6
$\begingroup$

In this paper it is mentioned that

... octenidine can release iodine radicals from the PVP complex, resulting in a tissue irritation as well as strong brown to violet discolouration in the border areas.

but elsewhere I read that

... it turns out that ΔG > 0 for radical iodination.

Is it really plausible, that octenidine releases iodine radicals from povidone-iodine?

I have seen and experienced the purple discoloration of combining octenidine with pvp-iodine. The stain parts after a few days. However, I have not found any reports of skin irritation so far, except for the quote above. Iodine radicals could cause skin irritation, so if the first statement ist true, that would at least in theory support the hypothesis that skin irritation could occur.

Bonus question: How would you explain the purple colour of the reaction?

Disclaimer: I am a medical doctor interested in understanding the chemical background of the drug interaction of these two bactericidal skin disinfectants, which is often warned about on the ward.

-- EDIT: Additional background info, helped me to understand the problem:

This journal article mentions in its introduction:

Use of iodine as an antiseptic has gone into disrepute as a result of its short lasting action and irritant properties. Strong solution of iodine is corrosive and can cause blistering and necrosis of skin, commonly referred to as chemical burns or irritant contact dermatitis. Iodine has thus been replaced by substances known as iodophores that contain an iodine molecule linked to a large molecular-weight organic compound. A slow release of iodine from the polymer contributes to a long lasting action and a low irritant profile of such compounds. Povidone iodine is one such iodophore that has poor irritant property despite preserved antiseptic efficacy, and hence is advocated as a non-irritant, non-toxic compound for surgical scrubbing and disinfection of endoscopes.

$\endgroup$
6
  • 1
    $\begingroup$ en.wikipedia.org/wiki/Octenidine_dihydrochloride $\endgroup$
    – Karl
    May 22 at 14:03
  • 1
    $\begingroup$ An antiseptic that is only safe to apply to the intact skin, do I get that right? Why would I ever mix it with PVP-iodine? (I'm a chemist, and just asking. Planned surgery?) $\endgroup$
    – Karl
    May 22 at 14:18
  • 1
    $\begingroup$ Hi @Karl, yes that's correct. Octenidine is commonly used as an alternative to other skin desinfectants, like PVP-iodine and chlorhexidine. It is only safe to apply to the intact skin, because in deeper tissue it can cause necrosis of the tissue, which would then have to be resected surgically. PVP-iodine is contraindicated for neonates and rarely used in the oral cavity. Octenidine and PVP-iodine shouldn't ever be mixed, but it does happen by accident every now and again. $\endgroup$ May 22 at 20:47
  • 1
    $\begingroup$ Thnx. I have no clue whatsoever why the described should happen. "Radicals" is most certainly nonsense. If copious amounts of hydrochloride strongly hydrolyse the lactame group in PVP, elemenal purple iodine will be freed. $\endgroup$
    – Karl
    May 22 at 21:35
  • $\begingroup$ What do they mean by "brown" discolouration? PVP-Iodine is very brown to begin with! (I get it, dicolouration from brown to violett.) $\endgroup$
    – Karl
    May 22 at 21:37

2 Answers 2

4
$\begingroup$

Elemental iodine (not iodine radical) is purple, and some iodine complexes are similarly colored: with starch, it's dark blue, almost black; with chitosan (a component arthropod exoskeletons) purple. Iodine's color also appears different in differing solvents.

You might experiment with a dilute solution of PVP-iodine and starch as a control, and with dilute PVP-iodine mixed with octenidine dihydrochloride (a surfactant, like soap) and starch, and see if the added surfactant makes for a much darker blue-black color. If so, that would indicate elemental (molecular) iodine is being freed from the PVP complex by octenidine dihydrochloride.

$\endgroup$
2
  • $\begingroup$ Thank you for the answer, would you agree with my interpretation of the second source, that it's more likely elemental iodine and not iodine radicals that are released from the complex? $\endgroup$ May 22 at 20:49
  • 1
    $\begingroup$ Iodine radicals do not hang around long enough to see a color in solution. There are some stable radicals, such as nitric oxide (which is a vasodilator, en.wikipedia.org/wiki/Nitric_oxide), but not iodine, wiley.com/en-us/… $\endgroup$ May 22 at 20:59
1
$\begingroup$

Reactions with $\Delta G>0$ most certainly do occur; in fact green plants literally run such a reaction, conversion of carbon dioxide and water to sugar and oxygen, all day.

The catch, of course, is you must put in work energy (not just heat energy) to run the reaction, which is of course the definition of "nonspontaneous". In the case of green-plant photosynthesis, the work energy is provided by sunlight, whose photons carry much higher energy than would be the case for thermal radiation at equilibrium.

Work energy can thus be provided by light. Or it can be provided by an auxiliary chemical reaction, which is certainly possible on skin. Identifying this energy source precisely in the case of this question would require more background information from the OP.

$\endgroup$

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.