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.