Bromine evolution

The suitability of boron-doped diamond as anode material for the generation of aggressive reagents, such as bromine, has been investigated. Vinokur et al. reported that the electron transfer at boron-doped diamond electrodes is strongly affected by the nature of the electrode process. Irmer-sphere processes such as the bromine evolution from bromide seem to be Id-netically slow, [73,120] in particular, when occurring positive of a potential of approximately —0.05 V vs. SCE. This is currently limiting possible wider applications of boron-doped diamond electrode materials. 

Use of higher concentration sulfuric acid ( 2.0 M) than traditionally employed is crucial to allow complete conversion of the 2-iodobenzoic acid to 1.2 In the submitters hands, use of lower concentrations ( 0.5 M) of sulfuric acid at 60°C led to exclusive formation of 1-hydroxy-1,2-benziodoxol-3(1 H)-one. When lower concentrations of sulfuric acid are employed, higher temperatures are required to effect oxidation, as judged by the initiation of bromine evolution. One of the factors leading to the lack of reproducibility in the preparation of 2 results from incomplete conversion to 1. 

Concentrated sulfuric acid Bromides, Br Reddish-brown coloration, followed by reddish-brown vapors (hydrogen bromide + bromine) evolution... 

Fig. 81 Utideipotential deposition of atomic bromine (large anodic peak) and the beginning of bromine evolution on platinum, in ].0 M Al Br and 0.8 M KBr in ethyl benzene, v = 120 niVlsec. Data from Elam and Gileadi, J. Electrochem. Soc. 126, J474 (1979).

Purification in many applications the use of unrecrystallized material has led to erratic results. Material stored for extended periods often contains significant amounts of molecular bromine and is easily purified by recrystallization from H2O (AcOFI has also been used). In an efficient fume hood (caution bromine evolution), an impure sample of NBS (200 g) is dissolved as quickly as possible in 2.5 L of preheated water at 90-95 °C. As filtration is usually unnecessary, the solution is then chilled well in an ice hath to effect crystallization. After most of the aqueous portion has been decanted, the white crystals are collected by filtration through a bed of ice and washed well with water. The crystals are dried on the filter and then in vacuo. The purity of NBS may be determined by the standard iodide-thiosulfate titration method. 

Janssen LJJ, Hoogland JG (1970) Mechanism of bromine evolution at a graphite electrode. Electrochim Acta 15 1677-1683. doi 10.1016/0013-4686(70)80088-3... 

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