Estimation of Aniline

Theory behind the reaction for estimation of aniline C6H5NH2

Aniline reacts with bromine to form tribromaniline which can be represented using the following equation
C6H5NH2 + 3Br2 → C6H2NH2Br3 + 3HBr
Equivalent mass of aniline is Molar Mass/6 = 15.5 gram equivalents

The actual reagents used for the reaction is not bromine solution, but bromine is liberated by a redox reaction. Potassium bromate when treated with potassium bromide in the presence of HCl liberates Br2. The liberated bromine is used in the titration
KBrO3 + 5 KBr + 6 HCl → 6KCl + 3Br2 + 3H2O

Reagents required

  • Sodium thiosulfate solution approximate concentration = 0.1N
  • Brominating mixture KBrO3/KBr approximate concentration = 0.1N
  • Potassium Iodide solution 10 %
  • Starch solution

There are two parts to this experiment

  • First determine the concentration of sodium thiosulfate – Standardization
  • Secondly estimate the Aniline solution

Standardization of Na2S2O3 – Sodium thiosulfate

Mass out 1.25 grams of potassium dichromate into a 250 ml standard flask. Dissolve the solid using minimum water first, when all the crystals have been dissolved add more water up to the graduated mark in the flask. Mix the solution to make it uniform before the experiment is done.
10 ml of this solution is pipeted into an Erlenmeyer Meyer flask, add about 3 ml of concentrated HCl (1M) followed by 5 ml of 10 % KI solution.
The liberated bromine is titrated against sodium thiosulfate solution taken in the burette. When the solution turns pale yellow add the starch solution, titration is performed until concordant values are obtained. (Alternate methods)

Estimation of Aniline

If you are given an aniline solution it is to be diluted appropriately first. 20 ml of this aniline solution and about 40 ml of the brominating mixture are pipetted into a stoppered 250 mL Erlenmeyer flask (conical flask) and diluted with about 20 ml of water and 5 mL of concentrated HCl (1M to 3M may be used). After closing the Erlenmeyer flask swirl the mixture properly to mix the contents for the next 30 minutes. Cool the contents of the flask and open the flask carefully without loosing any bromine liberated (Use an Erlenmeyer flask with a glass stopper pour some of the 10 % KI around the stopper and then open it carefully not to loose any bromine liberated, the KI will get sucked into the flask) and add 20 mL of 10% KI solution. Close the flask and shake it for 25 seconds allow the mixture to stand for 10 minutes.

Remove the stopper wash down the contents using a little distilled water and titrate the free iodine against sodium thiosulphate using starch as the indicator. When the colour of the solution becomes pale yellow add one ml of already prepared fresh starch solution.

Titrate the solution carefully and slowly until the blue color of the starch iodine complex disappears. The volume of sodium thiosulfate consumed is equal to the liberated excess bromine (hence care should be taken not to loose the bromine before adding KI)

Perform a blank titration using 20 mL of the brominating mixture and water, using the same procedure.


  • Normality of sodium thiosulfate = N1
  • 20 mL of brominating mixture is = V1 ml of Sodium thiosulfate
  • Volume of brominating mixture used = 40 mL
  • 40 mL of brominating mixture = twice the volume of thio = 2V1
  • 20 ml of iodine solution and 40 ml of brominating mixture = V2
  • Amount of sodium thiosulfate = aniline = (2V1-V2)mL
  • Normality of aniline= (2V1-V2)mL x N1/20X10
  • Divide the fraction by 10 if you made up the aniline solution in a 100 mL standard flask, if not only use 20 mL
  • The normality can be converted to appropriate units.