# Why is bleaching with chlorine permanent but with sulphur dioxide temporary?

I was reading the properties of chlorine when I came across this statement:

Chlorine is a powerful bleaching agent; bleaching action is due to oxidation. Chlorine bleaches vegetable or organic matter in the presence of moisture. Bleaching effect of chlorine is permanent. $$\ce { Cl_2 +H_2O -> 2HCl + O}$$ $$\ce { Colored Substance + O -> Colorless Substance }$$

I searched it up and found out that bleaching with $$\ce{Cl2}$$ is an oxidation and is made powerful with the help of the nascent oxygen, hence it is permanent, but bleaching with $$\ce{SO2}$$ is a reduction and hence it is temporary.

Could someone please help me understand why the oxidation is permanent and the reduction is not? Is it powerful to an extent of the reaction not being able to get reversed? If yes/no, is there any quantitative/qualitative measure for the same?

• Because our air is (re)oxidizing, maybe? Mar 13 '21 at 13:00
• Are you sure you refer to $\ce{SO2}$, sulfur dioxide, and not $\ce{S2O^{2-}4}$ like in sodium dithionite, which may, for example, be used to reduce temporarily blue indigo into yellowish / colorless leucoindigo (eventually oxidizing on air again) as used for blue jeans (see, e.g. here)?. Mar 13 '21 at 13:19
• @Buttonwood - I am referring to $SO_2$ only. I am not sure about how to relate the leuco dye mechanism with this, but it seems to me that atmospheric oxygen aids in the replacement of the oxygen that is lost in the bleaching action of $SO_2$ , which gives back it's original color. What about the $Cl_2$ bleaching mechanism though, do you have any clue if it is possible to re-obtain nascent oxygen? Thank you.
– rev0
Mar 13 '21 at 16:48

Everybody knows chlorine ($$\ce{Cl2}$$) is a bleaching agent acting on oxidation. However, what majority of us do not know is sulfur dioxide ($$\ce{SO2}$$) is also a bleaching agent, which in contrast to $$\ce{Cl2}$$, acting on reduction (though we know about other reducing sulfur agents such as sodium dithionite and thiosulfate). The well-known Encyclopaedia Britannica simply explain it as:

Wood and animal fibres are bleached by acidic reducing agents such as sulfur dioxide. In the pulp and paper industry chlorine dioxide, hydrogen peroxide, sodium peroxide, sulfur dioxide, sodium bisulfite, and sodium hydrosulfite are commonly used.

Bleaching agents, regardless of oxidizing or reducing, are formulations that whiten or lighten a substrate by solubilizing color-producing substances or by altering their light-absorbing properties. The decolorizing reaction generally involves the removal of chromophoric sites in which electron delocalization over conjugated double bonds has made the substrate capable of absorbing visible light. The bleaching agent will typically react by irreversibly cleaving or adding across these double bonds (Ref.1). If the bleaching is done by oxidation, the conjugation is broken irreversibly (e.g., converting double bonds to diols), and permanently because there is no reducing agents in air. Even sunlight helps the oxidation, which is the chief bleaching method of textiles (a process known as crofting) used before the discovery of the element chlorine in 1774.

If the bleaching is done by reduction, it is a different story. According to Ref.1:

A few bleaching compounds act by chemical reduction; these include sulfur dioxide ($$\ce{SO2}$$), sulfurous acid ($$\ce{H2SO3}$$), hydrogensulfite ($$\ce{HSO3−}$$), sulfite ($$\ce{SO3^2−}$$), and dithionite ($$\ce{S2O4^2−}$$), as well as sodium tetrahydroborate (borohydride) ($$\ce{NaBH4}$$). Their application is primarily in pulp and textile manufacturing, where the bleaching action is thought to occur by reduction of a chromophoric carbonyl group. Other applications include the bleaching of glues, gelatin, soap, and food products.

