Dilute nitric acid— Bubble air through nitric acid until the acid is colorless,then mix 1volume of the acid with 4volumes of water.

Standard phenol solution— Dissolve about 1g of phenol in about 100mLof water,and determine the actual C6H6Oconcentration as follows.Pipet 4mLof the solution into an iodine flask,add 30.0mLof 0.1Nbromine VS,then add 5mLof hydrochloric acid,and immediately insert the stopper.Shake the flask repeatedly during 30minutes,allow to stand for 15minutes,add quickly 5mLof potassium iodide solution (1in 5),taking precautions to prevent the escape of bromine vapor,and at once insert the stopper into the flask.Shake thoroughly,remove the stopper,and rinse it and the neck of the flask with a small quantity of water so that the washings flow into the flask.Add 1mLchloroform,shake the mixture,and titrate the liberated iodine with 0.1Nsodium thiosulfate VS,adding 3mLof starch TSas the endpoint is approached.Perform a blank determination.Each mLof 0.1Nbromine is equivalent to 1.569mg of C6H6O.Dilute a suitable volume of the solution with water to obtain a concentration of 250µg of C6H6Oper mL.

Procedure— Place about 2.5g of Cresol,accurately weighed,in a 250-mLvolumetric flask,add 10mLof sodium hydroxide solution (1in 10),dilute with water to volume,and mix.Pipet 5mLof this solution into a 200-mLvolumetric flask,add 45mLof water and 1drop of methyl orange TS,neutralize with Dilute nitric acid,added dropwise,then dilute with water to volume,and mix.Pipet 5mLof the neutralized solution into each of two 20-×180-mm test tubes,graduated at the 25-mLmark,and pipet 5.0mLof Standard phenol solutioninto each of two similar test tubes.To the contents of each tube add 5mLof Millon's Reagent,allowing it to flow down the inner wall of the tube,mix,place the tubes simultaneously in a boiling water bath provided with a rack so that the tubes do not touch the bottom of the bath,and maintain the bath at boiling temperature for 30minutes,accurately timed.At once remove the tubes from the bath,cool them immediately and thoroughly by placing them in a bath of cold water for not less than 10minutes,add 5mLof Dilute nitric acidto each tube,and mix.Add 3mLof a mixture of 1volume of formaldehyde solution and 50volumes of water to one of each pair of tubes,add water to fill all tubes to volume,shake thoroughly,and allow to stand for 16hours,during which time the added formaldehyde imparts a yellow color while the contents of the other 2tubes acquire an orange-red color.

5V/W,