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Author: Subject: Sulfur trioxide from sodium hydrogen sulfate
garage chemist
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Sulfur trioxide from sodium hydrogen sulfate

With this preparation I think I have found the easiest, cheapest and highest-yielding method of sulfur trioxide production for the amateur chemist.

The idea of making sulfur trioxide by heating sodium hydrogen sulfate has been brought up since the beginning of discussion on oleum production on this board, more specifically here:
The old Oleum & SO3 thread
But as far as I know, I am the first one to have success with it.

Theory
Sodium hydrogen sulfate gives off water when heated at 300- 500°C and turns into sodium pyrosulfate:

2 NaHSO4 -----> Na2S2O7 + H2O

This in turn gives off SO3 at 680- 880°C:

Na2S2O7 -----> Na2SO4 + SO3.

The temperatures stated are the ones I observed, they were measured at the outside of the test tube.
In literature you will find very much lower temperatures- I assume those are the temperatures at which the corresponding decomposition just starts, not the ones at which the decomposition goes at a useful rate.

The two reactions are not cleanly separated. Even when one heats at 480°C until absolutely no more water vapor is being produced, as I have done, some concentrated H2SO4 will come over from 500- 670°C, no matter how slowly the heating is carried out.
The yield of SO3 coming over at 680- 880°C is still extremely good, though.

But what makes the NaHSO4 method so much better than the ferric sulfate method, which operates at a similar temperature, is the fact that the SO3 does not decompose to SO2 and O2 at all.
The equilibrium for the thermal decomposition and formation of SO3 indicates that ca. 70% of the SO3 should be decomposed at 800°C- see the diagram that I posted in the ferric sulfate method.

I cite Ullmann's Encyclopedia:
 Quote: Pure sulfur trioxide is in fact extremely resistant to thermal decomposition because of kinetic inhibition, even at elevated temperatures where thermodynamic equilibrium is shifted heavily toward SO2 + O2. However, certain catalytically active substances are able to increase the rate of equilibration substantially. In the presence of metals such as platinum or of metal oxides and sulfates (e.g., of iron, copper, and, of course, vanadium) the decomposition approaches equilibrium at temperatures above ca. 700 °C.

The ferric sulfate/oxide in my ferric sulfate method catalyzed the dissociation of SO3.
No iron or other heavy metal compounds are present in the NaHSO4 method, hence no dissociation of SO3 here.

This explains the unusually high yield of SO3 obtained from NaHSO4 and the complete absence of any SO2 smell.

Precursor

The precursor is technical grade sodium hydrogen sulfate, purchased from the pool section of the home store.
It is used for lowering the pH of swimming pools, and comes in buckets of 1,5kg.

Apparatus and Procedure

100g NaHSO4 were filled into a quartz glass test tube with joint, the same one I had used for the ferric sulfate pyrolysis.

After melting, the melt filled only half of the test tube.

This was then slowly heated in the tube furnace. At 300-450°C there was a strong evolution of water vapor after melting.

It was held at 480°C for 45min, after which the steam evolution had ceased entirely.

Upon further heating from 500- 670°C, small amounts of concentrated H2SO4 slowly distill off. To capture and condense the vapors, I placed a round-bottom flask in front of the test tube:

A quartz joint with a bent tube would have been much better to use, but I didn't have such a thing.

At 680°C, SO3 evolution started. A quartz glass extension was put on the test tube, and a glass joint adapter carrying a bent glass tube was put on the extension. The glass tube led into a cylindrical receptacle cooled with water from the outside, so this was essentially the same apparatus I used for the ferric sulfate pyrolysis.
Since the SO3 is pure and not diluted with SO2, no ice is necessary for cooling.

