Sulphuric cell.

[goldsilverpro]

That's what I thought when I first looked at it. If so, the solution was probably allowed to get too hot. Too much amperage.

 

[solar_plasma]

@Jason and Barren Realms

I don't want to spread panic. But being prepared for the worst case scenario is not the most stupid thing.

Barren Realms I am not sure you are right. In my logic the temperature would make it even worse. The only fact that might prevent small steam explosions would be, if the condensed "water" is actually more or less dilluted sulfuric acid.

Fact is, some people have blown their probe glasses (obviously a bad choice) and I can only see two possible reasons: thermal stress and/or condensed water drops. I might be wrong.

Whatever you guys do, be prepared and stay safe!

 

[goldsilverpro]

In the plating industry, the old standard method of analyzing gold in a cyanide solution was to use about a 5ml sample and "wet ash" it in 100-150ml of very hot concentrated sulfuric (FUME HOOD!!!). It takes awhile but the end result is that all the gold ends up quite pure on the bottom of the 250ml erlenmeyer flask (in this case, an erlenmeyer is safer than a beaker - splattering becomes less of a problem due to the pinched top of the flask). The gold is then separated (fairly dangerous in itself), rinsed, dried, and weighed. NOTE: I haven't given all the information on how to exactly do this, so don't try it!

The main purpose of this post is the SO3.

Most of the published instructions for the above method say to heat until "copious" white fumes of SO3 appear (they are obvious - you can't miss them). Copious means abundant or present in large quantities. When evaporating weak sulfuric to the point where most of the water is gone, a certain concentration/temperature level will start producing these dense white SO3 fumes, in VAST quantities. If you breathe them or your skin is exposed to them, they react with the water in or on your body to produce sulfuric acid (H2O + SO3 = H2SO4). What I'm saying is that you must take every possible precaution against contact with the white SO3 fumes. An excellent fume hood is an absolute MUST when evaporating sulfuric. I would never do this outside because you can't predict wind changes. Also, the SO3 will react with the water in the air and produce a heavy H2SO4 atmosphere. If you're going to evaporate sulfuric, first get a good well-designed fume hood. Please!

 

[Barren Realms 007]

You are correct in being prepared for the worse case scenario.

First off if you are evaporating the water off you should not have a top on your beaker or container. The top needs to be left off so the water vapors can escape other wise it lengthens the evaporation process.

Second fact any water that will condense will be on the insides of your beaker or container and will roll back down the inside of your container into your solution and they will be at or close to the same temperature as what the solution you are evaporating off is so there will be little temperature difference.

This is not a guess or something I read. This is from practical experience of doing the process.

Breaking beakers is always a posability. But if you use quality glass ware the posability of failures is less likely. Or if your glassware is badly scratched it should not be used in this process.

 

[solar_plasma]

so there will be little temperature difference.

This is not a guess or something I read. This is from practical experience of doing the process.

Since you have seen it in real, then it is no condensed water, but dilluted H2SO4. Another possibility that I haven't tried (and don't want to): 85% sulfuric won't act the same like oleum, not even like 98%. Maybe both explanations will fit. So, it must have been the thermal stress, that blew those guys' probe glasses.

When you dillute concentrated sulfuric, an exotherm process is taking place (I need to read it again, but if I remember correctly, it is a chemical reaction). It is exotherm, if the water is cold or hot. If both are hot, water will vaporize even quicker.

@gsp
Great post! Thank you!

A larger quantity of hot smoking H2SO4, that won't stop smoking until it has cooled down by itself must be the absolute horror for everyone, who is not prepared for this. That is, what I always try to say: reactive chemicals get exponentially harder to control in relation to the quantity! 10ml can easily get covered with sand. 1l not.

 

[Barren Realms 007]

That is not necessarily true, you will have some water that condenses on the sides and falls back into the solution till you have evaporated the water out of the acid.

That is correct that it was probably thermal stress that caused the failure of the container. Either from a scratched beaker, cheap glass ware or improper glass ware used to do the process.

Yes pouring water into the sulphuric acid will cause an exothermic reaction and possible eruption of the solution if a lot of water is poured into a small amount of acid. If it is just a drop or 2 in a large quantity of acid there is less of a possibility of this happening but it is still the possibility of this happening.

Pouring your acid into the water also causes an exothermic reaction and less of a chance of an eruption of your solution unless you pour a lot of acid into a small amount of water. You can counter react this exothermic reaction to a degree by adding ice to your water and cooling the temperature of the water then add your sulphuric acid to the water. You will still have an exothermic reaction just not as sever of a rise in temperature.

 

[solar_plasma]

That is not necessarily true, you will have some water that condenses on the sides and falls back into the solution till you have evaporated the water out of the acid.

There is also condensing H2SO4. Yes, sulfuric doesn't vaporize on low temperatures, but at 200°C it will.

Yes pouring water into the sulphuric acid will cause an exothermic reaction and possible eruption of the solution if a lot of water is poured into a small amount of acid. If it is just a drop or 2 in a large quantity of acid there is less of a posability of this happening but it is still the posability of this happening.

This is not correct, a drop can cause an eruption, though I can't remember at which concentration 18M or 10M, - or even both.

Pouring your acid into the water also causes an exothermic reaction and less of a chance of an eruption of your solution unless you pour a lot of acid into a small amount of water. You can counter react this exothermic reaction to a degree by adding ice to your water and cooling the temperature of the water then add your sulphuric acid to the water. You will still have an exothermic reaction just not as sever of a rise in temperature.

I do not know the explanation, but water is a much better heat conductor than conc. sulfuric, if I remember right. If so, this alone could prevent the violent reaction.

