The bromous acid is an inorganic compound of formula HBrO2. Said acid is one of the oxacid acids 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 could not be isolated in the laboratory.
This instability, analogous to iodine acid, is due to a dismutation reaction (or disproportionation) to form hypobromous acid and bromic acid as follows: 2HBrO 2 → HBrO + HBrO 3.
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 → HBrO 2 + HCl
HBrO + H 2 O + 2e – → HBrO 2 + H 2
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 centigrade and a boiling point of 522.29 degrees centigrade. Its solubility in water is estimated to be 1 x 106 mg / L (Royal Society of Chemistry, 2015).
There is no registered risk in the handling of this compound, however, it has been found that it is 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 dependency of [Br-] was found. The reaction was also studied in acetate buffer, in the pH range 3.9-5.6.
Within the 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:
HBrO 2 + BrO 2 → HOBr + Br0 3 –
Rate constants of 800 ± 100 M -1 for the reaction:
2HBr0 2 → HOBr + Br0 3 – + H +
And an equilibrium quotient of 3.7 ± 0.9 X 10 -4 for the reaction:
HBr02 ⇌ H + + BrO 2 –
Obtaining an experimental pKa of 3.43 at an ionic strength of 0.06 M and 25.0 ° C (RB Faria, 1994).
Alkaline earth compounds
Bromic acid or sodium bromite is used to produce beryllium bromite based on the reaction:
Be (OH) 2 + HBrO 2 → Be (OH) BrO 2 + H 2 O
Bromites are yellow in color in solid state or in aqueous solutions. This compound is used industrially as an oxidative starch descaling agent in the refinement of textiles (Egon Wiberg, 2001).
Bromic acid or bromites can be used to reduce permanganate ion to manganate in the following way:
2MnO 4 – + BrO 2 – + 2OH – → BrO 3 – + 2MnO 4 2- + H 2 O
What is convenient for the preparation of manganese (IV) solutions.
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 are mixed to form a green color, which turns blue, purple and red, and then turns green and repeats.
The three solutions that are mixed are as follows: a 0.23 M KBrO 3 solution, a 0.31 M malonic acid solution with 0.059 M KBr and a 0.019 M cerium (IV) ammonium nitrate solution and H 2 SO 4 2.7M.
During presentation, a small amount of the indicator ferroin is introduced into the solution. Manganese ions can be used in place of cerium. The overall reaction BZ is the cerium-catalyzed oxidation of malonic acid by bromate ions in dilute sulfuric acid as presented in the following equation:
3CH 2 (CO 2 H) 2 + 4 BrO 3 – → 4 Br – + 9 CO 2 + 6 H 2 O (1)
The mechanism of this reaction involves two processes. Process A involves ions and transfers of two electrons, while Process B involves radicals and transfers of one electron.
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 by bromide ions in two electron transfers. It can be represented by this net reaction:
BrO 3 – + 5Br – + 6H + → 3Br 2 + 3H 2 O (2)
This occurs when solutions A and B are mixed. This process occurs through the following three steps:
BrO 3 – + Br – +2 H + → HBrO 2 + HOBr (3)
HBrO 2 + Br – + H + → 2 HOBr (4)
HOBr + Br – + H + → Br 2 + H 2 O (5)
The bromine created from reaction 5 reacts with malonic acid as it slowly enolises, as represented by the following equation:
Br 2 + CH 2 (CO 2 H) 2 → BrCH (CO 2 H) 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 Ce 3+ → Br 2 + 10Ce 4+ 6H 2 O (7)
And it consists of the following steps:
BrO 3 – + HBrO 2 + H + → 2BrO 2 • + H 2 O (8)
BrO 2 • + Ce 3+ + H + → HBrO 2 + Ce 4+ (9)
2 HBrO 2 → HOBr + BrO 3 – + H + (10)
2 HOBr → HBrO 2 + Br – + H + (11)
HOBr + Br – + H + → Br 2 + H 2 O (12)
The key elements of this sequence include the net result of Equation 8 plus twice Equation 9, shown below:
2Ce 3+ + BrO 3 – + HBrO 2 + 3H + → 2Ce 4+ + H 2 O + 2HBrO 2 (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 a 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) ions and bromine 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 complexes of iron and cerium.
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 oscillations are interrupted by chloride ion contamination (Horst Dieter Foersterling, 1993).
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- Ropp, RC (2013). Encyclopedia of the Alkaline Earth Compounds. Oxford: Elvesier.
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