The SO3 had a tendency to crystallize inside the glass tube, potentially leading to plugging. To remedy this, I increased the heating power on the tube furnace so that the SO3 came over faster and at a temperature above the melting point of solid SO3. Here is a picture of the SO3 in the tube melting:

Some crystals of solid SO3 deposited on the walls of the receptacle, while the bulk of the product collects below the cooling water surface:

The evolution of SO3 was complete at 880°C. I briefly heated to 930°C to melt the sodium sulfate into a single chunk:

And this is the product after solidification- an astounding 23,8g of SO3, from 100g NaHSO4, a 71% yield!

Conclusion
I think the yield of the NaHSO4 method speaks for itself. This is a much better method than the ferric sulfate pyrolysis.

Adding to that, the precursor can simply be bought OTC instead of having to prepare the ferric sulfate precursor beforehand.

Now we have a high-yielding and cheap method of SO3 production.
This should open up lots of possibilities- the synthesis of TNT, which requires oleum in the last step, springs to mind.

I have already explained in the other thread how useful SO3 and oleum are for the synthetic chemist. Thionyl chloride, dimethyl sulfate, chlorosulfonic acid, Oleum as dehydrating agent etc... are just a few applications of SO3.

If you choose to make SO3, please make yourself aware of the dangers of this substance first.
The warning "Extremely corrosive" does not really express the dangers appropriately. Oleum destroys many plastics, like PE and PP, that are completely resistant to 98% H2SO4- only glass and fluoropolymers like PTFE are really safe.

The reaction of SO3 with water is explosively violent. SO3 must only be diluted with concentrated H2SO4!

SO3 and Oleum also create an extremely dense fog when handled in open air, due to reaction of gaseous SO3 with aerial moisture forming droplets of H2SO4. If using a fume hood, be aware that all this thick fog will come out the exhaust! Most filters are incapable of precipitating H2SO4 fogs!

[Edited on 24-3-2008 by garage chemist]

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DJF90
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Awesome write-up garage chemist. A couple of questions I'd like to ask though. Would a bunsen/other gas torch flame be sufficient, so long as the required temperatures can be achieved? What alternatives are there to the quartz test tube?
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I have not been able to reach the required temperature with a bunsen burner in my many experiments with NaHSO4 years ago.
I only got to the stage where conc. H2SO4 slowly distills off, never to the SO3 evolution.
If you can reach the required temperature with a burner, why not. But you should definately do the pyrosulfate formation separately in an open crucible beforehand, as quite long heating is necessary for this.

I am afraid standard borosilicate glass becomes soft at 600-700°C. Can anyone confirm this?

There is a glass called Supremax that can take temperatures of up to 1000°C, this would be very useful here. You will have to look for it, it certainly isn't common.
Quartz glass is more common, and a glassblower can make custom parts like this test tube and the extension from it.

A fired and glazed retort made of clay or porcelain would be another good container for the SO3 production. If you know someone who does pottery as a hobby, you could ask him/her to make such a thing for you, or make it yourself and have him/her fire and glaze it.
This could potentially be cheaper than a quartz glass test tube.

Metals are unsuitable as a container material as far as I know, as SO3 is a powerful oxidiser especially at elevated temperatures and already at room temperature.

[Edited on 23-3-2008 by garage chemist]

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DJF90
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I was at the hardware store the other day, and noticed the propane/butane blowtorches. On some of the packaging it listed the temperature that could be achieved by the torch. The ones I was looking at had temperatures of 1300C, 1370C and 1600C, and so should be more than sufficient for this purpose. The main problem for me would be the reaction vessel. I also believe that borosilicate glass starts to become soft in the range of 600-800C, rendering it unuseful for this particular use. I'm assuming any metal reaction vessel is also unusable due to the catalytic effect it may provide on the decompostion of the sulphur trioxide. Would "fire cement" (a refractory, probably good up to 1300C) be suitable for making a retort? Unfortunately I do not know the composition, but I would expect that it contains substances like alumina, silica and magnesia. These will react with the sulphur trioxide yes? What do you suggest to glaze the retort with? Sorry, I do not know much about pottery
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The stated temperatures on the blowtorches don't mean anything. They are just the flame temperatures.
I can easily melt thin glass tubes with my bunsen burner, but a 100ml flask with sodium pyrosulfate I can't get to 700°C with it.
What matters is the total heat output and the size of your vessel.