 

[rickbb]

Maybe I've misread some information in this post, (wouldn't be my first time). But;

For evaporating water off of sulfuric wouldn't it be safer to use a lower temperature and let it take longer?

I concentrate battery acid by using a 1L beaker in a sand bath in a slow cooker, (crock pot). At 190F and let it run overnight I can bring 1L of 30% battery acid down to 300ml that will not reduce any further, leaving a viscous, oil like concentration. All done in a fume hood of course and I never see any of the white SO3 smoke, just small amount of water vapor which gradually reduces in volume as it concentrates down.

I don't see a need to do this at 200C if just concentration is the goal. Was it recommended to separate the gold from the copper in the solution?

 

[solar_plasma]

:shock: I didn't know, it would work like this. Sounds comparably safe to me. Would be fine, if some seniors could evaluate this.

30% is by weight, not volume, so do you know the end concentration or density?

 

[solar_plasma]

I agree.

I try this drop thing on a little quantity of cold 18M in an erlenmeyer under safe conditions tomorrow and find out if it is a myth.

 

[solar_plasma]

I googled a little. At least for 98% sulfuric it is a myth, look here: https://www.youtube.com/watch?v=_f0vK8Wffgg

Though we weren't talking about 18M, I will try, if it won't work here either.

The drop thing is a widely believed myth and you can read it everywhere. Interesting. Thank you Barren Realms! (By the way, since I also use conc. sulfuric over 30 years, 19 years professional, this is obviously a bad argument, since I was wrong :mrgreen: )

 

[Barren Realms 007]

I would not consider you as being wrong. It's just a matter of learning the limitations of what we do.

Edit: I'm also not suggesting that anyone go and try this just for the fun or experience. That is when mistakes and accidents happen.

 

[jason_recliner]

I am not a chemist, nor do I play one on a weekly radio show. But I believe it's not a myth; it just needs the right conditions.

I found a you-kill-yourself video recently which I can't dig up right now; I'm at work. It had a guy in the desert trying to add water to all sorts of acids including small volumes of water into large volumes of concentrated sulphuric. He was measuring temperature rise, and while he got some, everything stayed below about 170°F I think (about 80 for the rest of us).

What he failed to try was to put water into hot acid. As a firefighter I know only too well what happens when well-intending people throw water onto an oil fire. You can quite literally lose face doing this. It's reasonable to expect if water is dropped into acid over 100°C it quickly becomes boiled and will similarly steam and spurt nasty, hot acid everywhere. It's not the acid spurting. It's the water.

Sulfuric acid will adsorb water through its surface (or absorb it at the surface? Whatever). But I think I can say that presence of a fine fog is not going to acid to spray everywhere, rather gently dilute it while also providing some cooling. But even if I believed it were safe to drop small amounts of water into sulphuric acid, I would still probably not be willing to make any definite statements to the effect.

Now, GSP made an excellent point about the SO3 cloud. I had considered that, but only partly, out of ignorance. For me it was just "don't want any sulphuric vapours". Further reason to not evaporate to 90%. In my case, I may or may not have lost some acid to vapour, outside, over about 2 hours. At no time was there any condensation visible on the sides of the spotlessly clean borosilicate beaker.

Part of my contingency plan was a long stick to turn off the gas if required from upwind (and that method tested), plus a charged hose nearby preset to fine fog/mist. As scarey as it sounds keeping water on hand for acid, my plan was for in case of something worse than a bit of acid splash. Though I did not know it fully at the time, this fog would also apparently be good for reducing SO3 to localised acid rain, which I'd rather on my property than in my skin. Not that I likely got anywhere near massive clouds of SO3 at 78%.

I can't say I would recommend evaporating sulphuric acid if it is at all avoidable.

Also, taking a look at my settled powder, it does look an awful lot like copper...

 

[solar_plasma]

Also being a hazmat firefighter (doesn't mean much, we have a lot of books we use before doing ANYthing) I agree with you on throwing down hygroscopic vapors always might be considered a plan. I do not agree on spraying water on the surface (even if it would work). It would free extra energy. And it would be very hard to argument for having done this in front of any court, which only will look at the [stt]W[/stt] in the hazmat diamond. If it is a closed metal tank without leakage, cooling the outside from a safe distance might be investigated as an option. In your experiment I would still see sand as the state of the art.

edit: On the other side, right is, who has succeeded. So, it is hard to determine if it is this or that and not one accident is like the other. We have a saying that translated says something like "Live the case!" (=Lebe in der Lage!)

 

[solar_plasma]

I tried drops an splashes of water on what once has been 18M (I doubt it still is, since it is a maybe 20 years old bottle, need to measure) and nothing happened beside it got warm. So, I learned something new.

Nevertheless, don't try this at home. As jason said, it might be something else under some circumstances.

 

[Barren Realms 007]

:lol: See, it is possible to teach an old dog new tricks.

 

[solar_plasma]

When we stop getting better, we're not good anylonger.

 

[rickbb]

30% is by weight, not volume, so do you know the end concentration or density?

No, I don't have the means, (or knowledge) to accurately measure the concentration. But it looks and works like 98% that I have purchased.

 

[solar_plasma]

If you know the weight of a given volume, you can calculate the density. Then you can find the respective density in a chart density vs. concentration.

weight (g) / volume (ml) = density (g/cm³)

 

[jason_recliner]

I used this: http://www.sschemical.com/wp-content/uploads/2013/05/Conversion_Table.pdf
Although on looking at it again I realise I made an error. I accidentally read from the "% 66 Baume" column instead of "% H2SO4". So 78% of 93% acid. Understanding the high concentrations takes a bit of getting your head around for new players!
So my evaporated acid was only ~73%, not ~78%, which is becoming a fair way short of the ideal 85%. No wonder I have a copper powder collection!