Bromine

Hydrolysis

Description of key information

Bromine is unstable in water hydrolysing rapidly. The rate constant for bromine hydrolysis is in the range of 97sec-1 (Becker RH et al., 2001, Beckwith RC et al. , 1996).
The identity of the degradation products of bromine have been outlined in Lewis S et al. (1994). In aqueous solutions made with pure water, bromine hydrolyses to hypobromous acid (HOBr) and hydrogen bromide which further dissociate to give hypobromite ions (OHBr-) and hydrogen ions (H+) and bromide ions with the degree of dissociation being highly pH dependent.  The half life in diluted aqueous systems was calculated to be 0.007 s.

Key value for chemical safety assessment

Half-life for hydrolysis:

0 min

Additional information

Bromine is unstable in water hydrolysing rapidly. The rate constant for bromine hydrolysis is in the range of 97 sec^-1(Becker RH et al., 2001, Beckwith RC et al, 1996).

The identity of the degradation products of bromine have been outlined in Lewis S et al (1994). In aqueous solutions made with pure water, bromine hydrolyses to hypobromous acid (HOBr) (free bromine) which further dissociates to give hypobromite ions (OHBr-) and hydrogen ions (H+), and bromide. The degree of dissociation being highly pH dependent. An overview of the dissociation of hypobromous acid over various pH values are given below:

Effect of pH on Hypobromous acid Dissociation:

 

pH	Bromine
6.0	5 % Br2 95 % HOBr
7.0	99 % HOBr 1 % OBr^-
8.0	90 % HOBr 10 % OBr^-
9.0	50 % HOBr 50 % OBr^-

As under the exposure conditions covered in the registration dossier concentrations of bromine are low, water is in excess and the hydronium ion concentration is buffered to physiological pH values, the equilibrium is shifted to the hydrolysis products. The above reaction is pseudo first order. With the rate constant k1 of 97 s-1 a half-live of ln2 / 97 = 0.007 s can be calculated which confirms the rapid hydrolysis of the substance.

In natural waters, bromine reacts with ammonia and organic nitrogenous materials to form bromamines.. Bromine will react quickly with other organic materials in water, to form other chemicals analogous to chlorinated products (e.g. bromoform, bromophenols, bromoaldehydes, bromoacetonitriles, bromoacetic acids, bromoketones, bromohydrins). In seawater, the likely predominant species will be inorganic bromamines (especially dibromamine) because of the relatively high ammonia concentrations. As a result, because of the limited stability of inorganic bromamines, it is likely that the total bromine residual will decay fairly rapidly to bromide. In river waters, with typically low ammonia concentrations, and relatively high organic carbon contents, it is likely that higher concentrations of inorganic and organic bromamines would result. In neither case, is there likely to be any free bromine remaining after a short contact period. This could be difficult to confirm since standard analytical procedures are incapable of distinguishing free and combined bromamines (i.e. incapable of distinguishing hypobromous acid and the inorganic bromamines. If any free bromine should persist in an open environment, it would be destroyed by photolysis yielding bromide, and possibly bromate.