The best way is to do an experiment than to speculate. Just do it!

Due to  its more rigid structure than other organic dyes,rhodamine B    is stable or exhibits an extremely slow  degradation   under visible light irradiation  .

Some dyes may be experienced to degradation upon exposure to sunlight especially in alkaline solutions. But, as prof. Yurii suggested,  just try to do it.

Thank you very much. As my experiment, the UV-Vis absorption spectra of RhB dye increase after expose under visible light (555 nm). It means RhB is not degraded. Could you please explain this phenomenon?

Dye may has an isosbestic point ,equal absorption for all pH values at specific wavelength, at which  the total absorbance of a dye does not change during a chemical reaction. Dye may show high assembly(dye aggregates) under light causing an increase in the absorptivity or cause the spectrum to become more intense.

You should carry out all exps in buffered solutions to keep pH constant

Rhodamine B is ozo dye, stronger than methylene blue dye. It does not  degrade itself without any catalyst or semiconductor in visible light or even in UV light. 

Dear Lakshmana, 

is your claim experimentally confirmed or just is a guess?

It is not degraded by visible light

Dear Le Anh Thi 

Degradation of an organic dye pollutant under visible-light irradiation depends on the power of the light. With increasing the light power, the degradation of the pollutant increases.  

Dear Le Anh Thi

Increase of the absorbance under visible-light irradiation can be related to dissolution of RhB during the photolysis condition. Please prepare the RhB solution about 24 h before the experiment. It the problem persist, please decrease concentration of the pollutant.

An increase in the absorbance may also be due to evaporation of the water in your solution, leaving behind a more concentrated solution. try controlling your reaction temperature and also if possible  increase the volume of your RhB solution.

Generally, If  aqueous solution of Rh B dye is placed under the visible light irradiation long time, the dye can degrade slightly and absorption spectra is slightly decreased. It also depends upon the distance between the light source and dye as well as light intensity, volume and concentration of dye solution. However, in short periods, the Rh B is difficult to degrade. 

Dear Le Anh Thi,

It can happen by both possibilities of dissolution as well as heating effect. You can use proper photoreactor with water circulation and check dye solution also to avoid this. 

Thank you very much!

 UV-VIS absorption decrease on exposing Rh B under visible light. because all organic dyes get degraded in presence of UV-Vis light .. 

when catalyst is added it  only increases the rate of degradation.

RhB is a stable dye and can not be degraded without the assistance of semiconductors under visible light illumination. The reason behind the increase in absorption intensity of RhB is the evaporation of water during illumination. You should maintain the temperature of the reaction while conducting experiments.