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Caro's Acid-an archive

Hermes_Trismegistus - 2-4-2004 at 08:04

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Equilibrium producing Caro's Acid

H<sub>2</sub>O<sub>2</sub> + H<sub>2</sub>S0<sub>4</sub> <--> H<sub>2</sub>SO<sub>5</sub> + H<sub>2</sub>0
Reaction producing peroxyacetic acid

H<sub>2</sub>O<sub>2</sub> + CH<sub>3</sub>COOH --> CH<sub>3</sub>COO-OH + H<sub>2</sub>0

[Edited on 2-4-2004 by Hermes_Trismegistus]

Hermes_Trismegistus - 2-4-2004 at 08:10

Piranha bath/ Bomb Piranha bath, otherwise known as bomb or Caro's acid, is a mixture of sulfuric acid and hydrogen peroxide with a little water. There are various recipes. As the name suggests, this is a very aggressive mixture, potentially unstable if used carelessly, so only make it up and use it in a proper chemical workstation. The webmaster prefers to use a lower strength H2O2, say 10 or 20 volume, concentrated sulfuric acid, and to add no extra water. Put the peroxide in the beaker first, and pour in an equal volume of the acid very slowly. It is best to use a beaker of large enough cross-section so that the substrate can lie flat, and to have a total depth of etchant of less than a centimetre.

Piranha bath is usually heated to boiling point. Add the substrate to the beaker and then turn on the heater at a low setting. Typically, the substrate is left immersed for a time from 5 minutes to an hour. It is best to use teflon tweezers, as piranha bath attacks tweezers made of other polymers or nickel.

Hermes_Trismegistus - 2-4-2004 at 08:18

Caro's acid: permonosulfuric acid (i.e., peroxymonosulfuric acid), H<sub>2</sub>SO<sub>5</sub>, first prepared by Heinrich Caro in 1898

Caro's acid (peroxomonosulphuric acid, H<sub>2</sub>SO<sub>5</sub>;) is the peroxide of sulphuric acid, a powerful oxidant and a dangerously explosive mixture with certain organic compounds


Two peroxo acids of sulphur are- peroxomonosulphuric acid, H<sub>2</sub>SO<sub>5</sub>, and peroxodisulphuric acid, H<sub>2</sub>S<sub>2</sub>O<sub>8</sub>.-

The peroxodisulphate is an oxidising agent and a free radical generator.

[S<sub>2</sub>O<sub>6</sub>]‾² + 2<sub>e</sub> ® <sub>2</sub>SO<sub>4‾² ; E° = 2.01v

Peroxomonosulphuric acid, also known as Caro's acid, is prepared from the hydrolysis of the peroxodisulphuric acid.

O<sub>2</sub>S(OH)-O-O-S(O)<sub>2</sub>(OH) + H<sub>2</sub>O ® O<sub>2</sub>S(OH)-O-O-H + H<sub>2</sub>SO<sub>4</sub>

A more-effective way to make Caro's acid
A new process for making peroxymonosulfuric acid (H<sub>2</sub>SO<sub>5</sub>;), commonly known as Caro's acid, has made its commercial debut at FSTI in Austin, TX. Where the powerful oxidizing agent is being produced for use in the semiconductor industry.

Our Caro's acid generators, reacts 70% hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>;) with concentrated sulfuric acid, but it has an 80% yield, compared with 30-45% for conventional methods. This is achieved by keeping the temperature of the highly exothermic process below 30 degrees C, above which the H<sub>2</sub>SO<sub>5</sub> quickly decomposes. FSTI controls it mainly by means of a specially designed heat exchanger that is inert to the extremely corrosive reaction mixture.

Other processes do not control the reaction temperature, so the weaker acid solutions they produce cannot operate with the same efficiency.

Mendeleev - 8-4-2004 at 20:33

I assume Caro's acid undergoes 100% dissociation? Also could it be used as a souped up nitration catalyst replacement for sulfuric acid?

tom haggen - 27-10-2004 at 17:06

I'm having a hard time understanding what gets the reaction started. Is it heat or is it cold?

I'm sorry, I thought I was clear on that point.

