Bromine number

In chemistry, the bromine number is the amount of bromine (Br2) in grams absorbed by 100 g of a sample. The bromine number was once used as a measure of aliphatic unsaturation in gasoline and related petroleum samples, but this assay has fallen into disuse with the introduction of spectroscopic and chromatographic analyses.

Concept
It is based on the following reaction:
 * R2C\dCR2 + Br2  ->  R2C(Br)\sC(Br)R2

Bromine is deeply colored but R2C(Br)\sC(Br)R2 is not. Thus, the consumption of bromine can often be gauged visually. Alternatively and more quantitatively, the bromine consumed by a sample can be determined by iodometry.

The bromine number indicates the degree of unsaturation of a sample. The technique can be subject to a variety of problems, whereby the sample consumes more or less bromine than predicted by the equation above. For example, some alkenes dimerize in the presence of bromine. Allylic bromination is another problem.

The bromine number is similar to the iodine number, which is a similar technique used in evaluating the unsaturation of fats and fatty acids. Iodine is less reactive toward the tri- and tetra-substituted double bonds, found in petroleum-derived samples. The C=C double bonds in fats and fatty acids are exclusively disubstituted.

The bromine number is usually determined by electrochemical titration, where bromine is generated in situ with the redox process of potassium bromide and bromate in an acidic solution, using a mercury catalyst to ensure the complete bromination of all olefins.

Related test methods

 * Acid value
 * Epoxy value
 * Hydroxyl value
 * Iodine value
 * Peroxide value
 * Saponification value