What is the mechanism of temporary bleaching of flowers by SO2 ? Can H2S be used?

Question

There are many claims on the web, explaining that the bleach action of Chlorine gas in discoloring of flower proceeds as follow and require moisture for it : $$\ce{ Cl2 + H2O <=> HOCl + HCl <=> HCl +nascent O}$$

And this nascent Oxygen reacts with the $\pi$ conjugated molecules of flowers, discoloring them.

Similarly, temporary discoloring by SO2 was explained as a result of nascent Hydrogen, something like : $$\ce{ SO2 + 2H2O <=> H2SO4 + 2 nascent H }$$

And this nascent Hydrogen can also discolor the flowers.

My colleague and I had a debate where I claimed if $\ce{SO2}$ can discolor the flower so will $\ce{H2S}$. And if $\ce{SO2}$ can produce the "nascent hydrogen", so will $\ce{H2S}$ because it is a better reducing agent than $\ce{SO2}$.

I searched the web and didn't found much information either backing the nascent Hydrogen hypothesis or discoloring by $\ce{H2S}$ or by $\ce{SO2}$. Though I found one or two articles explaining how the nascent hydrogen hypothesis came and how it got discredited. Modern theory can explain all the reactions of such "nascent hydrogen" by assuming $\ce{H2}$ molecules at high temperatures, proving that they are nothing but non-equilibrated states of H2 which are just formed.

So, my question is : Can we claim that if one reducing agent like $\ce{SO2}$ can discolor a flower, other strong agents like $\ce{H2S}$ will surely discolor it?

Answer 1

The critical point in the understanding of the bleaching process is the property of the dye molecule itself. Textile dye molecules or natural pigments are large conjugated systems that absorb light in the visible region (400 to 700 nm). Therefore, any chemical reagent which destroys this conjugation or destroys the dye molecules into smaller soluble units without significantly destroying the substrate (cloth, plastic, flowers, unfortunately even food is bleached in some countries!! etc.) gets the label of a bleaching agent. This includes chlorine-based bleaching agents, peroxide-based bleaches, enzymes, and reducing agents.

It is not necessary that the bleaching mechanism is well understood. The reducing bleaches probably destroy the carbonyl group if it is present in the dye molecule. See for example Bleaches and Sterilants by Marianna A. Busch, Kenneth W. Busch, in Encyclopedia of Analytical Science (Third Edition), 2019. Will hydrogen sulfide reduces a dye? You can try Googling along these lines. H2S is a relatively weak reducing agent so I don't think it can destroy/reduce organic dyes.

In older books, from the 1920 to-40s, you will see an extensive usage of the word "nascent" terminology, and some parts of the world which have not updated their high school chemistry syllabus in the last 50 years still use this terminology in their school books. Their university syllabs are very good but nobody bothered to change school and college texts. However, old works are not to be ridiculed as Wikipedia claims; they should be respected as they represent the thought process of our learned scholars to explain the phenomenon which is not fully understood as yet. I already addressed the current usage of nascent terminology. It still occasionally appears in respected journals. See this previous post Is there any evidence, any evidence at all, that nascent hydrogen actually exists?