Estimations using Potassium Bromate

The method depends on the direct bromination of a substance by means of nascent bromine liberated from potassium bromate and potassium bromide in hydrochloric acid solution. 

The corresponding sodium salts, which are cheaper, serve equally well. Excess of potassium bromide and hydrochloric acid is added to a solution of the substance to be estimated and the bromate, in the form of a volumetric solution (usually N/5), is run in. The end point (shown by the presence of free bromine) is detectable, quite sharply, by spotting on starch-iodide paper as external indicator. Alternatively the end point may be determined by adding a known amount of bromate in excess, then potassium iodide, and titrating the liberated iodine with thiosulphate. 

Preparation of Nj5 Bromate Solution.—5-567 gms. (1/30 mol.) of potassium bromate are dissolved in water to 1,000 c.cs. 25 c.cs. are withdrawn, diluted to 200 c.cs. with water, 3—5 gms. of potassium bromide (free from bromate), 3—5 gms. potassium iodide (free from iodate) and 5 c.cs. cone, hydrochloric acid (free from chlorine or iron) added. The liberated iodine is titrated with thiosulphate in the usual way, and the bromine equivalent of the bromate solution thus obtained. 

The reagent is particularly suitable for the determination of amines and phenols and their derivatives, and also for unsaturated compounds. Temperature has a marked effect on the rate and on the extent of bromi-nation. It is necessary to know or to determine what bromo compound is being formed. 

In the case of amines and phenols, bromine enters the ring in the ortho- or para- position if there is no unoccupied ortho- or para-position bromination does not take place except when this position is occupied by a carboxyl or sulphonic group, and in such case the group is split off and replaced by bromine. 

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A different approach is used in the estimation of phenols. Bromine is generated by reaction of potassium bromide with a defined volume of a standard solution of potassium bromate according to the following equation ... 

In acidic medium, potassium bromide and potassium bromate produce bromine stoichiometrically, and this is the basis of a titration method [82], which has been used to determine double bonds in polymethylacrylate. After bromination, excess bromine is estimated by the addition of potassium iodide and the iodine produced by the reaction is titrated with standard sodium thiosulfate to the starch end-point. 

General Procedure.—Dissolve 0-2—0-5 gm. of the compound to be estimated in 200 c.cs. of water, using a slight excess of hydrochloric acid for amines or a similar excess of caustic soda in the case of phenols, carboxylic- and sulphonic-acids. To the solution thus prepared 10 c.cs. of a 20% solution of potassium bromide and 5—10 c.cs. of concentrated hydrochloric acid are added the mixture is brought to the required temperature and N/5 bromate solution run in slowly until a sample withdrawn gives a reaction on starch-iodide paper. The presence of free bromine should be again tested for after 5 minutes. 

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