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Hydrolysis and identification of products

Hydrogen bromide (HBr) is very soluble in water (Annex VII Section 7.7 Solubility in water) forming hydrobromic acid which dissociates to ions (H+and Br-). It forms an an azeotropic mixture in water. A saturated solution is 66.5 % at 25 ºC. HBr in water acts as a strong mineral acid (i.e., hydrobromic acid) and dissociates readily to form hydrogen and bromide ions.

Photolysis in air

Falconer and Sunder (1968) showed that the photolysis reaction of HBr could progress via one or more of the following reaction schemes:

[1]       HBr +hν → HBr*

[2]      HBr* + M → HBr + M

[3]       HBr* → H(2S) + (Br(2P3/2)

[4]       [HBr + HBr]c+hν → [HBr…H…Br]c

[5]       [HBr…H…Br]c→ HBr + HBr

[6]       [HBr…H…Br]c→ H2+ 2[Br]c

[7]       [HBr…H…Br]c→ HBr + H+ [Br]c

[8]       H + H + M → H2+ M

[9]       H + HBr → H2+ [Br]c

[13]     [Br2+ HBr]c+hν → [HBr…H…Br]c

[14]     [HBr…H…Br]c→ Br2+ HBr

[15]     [HBr…H…Br]c→ H + Br2+ [Br]c

[16]     H + Br2→ HBr + Br

Dotted lines indicate complexes and the subscript c indicates caged or matrix restrained species. 

Baumfalk R, et al (1997) measured kinetic energies the dissociation reaction and obtained values of 1.36 ± n0.06 eV (Br) and 0.90 ± 0.05 eV (Br*) which were in good agreement with the calculated values of 1.35 eV and 0.89 eV obtained using the following equation:

              hν(243.1nm) –D0(HBr) =Ekin(H) +Ekin(Br) +Eint(Br)

WhereD0is the dissociation energy of the HBr molecule andEkinandEintthe kinetic and internal energy of the fragment atoms.

The H atom photofragmented with zero kinetic energy after the photo-dissociation


Ready biodegradation

HBr is an inorganic substance so does not undergo biotic degradation. 


Further biodegradation studies

HBr forms hydrobromic acid in water. The toxic effect seen to aquatic organisms is from the acidity of the resulting solution, which is a function of the hydrogen bromide concentration.

As adequate information on HBr degradation is available and it is not technically feasible or scientifically unjustified biodegradation simulation studies would not be performed.



HBr has an estimated log octanol/water partition coefficient of 0.6287 (Annex VII Section 7.8), so it has a low potential for adsorption. HBr in contact with water forms hydrobromic acid, which dissociates into hydrogen and bromide ions. Thus, adsorption of hydrogen bromide itself is unlikely. 

Bromide, the primary degradant, occurs naturally in the environment. Negative anions such as bromide are known not to adsorb to soil and bromide has been used to monitor ground water flow through soil as it is not known to significantly adsorb to soil.


Henry’s Law Constant

Brimblecombe, P., and Clegg, S.L. (1998) reported a Henry's law constant of hydrobromic acid = 1.32 x 109mol2 kg-2 atm-1at 25 ºC



HBr is an inorganic substance. Upon contact with water, HBr reacts rapidly to form hydrobromic acid which dissociates to hydrogen and bromide ions. It is not feasible to perform the test on HBr itself. An estimation of the log Kow for hydrogen bromide is 0.6287 (Annex VII Section 7.8), so it has a low potential for bioaccumulation.