5
$\begingroup$

I have blue litmus paper, and if I put it in an acid it turns red. I also have red litmus paper, and if I put it in a base, it turns blue. I know the question about litmus was asked and answered and I like the answer: How does the litmus pH indicator work? However, damp litmus paper also becomes bleached in presence of chlorine gas. I believe it has nothing to do with the mechanism described in aforementioned question. I wonder what the mechanism is. Wikipedia actually suggests:

For instance, chlorine gas turns blue litmus paper white – the litmus dye is bleached, because of presence of hypochlorite ions. This reaction is irreversible, so the litmus is not acting as an indicator in this situation.

I also found an equation which describes formation of hypochloric acid (source): $$\ce{Cl2 (g) + H2O (l) -> HClO (aq) + HCl (aq)}$$

The problem that I have now is as follows: We know that litmus paper gets discolored because of contact with hypochloric acid. But what is actually the chemical reaction for that?

|improve this question
$\endgroup$
  • $\begingroup$ I'm not sure about this (plus it doesn't give sufficient details) so I am not posting this as an answer: Litmus is a mixture of dyes extracted from lichens, and most of these dyes/indicators have colour due to a extended pi conjugated system. The acid-base environment alters the pi conjugated system and causes a shift in absorbance, causing colour change. The HOCl is the bleach in the reaction, and it probably functions by destroying the extended pi conjugated system. $\endgroup$ – getafix Mar 23 '16 at 4:41
  • $\begingroup$ Azolitmin is known to be one of the major components of litmus: azolitmin I have no access to SciFinder at the moment, but I do recall hydantoin imides to be exquisitely sensitive to oxidizing agents, and I don't see azolitmin being an exception. $\endgroup$ – user28178 Mar 23 '16 at 5:53
  • $\begingroup$ @tovok Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. I have converted your answer into a comment for now. With a bit more rep, you will be able to post comments on any question/answer. $\endgroup$ – Martin - マーチン♦ Mar 23 '16 at 8:20
  • $\begingroup$ I just came across a mention of the easiness with which the double bond is disrupted by halogens, in the 10th grade textbook by Rudzitis (Russian). Nice. $\endgroup$ – CowperKettle Mar 24 '16 at 9:48
3
$\begingroup$

I can supply some details now, and hopefully this ought to qualify as an answer.

As I mentioned earlier litmus is a mixture of 10-12 dyes (CAS number: 1393-92-6).

The acid-base indicator properties of litmus are primarily due to 7-hydroxyphenoxazone chromophore (pictured below) enter image description here

The answer you linked to discusses the acid-base indication mechanism in some detail, so I shall skip over that.

Anyway, what this serves to establishing that it is indeed a extended $\pi$-conjugated system that we are dealing with in the chromophore.

Now, $\ce{HOCl}$ would bring about halohydrination (basically an electrophilic addition) across the $\pi$ bonds, thus disrupting the conjugated system.

Halohydrins are compounds that contain an $\ce{-OH}$ and $\ce{-X}$ groups on adjacent carbons.

This image describes the general mechanistic scheme in a simpler case: enter image description here

Anyway, after said addition reaction has taken place in the chromophore, the $\pi$-conjugated system is disrupted, and thus, we see an absence of colour.

|improve this answer
$\endgroup$
  • $\begingroup$ Thank you, getafix! Did I understand you correct, that there is no reaction with HOCl, but instead there is a reaction with Cl2, followed by reaction with H2O? $\endgroup$ – Sleepy Hollow Mar 23 '16 at 12:15
  • $\begingroup$ Well, the chlorine+water is furnished by the HOCl (so to speak). The reaction of chlorine you put down with water in your question is reversible. $\endgroup$ – getafix Mar 23 '16 at 12:26
  • 3
    $\begingroup$ @SleepyHollow - the reaction is probably with the hypochloride ion (ClO minus), since HClO dissociates in water. Here's a nice description of electrophilic addition, showing how the breakdown of the pi-bond occurs and what follows that. $\endgroup$ – CowperKettle Mar 24 '16 at 9:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.