These reduced chromophores would be regenerated slowly by the atmospheric oxygen (recall oxidation and reduction of indigo dye as Buttonwood explained in comment section). Thus, the bleaching by reduction is not permanent in most cases.

Late addition: After I submitted my answer, I have seen the conversation between OP and other commenters. I felt OP have not so correct idea of bleaching. Bleaching action is not removing oxygen or obtaining nascent oxygen. It is about the breaking the chromophore responsible for the color. I think I have explained the principle quite clearly in my answer above. Basically, regardless of bleaching of oxidation or reduction, it breaks the conjugation of the chromophore, which is responsible for the color (you want to get rid of). If it is done by oxidation, the color can't get back because no reducing agents in our atmosphere. Yet, if the bleaching is done reduction, the relevant chromophore would able to get its conjugation back with help of atmospheric $$\ce{O2}$$ (or $$\ce{O^.}$$).

Also, I like to point to a nice demonstration of bleaching by $$\ce{SO2}$$ here.

References:

1. Marianna A. Busch, Kenneth W. Busch, in Encyclopedia of Analytical Science (Third Edition); Alan Townshend, Editor-in-Chief; Elsevier Science Ltd: Amsterdam, Netherlands, 2019 (ISBN: 9780081019832).
• Hey, thank you for your answer. I would just like to include here that I have not misunderstood bleaching per se, I was trying to correlate oxidation and reduction with its permanence. I saw the visual in your link in the Late Addition and I have gotten the $SO_2$ bleaching part of it clear now, thank you.
– rev0
Mar 13 '21 at 17:41

The bleaching action of Chlorine is permanent because it involves the process of oxidation. $$\ce{Cl2}$$ reacts with water to produce nascent oxygen which combines with the material in question (by oxidising it, as pointed out by @Mathew Mahindaratne ) and makes it colourless.

The bleaching action of $$\ce{SO2}$$ is temporary because it involves the process of reduction. $$\ce{SO2}$$ removes oxygen from the coloured substance to make it colourless. Atmospheric oxygen slowly takes place of the removed oxygen, because of which the material regains it's colour.

• Hey, I'm sorry, I don't seem to get "why" this happens. I get it that bleaching with $Cl_2$ produces nascent oxygen, which makes the material in question colorless, but why? Is it because nascent oxygen cannot be re-obtained? In case of $SO_2$ it makes sense that there is an actual replacement of the original oxygen, so thank you for that, but for the $Cl_2$ part, I don't get it yet. Could you please elaborate on that if you know it please? Thank you
– rev0
Mar 13 '21 at 16:32
• Well, since the material gets oxidized with $\ce{Cl2}$ , and our atmosphere is an oxidizing atmosphere at present, it makes sense that the material would remain oxidized (and thus bleached). If our atmosphere was a reducing atmosphere (the Oparin-Haldane Theory states that early Earth had a reducing atmosphere), then intuitively, it would make sense that $\ce{SO2}$ would be a permanent bleaching agent and $\ce{Cl2}$ a temporary one. Mar 13 '21 at 16:52
• Woah that explains it well! Thank you. I read about the Oparin Haldane theory once you mentioned it. It seems very interesting.
– rev0
Mar 13 '21 at 17:01
• @Meta xylene: Regardless of oxidation is by $\ce{O2}$ or $\ce{O^.}$, it breaks the conjugation of chromophore, which is responsible for the color. Reduction do the same, but relevant chromophore would able to get its conjugation with help of atmospheric $\ce{O2}$ (or $\ce{O^.}$). Mar 13 '21 at 17:22
• Yes, which was my point. The oxidising atmosphere means that reducing bleaching agents only have temporary effects. I do agree that chromophore breakdown is responsible for it (wasn't aware of the same, thanks), but the question was about why one was permanent and the other temporary. But I'll edit my response to make it clear that it's oxidation and not nascent oxygen that does the bleaching :) Mar 13 '21 at 17:27