Hermes_Trismegistus - 27-10-2004 at 17:13

The reaction proceeds when the two reagents (liquid) are MIXED together.

I don't know how I can express the importance of slowly, heat is produced.

I do recommend however, that you wear eye protection. pirahna bath spatter + eyeballs = ray charles.

tom haggen - 27-10-2004 at 17:32

This is achieved by keeping the temperature of the highly exothermic process below 30 degrees C,

Sorry i'm an american. I see 30 degrees and I think cold. wasn't really paying attention. you and your damm metaphors:P By the way whats that reaction you have in your first post with acetic acid and hydrogen peroxide? Do you think it will store in an HDPE container?

[Edited on 28-10-2004 by tom haggen]

[Edited on 28-10-2004 by tom haggen]

Hermes_Trismegistus - 27-10-2004 at 18:26

Originally posted by tom haggen
Sorry i'm an american. ...........whats that reaction you have in your first post with acetic acid and hydrogen peroxide? Do you think it will store in an HDPE container?

I think it's about damn time somebody apologised for that!!!:D

and the peroxyacetic is just an acetic analogue of caro's acid (think of it as acetic's evil in a B movie)

That would certainly pep up the ex wife's italian dressing for her!

Hermes_Trismegistus - 31-10-2004 at 12:55

Caro's acid

Further Research indicates that peroxyacids aren't stable for any real length of time and should be made in situ, also the reaction requires high test acids/peroxide and low heat, and that they must be made with an almost overwhelming excess of peroxide as some peroxide is destroyed during conversion.

Because of this, the stronger the peroxide, the less dilute the final sol'n, the better the chances of conversion.

tom haggen - 1-11-2004 at 11:41

I synthesized some of this acid last weekend. I used one to one ratio of H2O2 to H2SO4, and kept the temperature below 35 degrees celcius. I have to say that it was very difficult to keep the temperature down. Also I was using this chemical on some American coins, and the results were violent sizzling sounds. Maybe next weekend I can post some pictures. On one of the quarters I did. I left it over night, and it left crystalline like patterns on the coin.

[Edited on 1-11-2004 by tom haggen]

Pyranha Solution...

countrychemist - 2-10-2005 at 15:19

This summer in a research lab, I used a solution that we commonly called Pyranha Solution. It was prepared by mixing in a 3:4 ratio of Concentrated H2SO4 and 30%H2O2. It was an excellent cleaner for the glassware I was using. I am curious what this acid really is...does the H2O2 continually replace the H+ as it is used from the H2SO4 or does it make another compound like a persulfuric acid ?? Just curious because I am wanting to use it for a heat of neutralization experiment for Physical chemistry... Thanks

Blind Angel - 2-10-2005 at 15:27

It make Peroxysulfuric Acid, H2SO5, search on this forum for Caro's Acid, i think there is a good thread on it already.

Fleaker - 2-10-2005 at 17:22

Peroxysulfuric, Caro's acid, Pirrahna solution are just a few names for it. Excercise caution while using it, it's not called Pirrahna solution for naught, it will digest your flesh with remarkable speed.

I should like to note, peroxysulfuric acid is a great and controllable way to make elemental bromine and iodine from their respective halide salts.

Chris The Great - 2-10-2005 at 20:09

I always just pour my 35% H2O2 into an equal to double volume of sulfuric acid and swirl it to mix. Tada! It bubbles alot from decomposition however, but dissolves anything organic (leaves, branches, tables) in seconds! I found out copper induces violent decomposition however, I wanted to see it eat copper and it merely boiled up all over my table and ate a bunch of deep holes in it! However, the wood was very clean after the charcoal was washed off.....

Safety gloves and the like are a MUST! I have no doubt it would eat holes right through your skin before you made it to the sink.

A solution this concentrated and hot is not needed for cleaning glassware but it sure is cool to watch it completely dissolve a chunk of organic matter into nothing in mere seconds!

