[T-bone]

Hi,

The question on how to mix Piranha acid (H2SO4 96-98% with H2O2 30%) lead to some heavy debating here in the office/lab. Maybe you guys can provide me with some new insights.

I was taught to "always add the acid... except when making piranha acid". Some googling will lead you to procedures from renowned universities (MIT, Princeton, Cambridge) where it is also advised to always add the H2O2 (30%) to the H2SO4 (96-98%). However, where I'm working now, people have years of experience in mixing large quantities of Piranha acid for SPM cleaning in semiconductor applications (e.g. photoresist strip) and they insist on adding the H2SO4 to the H2O2.

Granted, doing it 'my way' and adding water (70% of the peroxide) to the concentrated acid is highly exothermic. Given the lower boiling point of water, this is what is considered to be most 'dangerous' in the mixing process according to my peers.

I'm having difficulties figuring out why exactly the before-mentioned universities (and my own) insist on adding the H2O2, there are not too many real arguments to be found online afaik. I thought it would be because a small quantity of H2SO4 will dehydrate a large amount of H2O2 (in the early stages of mixing). If I'm not mistaken, 96% H2SO4 will dehydrate roughly up to 1:1 in volume of water before reaching full dissociation of the first proton (H2SO4 + H2O --> HSO4- + H3O+). High concentration (>50%) H2O2 is basically rocket fuel (HTP, >70%), which is potentially more dangerous in your beaker than the 'controllable' temperature increase from mixing the other way around.
I'm starting to doubt my train of thought here, especially because I don't know much about the mixing dynamics (eventually you will have diluted the H2O2 with H2SO4, so H2O2 will never be >50% in the final solution) and because H2SO4 will also protonate H2O2 to H3O2+ along with H2O to H3O+ etc.

Am I overlooking something obvious? Could you guys explain to me why adding concentrated H2SO4 to H2O2 instead of the other way around, is a bad idea?

Thanks in advance!

[Arkcon]

I'd like to help, but your block of text leaves me not knowing where to begin.  But lets try:

Do it the way you oughta, always add the acid to the water.

We've covered this topic many times on this forum.  Its very common for people to get bogged down with equilibrium at this point.  But that's just bizarre to me -- I guess school just doesn't teach the reason why.  We do this because of the heat released by sulfuric dissolving in water.  We want to drop the denser, more viscous liquid into the water, with its higher heat capacity to absorb the heat.

What happens if we revers that?  There's a chance the water will vaporize and spatter the sulfici acid acid around.  Yes, sulfuric is denser, and below the water, but the vaporizing water may still splatter it.

So what if you have a ton of sulfuric acid to dispose of, and you can't lift it over a lager vessel of water?  You add water to acid, carefully, avoiding spatter as best you can.

Now, what to do with piranha solution?  I don't know.  Its still mostly water, so you still want its thermal mass is still available for the heat of solution of sulfuric acid.  But maybe your group wants a different sort of control, maybe to not lose water or peroxide.  Or maybe you're already being extra careful, given that this is piranha solution, so it doesn't matter.

But your protonation of H2O2, even if it happens (I'd have to look it up,) is just a distraction.

[Dan]

I have never made or handled piranha, but will comment that it is indeed generally recommended to add peroxide to sulfuric acid (the reverse of the "normal" order of addition), even often stated that you should never do the reverse. I have never seen an explanation for this counter intuitive order of addition, so it is possibly just something people do because the same procedure has been copy-pasted all over the internet and that's what everyone does.

I thought it would be because a small quantity of H2SO4 will dehydrate a large amount of H2O2 (in the early stages of mixing). If I'm not mistaken, 96% H2SO4 will dehydrate roughly up to 1:1 in volume of water before reaching full dissociation of the first proton (H2SO4 + H2O --> HSO4- + H3O+). High concentration (>50%) H2O2 is basically rocket fuel (HTP, >70%), which is potentially more dangerous in your beaker than the 'controllable' temperature increase from mixing the other way around.

