic acid by bromate ions in dilute sulfuric acid as presented in the following equation:

3CH2 (CO2H)2 + 4 BrO3– → 4 Br– + 9 CO2 + 6 H2O (1)

The mechanism of this reaction involves two processes. Process A involves ions and two-electron transfers, while process B involves radicals and one-electron transfers.

The bromide ion concentration determines which process is dominant. Process A is dominant when the bromide ion concentration is high, while process B is dominant when the bromide ion concentration is low.

Process A is the reduction of bromate ions to bromide ions in two electron transfers. It can be represented by this net reaction:

BrO3– + 5Br– + 6H+ → 3Br2 + 3H2O (2)

This occurs when solutions A and B are mixed. This process occurs through the following three steps:

BrO3– + Br– +2 H+ → HBrO2 + HOBr (3)

HBrO2 + Br– + H+ → 2 HOBr (4)

HOBr +Br– +H+ → Br2 + H2O (5)

The bromine created from reaction 5 reacts with malonic acid as it slowly enolizes, as represented by the following equation:

Br2 + CH2 (CO2H)2 → BrCH(CO2H)2 + Br– + H (6)

These reactions work to reduce the concentration of bromide ions in the solution. This allows process B to become dominant. The overall reaction of process B is represented by the following equation:

2BrO3– + 12H+ + 10 Ce3+ → Br2 + 10Ce4+ 6H2O (7)

And it consists of the following steps:

BrO3 – + HBrO2 + H+ → 2BrO2 • + H2O (8)

BrO2 • + Ce3+ + H+ → HBrO2 + Ce4+ (9)

2 HBrO2 → HOBr + BrO3 – + H+ (10)

2 HOBr → HBrO2 + Br– + H+ (11)

HOBr + Br– + H+ →Br2 + H2O (12)

The key elements of this sequence include the net result of Equation 8 plus two times Equation 9, shown below:

2Ce3+ + BrO3 – + HBrO2 + 3H+ →2Ce4+ + H2O + 2HBrO2 (13)

This sequence produces bromous acid autocatalytically. Autocatalysis is an essential feature of this reaction, but it does not continue until the reagents are exhausted, because there is second-order destruction of HBrO2, as seen in reaction 10.

Reactions 11 and 12 represent the disproportionation of hyperbromous acid to bromous acid and Br2. Cerium(IV) and bromine ions oxidize malonic acid to form bromide ions. This causes an increase in the concentration of bromide ions, which reactivates process A.

The colors in this reaction are formed mainly by the oxidation and reduction of iron and cerium complexes.

Ferroin provides two of the colors seen in this reaction: as [Ce (IV)] increases, it oxidizes the iron in ferroin from red iron(II) to blue iron(III). Cerium(III) is colorless and cerium(IV) is yellow. The combination of cerium(IV) and iron(III) makes the color green.

Under the right conditions, this cycle will repeat itself several times. Glassware cleanliness is a concern because the oscillations are disrupted by chloride ion contamination (Horst Dieter Foersterling, 1993).

References

bromous acid. (2007, October 28). Retrieved from ChEBI: ebi.ac.uk.
Egon Wiberg, NW (2001). Inorganic Chemistry. london-san diego: academic press.
Horst Dieter Foersterling, MV (1993). Bromous acid/cerium(4+): reaction and HBrO2 disproportionation measured in sulfuric acid solution at different acids. Phys. Chem 97(30), 7932–7938.
iodous acid. (2013-2016). Retrieved from molbase.com.
National Center for Biotechnology Information. (2017, March 4). PubChem Compound Database; CID=165616.
B. Faria, IR (1994). Kinetics of Disproportionation and pKa of Bromous Acid. J. Phys. Chem. 98(4), 1363-1367.
Ropp, R. C. (2013). Encyclopedia of the Alkaline Earth Compounds. Oxford: Elvesier.
Royal Society of Chemistry. (2015). Bromous acid. Retrieved from chemspider.com.
Stanley, AA (2000, December 4). Advanced Inorganic Chemistry Demonstration Summary oscillating reaction.

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