Would a controlled formation of peroxysulfuric acid be a good alternative oxidizer for some reactions, for example replacing KMnO4? (With good cooling of course!)
For example instead of using KMnO4 to oxidize toluene to benzoic acid, use dilute sulfuric acid and slowly add H2O2 while stirring and keeping the temperature down? Hope this isn't too off topic.
It looks like a cheap alternative for those of use who can't easily get stuff like KMnO4 because of the war on drugs but can nab a liter (or four) of concentrated H2O2 at the local store.

Swany - 2-10-2005 at 20:46

Personally, I love this reagent. It is fairly easy to make, and when the temp. is kept below 30C and 60% H2O2 is used, It is patently insane. It is amazing how much fun one can have by testing the mettle of everyday objects and materials with this... stuff. Mums petunias were no match. :P

I think it would be an interesting alternative to the KMnO4, although KMnO4 is easy for me to get. It would be extremely tough to add it to anything organic and not make it procede to react with as much vigour as it pleases. It would be a very interesting, non-conventional, oxidiser.

A little history

bio2 - 2-10-2005 at 22:36

.........It would be a very interesting, non-conventional, oxidiser............

Actually quite conventional.

100 Years of Baeyer–Villiger Oxidations
Michael Renz[a] and Bernard Meunier*[a]

The transformation of ketones into esters or of cyclic ke-
tones into lactones by peracids was discovered as early
1899 by Adolf von Baeyer[1] and Victor Villiger[2], when
they were working on the ring cleavage of cyclic ketones
(terpene derivatives).[3] On the treatment of menthone, car
vomenthone, and camphor with a new oxidant without sol-
vent for 24 h at room temperature, the corresponding lac-
tones of menthone and carvomenthone were obtained
40-50% yield and 15-20% of the starting material was re-
isolated (Scheme 1). For camphor, the mass balance was
lower than 50%. Additionally, they mentioned that this new
oxidation reaction works also with several small-ring ke
tones, but the corresponding lactones could not be isolated
and no further experimental details were given.[3,4] The
newly discovered oxidant that they employed was potas-
sium monopersulfate (KHSO5). Their attention was drawn
to it by a publication of Caro the year before.

In 1898, Caro[5] detected a nitrobenzene odor in the oxi- cardation
of aniline with ammonium persulfate [(NH4)2S2O8]
(persulfate stands for the symmetrical peroxodisulfate
22).[6] This product had never been observed before
not with that oxidant nor with any other oxidizing agent.
So he looked for an impurity in the ammonium persulfate
and found out that the addition of ammonium or potass-
ium persulfate to concentrated sulfuric acid produces an oxidant
which converts aniline into nitrosobenzene. He con-
cluded that the persulfate was protonated by sulfuric acid
and then transformed into the “Nitrosobenzol liefernde
Substanz” (nitrosobenzene producing substance; see Table
1 for a correlation between historical, mostly German
names and current English ones of peroxide derivatives and
peracids). Furthermore, he confirmed that electrolyzed sulfuric
acid did not contain any persulfuric acid after two days
but the “Nitrosobenzol liefernde Substanz”.

Baeyer and Villiger then reported the first preparation of
this new oxidant and explored its composition. The “dry
reagent” was prepared by adding 11 g (0.11 mol) of concentrated
sulfuric acid to 10 g (0.037 mol) of a potassium per-
sulfate powder in a mortar. After 10 min, 30 g (0.17 mol)
of potassium sulfate was added and the mixture grated until
a very dry powder was obtained. The salt had a composition
of KHSO5 · 7 KHSO4 · 2 K2SO4 and was stable in
the absence of humidity. Additionally, they prepared a solution
of this reagent by addition of sulfuric acid and water
which was used for the camphor oxidation.[7]
During their studies on the structure of Caro9s reagent,
for which they suggested the formula KHSO5 in their second
publication,[8] they looked for comparable oxidants
and discovered en passant the organic peracids.[9] Baeyer
and Villiger treated dibenzoyl peroxide with sodium ethox-
ide and the sodium salt of perbenzoic acid precipitated as
colorless powder (Scheme 2).[4]

agent_entropy - 24-9-2006 at 20:42

Originally posted by Fleaker
I should like to note, peroxysulfuric acid is a great and controllable way to make elemental bromine and iodine from their respective halide salts.

Could you provide the details of such a procedure?