The present experiment is a convenient preparation of bromine by the reduction of 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH) in aqueous solution by sodium metabisul‘te/sulfur dioxide. This halogenated hydantoin is readily available and inexpensive due to its mass use as a disinfectant/bactericide in the consumer market. When BCDMH in reduced slowly in water, dimethylhydantoin is the end product releasing both bromine and chlorine
C5H6N2O2BrCl + 2H2O → HOBr + HOCl + C5H8N2O2 .(7.1)
It is interesting to note that with a moderate reducing agent chlorine is reduced much more slowly than bromine, presumably due to the stronger N-Cl bond of
the halogenated amide, so the reaction with a concentrated solution of Na2S2O5 is rapid, and leads quantitatively and almost exclusively to the reduction of bromine
Sulfur dioxide can reduce bromine further to bromide in a method used to prepare hydrobromic acid :
SO2 + Br2 + 2H2O → 2Br− + SO42− + 4H+. (7.3)
Hence, hydrogen bromide as well as sulfuric acid (formed in the gas phase) distill over with the water and bromine from the reaction mixture. This results in a loss of product both directly, and due to the fact that the bromide ion greatly enhances the solubility of bromine in the aqueous phase ; hence, the amount of metabisul‘te used in this experiment is adjusted to be slightly above that required by stoichiometry, at which point a quantitative yield of bromine is obtained. The small amount of HBr that is formed is bene‘cial in preventing the formation of a bromine clathrate with water of stoichiometry Br2.~8H2O , which can block the condenser.