Following Links and papers will be useful for you

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Dear Mr. Konari, this phenomena is due to dissociation of acetylsalicylic acid. The pKa of aspirin is 3.48 (http://www.csudh.edu/oliver/chemdata/data-ka.htm). This means that in this pH, 50% of aspirin is in molecular form and 50% dissociated. In neutral solvent (~pH 7) more species are dissociated than in pH strongly acid. The molecular specie needs more energy to excite the electrons from fundamental state to another molecular orbital of higher energy while dissociated species, not.(orbitals from HOMO to LUMO). Like lower wavelenght has higher energy (Planck equation: E=hf) molecular species absorb UV radiation in lower wavelength. See: https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy). This also happens with others organic acids such as ascorbic acid (see my paper about ascorbic acid, you will see the same desviation in pH function: Use of column with modified silica for interfering retention in a FIA spectrophotometric method for direct determination of vitamin C in medicine).  I hope to have helped you. Kind regards.

Thank u Julio cesar Sir  ....For spending valuable time...................and clearing my doubt.......... 

Mr. Konari,

Due to the effect of the ionic strength in your case. If you would like to observe removing of the n-pi conjugation due to the protonation, then you should try to measure your compound in conc. H2SO4, for example.