good answer chakravarthi. Because of the formation of Benzoquinone
Plz follow below link
http://www-lib.uwyo.edu/showcase/files/original/solid-retort-water-interactions---the-fate-of-organics-and-inorganics_f731dd5d0e.pdf
Phenols undergo oxidative polymerisation., resulting in increasingly large conjugated systems of double bonds. This results in red-shifted absorbance maxima and the formation of yellow and red-coloured compounds (which appear brown in complex mixtures). Examples in nature include lignin and tannins. Molar extinction coefficients may also increase, as in the case of conjugated dienes.
Atmospheric oxygen contains unpaired electrons, so solutions of phenolic compounds tend to go brown over time. This is a pain, because the concentration of reactant has gone down, and you might even have to purify it or replace the stock. If you are going to leave the stock solution unused for several months you might consider bubbling nitrogen gas through the solution before storage.
thanks for all, In fact my experiments handle the effect of advanced oxidation on phenol and the assessment of degradation by spectrophotometer .. the results had given me for examples this result of absorbance by time 0.3-0.65-0.38-0.2-0.1
and my question for the second results
Due to double bond formation.Change of colour is due to the further reduction of hydroquinone and pyrocatechol,oxidation products of phenol(Carbolic acid).
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