This - adding peroxide to acid minimises peroxide concentration during mixing - seems a logical rationalisation to me.

[T-bone]

Thanks for your replies!

I'd like to help, but your block of text leaves me not knowing where to begin.  But lets try:

Sorry about that, I just tried to be as detailed and complete in my explanation as possible.

Do it the way you oughta, always add the acid to the water.

We've covered this topic many times on this forum.  Its very common for people to get bogged down with equilibrium at this point.  But that's just bizarre to me -- I guess school just doesn't teach the reason why.  We do this because of the heat released by sulfuric dissolving in water.  We want to drop the denser, more viscous liquid into the water, with its higher heat capacity to absorb the heat.

What happens if we revers that?  There's a chance the water will vaporize and spatter the sulfici acid acid around.  Yes, sulfuric is denser, and below the water, but the vaporizing water may still splatter it.

I take it you're in the "add the acid" camp when mixing Piranha. 
Maybe this was not clear in my original post: what you mentioned above is exactly my point of what is considered the most 'dangerous' phenomenon by my colleagues. Don't get me wrong, if I didn't know about all the procedures from different universities stating it should NEVER be mixed in the 'regular' way, I would also add the acid. But, I guess there must be another phenomenon which is more dangerous than the highly exothermic process of adding the peroxide? My question is: what is the other phenomenon?

So what if you have a ton of sulfuric acid to dispose of, and you can't lift it over a lager vessel of water?  You add water to acid, carefully, avoiding spatter as best you can.

I agree. But in the specific case of mixing piranha acid, the sulfuric acid to peroxide ratio is roughly between 7:3 and 3:1 in most applications I know. So, in this case you're actually making it harder for yourself to add the acid since it's the larger volume. (Edit: just to be clear: my 'experienced colleagues' generally make larger volumes and "add the acid". Universities will probably have students make smaller quantities and add the peroxide)

Now, what to do with piranha solution?  I don't know.  Its still mostly water, so you still want its thermal mass is still available for the heat of solution of sulfuric acid.  But maybe your group wants a different sort of control, maybe to not lose water or peroxide.  Or maybe you're already being extra careful, given that this is piranha solution, so it doesn't matter.

But your protonation of H2O2, even if it happens (I'd have to look it up,) is just a distraction.

Higher starting temperature will definitely increase the peroxide decomposition, so that is something to consider in the sense of control. When adding the peroxide to the acid you will end up with a higher temperature solution. I'm not sure how high the impact would be on e.g. 'lifetime' or effectiveness in general. When mixing it in one or the other way around I mean. Good point though.

I think Piranha acid in general is not something to take lightly. All the procedures I can find, point out to BE extra careful. Although I must say I have noticed that when working in 'correct' environments (clean glassware, fume hoods, protective clothing etc.) it is very easy for people to underestimate the power of Piranha acid.

I have never made or handled piranha, but will comment that it is indeed generally recommended to add peroxide to sulfuric acid (the reverse of the "normal" order of addition), even often stated that you should never do the reverse. I have never seen an explanation for this counter intuitive order of addition, so it is possibly just something people do because the same procedure has been copy-pasted all over the internet and that's what everyone does.

I was considering the same thing, the copy-pasting. But where does it come from? There must have been some kind of incident I expect?
I can't seem to find serious incidents related to mixing in the 'wrong way', it's mostly about explosions when storing Piranha acid. Or accidentally adding acetone to the mixture (really bad idea...).


I really appreciate your input! So far, it looks like I haven't overlooked anything obvious.
Hopefully we can figure out the logical background of this seemingly counter-intuitive procedure. Or maybe we can remove a wet monkey story (or chemical myth, or whatever you want to call it) from our textbooks.

[cbrose]

https://www.drs.illinois.edu/SafetyLibrary/PiranhaSolutions

[davidtalaga]

I believe that the reason is that adding peroxide results in a higher yield of peroxysulfuric acid than the other way around. Peroxysulfuric acid is the active species in Piranha solution.