Dear Mahmoud,

The most precise spectrophotometric method for the determination of aniline in water is by coupling aniline with diazotizedp-nitroaniline in the presence of ethanol and sodium hydroxide to form an intense red water-soluble azo-dye which shows maximum absorption at 505 nm and greater stability and sensitivity in the presence of cetyltrimethylammonium bromide. Beer's law is obeyed over the aniline concentration range 0.2–2.4μg/ml in the solution measured; the molar absorptivity is 3.46×104l· mole−1·cm−1, and the relative standard deviation 1–2%, depending on the aniline concentration.

http://link.springer.com/article/10.1007%2FBF01199282

Regarding the effect of pH on the lamda max:

Please note that aniline in basic medium exists as free form PhNH2 whereas in acidic medium aniline is converted to anilinium salt (PhNH3 +). The lamda max for the free aniline will be different from that of the anilinium salt due to variation in the chemical structures.

Hoping this will be helpful,

Rafik

Dear Rafik

I request kindly to refer to some paper in which this method has been used.

Thank you

Dear Mahmoud,

Herein the paper:

Original Papers  Microchimica Acta

September 1986, Volume 90, Issue 5, pp 417-423

Spectrophotometric determination of aniline in aqueous solution by azo-dye formation with diazotized p-nitroaniline

S. A. Rahim , Nada D. Ismail , W. A. Bashir

And the following link contains the abstract of the mentioned paper:

http://link.springer.com/article/10.1007%2FBF01199282

Rafik