A number of phenols can be substituted rapidly and quantitatively with bromine produced from bromate and bromide in acid solution. The determination involves treating phenol (see Note) with an excess of potassium bromate and potassium bromide; when bromination of the phenol is complete the unreacted bromine is then determined by adding excess potassium iodide and back-titrating the liberated iodine with standard sodium thiosulphate.
Prepare an approx. 0.1N standard solution of potassium bromate by weighing accurately about 1.65 g of the analytical grade reagent, dissolving it in water and making it up to 500 mL in a graduated flask. To determine the purity of a sample of phenol, weigh out accurately approximately 0.3 g of phenol, dissolve it in water and make the volume to 250 mL in a graduated flask. Pipette 25 mL volumes of this solution into 250 mL stoppered (ground-glass) conical flasks. To each flask pipette 25 mL of the standard potassium bromate solution and add 0.5 g of potassium bromide and 5 mL of 3M sulphuric acid. Mix the reagents and let them stand for 15 minutes, then rapidly add about 2.5 g of potassium iodide to each flask, immediately re-stoppering and swirling the contents to dissolve the solid. Titrate the liberated iodine with the standard 0.1M sodium thiosulphate until the solution is only slightly yellow, then add 5 mL of starch indicator solution and continue the.
Calculate the amount of excess bromate from the amount of thiosulphate needed for the back-titration of the free iodine, and hence the quantity of bromate which reacted with the phenol.
Note: Other phenols which undergo this type of reaction include 4-chlorophenol, m-cresol (3-methylphenol) and 2-naphthol.