ESTIMATION OF PHENOLS AND RELATED COMPOUNDS
In oxidation-reduction methods bromine is employed as an oxidizing agent in place of iodine, because it is reduced quantitatively be the readily oxidized pharmaceutical organic substances in a reaction which results in either water-insoluble bromine substitution products, for instance :
or corresponding water-insoluble bromine-addition products, such as :
However, the standard solution used does not have bromine (Br2) as such but it does contain an equivalent amount of potassium bromate and an excess of potassium bromide and the resulting mixture on subsequent acidification liberates bromine. The reaction may be expressed as follows :
The liberated bromine helps in oxidizing iodide to an equivalent amount of iodine as shown below :
The free iodine thus produced is titrated with previously standardized sodium thiosulphate solution as depicted below :
In oxidation-reduction assays the use of bromine is judiciously carried out as an oxidizing agent effectively for such specific compounds which ultimately results into the formation of both bromine substitution and bromine additive compounds. These products of reaction are produced quantitatively and are mostly water-insoluble in characteristics ; and more interestingly they take place in an acidic medium.
As it has been discussed earlier, iodine cannot be used directly as an oxidizing agent in such type of assays, whereas the liberated iodine quantitatively produced by the oxidation of iodide with bromine (excess) may be assayed by titrating against sodium thiosulphate solution.