We study reactions in 96 % sulphuric acid. The glassware in Pyrex and the stoppers in Teflon are cleaned very carefully but we always observe the slow and not reproducible appearance of a peak of absorption at 320 nm in the used sulphuric acid. To clean the glassware with piranha solutions does not avoid it. This peak disappears if we add an oxidizer but it is not SO2 which gives a peak at 280 nm. What could be the origin and the nature of this peak?
It is necessary to learn how obtained sulfuric acid catalyst. Probably appearance of a peak of absorption at 320 nm comes from the absorption of impurity dissolution of the catalyst.
Best regards.
Hydrogen sulfite does not have a maximum absorption at 320 nm as well. My thinking is that you might have metal contamination.
Professor Schmitz, I tried researching some monographs on sulfuric acid manufacture and purification to find some ideas to your problem. But that gradual appearance only in used acid seems very specific. Personally, I've had bad experiences with a few deliveries of sulfuric acid. It is rarely suspected, but it can cause a lot of trouble, so I got used to testing all acid we got. We always ordered p.a. quality from established suppliers, but there were a few cases where contaminations were very obvious. One batch even smelled of sulfur (rotten eggs) and it completely inhibited our BZ oscillations. The identity of that impurity remained unresolved, there was nothing in the certificates that could be assigned to this contaminant. As far as I know, the most abundant impurities in p.a. H2SO4 are oxidizable impurities assumed to be SO2, then NH4+, iron and lead. Lower grades can possibly contain some organic matter, and I also found that nitrogen-oxygen compounds may be present in acids manufactured by certain methods, but I never found this in certificates. As for the organic matter, there is an easy test. Heating the acid to fuming (with great care) will irreversibly decompose the organics and color will be manifested. Of course, p.a. grade acid should not give this at all. Another issue I have heard of is that if the acid is stored in a plastic container, the plastic is easily penetrated by volatile substances. For instance, if bromine or iodine, or their volatile compounds, are stored nearby, that could be trouble. That is all I can think of at the moment. I am not sure if it helps, unfortunately, I cannot help with something more specific. I wish you good luck with this issue! Best, Erik.
Dear Erik,
I thank you for your interesting answer. I don't believe that my problem comes from impurities in the H2SO4. The impurities you mention are oxidised rather quickly by 96% H2SO4 and give actually a peak at 320 nm. A new bottle of the analytical grade H2SO4 we use does not give this peak. However, if we add a very small amount of methanol or acetone, the peak appears after a few minutes. We have also performed a simple experiment. Let an open vial containing 96% H2SO4 without 320 nm peak close to another vial containing concentrated NH4OH. After a few hours, the H2SO4 shows a peak at 320 nm!
It is strange that this peak seems to have never been mentioned in the literature.
Best wishes, Guy
That is incredibly interesting, I will be following your progress on this, and I wish you the best of luck in resolving the problem! Best wishes, Erik
Cher Guy,
J'ai cherche la cause d'apparition du peak d'absorbtion a 320 nm. Je soupose qu'il s'agit d'une impurite de la fabrication de H2SO4 ou maime l'existence de SO3!! Et puis, j'ai trouve dans la literature que l'alchool amilique presente un peaq dábsorbtion meme a 320 nm! Mais, d'ou alcohol amilique libre dans H2SO4?
Je regret que je suis dans l'imposibilite de vous doner un bon reponse!
Avec la meme consideration,
Gelu
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