Why after the adsorption of methyl orange dye by my adsorbent, in addition to the color peak in 464 nm, another peak has appeared in 372nm n in theuv-vis spectrum. It should be noted that this additional peak can be seen only in low adsorbent dosage, higher color concentration and shorter conatct time!
thanks in advance for your help
Looking at the structure of methyl orange, the N=N bond can be cis or trans (E or Z). If you started with one pure isomer, it is likely the absorbent is equilibrating the two isomers. I have synthesized similar dyes and have had to deal with isolating each isomer. Both are dyes but they absorb at different wavelengths.
A second explanation could depend on the pH. Since it is a pH indicator dye, you could be at a pH where the basic and acidic forms are in equilibrium. It is red below 3.1 and yellow above 4.1. Both species should exist in equalibrium in a pH range around those values.
This reference points out that cis -methyl orange fluoresence originates through initial absorbtion around 375 nm so I think the first explanation holds water. https://www.sciencedirect.com/science/article/abs/pii/S1386142514001991
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