As a general rule, high temperatures are much easier reached with electrical heating than with a burner.
You need to get some Nichrome resistance wire, coil it around your vessel and thermally insulate it with kaowool, just as I have done with the ceramic pipe of my tube furnace.
Nichrome wire is good for max. 1200°C surface temperature, meaning at least 1000°C in the vessel.

Yes, the catalytic effect of almost all heavy metals makes metal vessels unsuitable here, in addition to the corrosion issues.

I don't know if fire cement would be resistant. I think it would be very difficult to make a gastight retort from this.

There certainly are forums dealing with pottery, as it probably is a more popular hobby than chemistry. Just search for one and ask there how they would make such a retort and a gastight connection to a glass pipe for it.
I don't know much about pottery myself either.

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microcosmicus
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The issue here is not so much temperature as heat.
Pretty much any flame is going to achieve 900C.
However, with an open flame most of the heat is
going to be lost to surroundings, so little actually
stays in the tube to heat it. The best remedy for this
is to make some sort of oven and confine the flame
in there --- you can make yourself a nice little
furnace to accomodate your test tube or retort
with some firebrick and furnace cement.

Fire cement is likely porous so wouldn't work too
well. Use it for building a furnace instead. As for
glaze, that is simply a layer of glass deposited on
the ceramic surface to make a non-porous layer.
I would suggest clear glaze over colored ones
just in case the colorant might leach out and
react (after all, we usually make labware from clear
glass) but, other than that possible glitch, there
should be no problem --- the glaze is not going to
react with SO3 any more than with the glass used
in the set-up above.

As for how to manufacture such a retort, you could
pinch the clay into shape by hand, turn it on a
wheel, or make a plaster mold and cast in with
slip. Then you would have to dry it, fire it, glaze it,
and fire the glaze. Just as with glassblowing, the
question here is whether you are willing to learn
a new craft and set up a shop with tools and
equipment (notably the kiln) or rather find (and
perhaps pay, at least worth a bottle of beer) a
craftsman to do it for you. Personally, I do both
glassblowing and pottery as hobbies parallel to
my scientific hobby and can say that, if you are
good with your hands and enjoy that sort of thing,
then doing it yourself is a good idea and doubles the
fun (both make and use the apparatus) but if
ceramics does not suit your style, you would do
better to find a potter or look around for a ceramic
item that could be used as a retort. The price of
equipment and materials spent in learning is not
likely to come out to less than finished items if
you only need a few things.. Also, if you want
to try your hand at making it, ther might be a "make
your own pottery" place where you can go and
make the retort and they will fire it for you.

[Edited on 23-3-2008 by microcosmicus]
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This is the best synth of SO3 I have ever seen on the net. I do not have such a nice heating setup, but could it be done with a normal test tube (16 mm wide, 16 cm length) made of supremax glass and heated with a big propane blow torch at the bottom? Although this does not work with 100 grams, it could work with 10 grams, making 2 grams of SO3, which I lead directly into some conc. H2SO4.

If I only could make a few tens of ml of 30% oleum, then I would be very happy with that. I can obtain oleum, but I have to pay EUR 50 for half a liter, and I have to pick it up in a place 250 km from my home. Not very good, so this cheap source of oleum would be very good.

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If you only need such small amounts of SO3, the P2O5/H2SO4 method would be a much better choice, as this can actually be done in normal glass flasks. 100g P2O5 in 75ml conc. H2SO4 are a good ratio I hear- look at the synthesis of chlorosulfonic acid on versuchschemie.de.

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The_Davster
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 Quote: Originally posted by woelen I can obtain oleum, but I have to pay EUR 50 for half a liter, and I have to pick it up in a place 250 km from my home.

Wow! Thats a pretty good price...we ordered some from aldrich at work, and it was several hundred dollars for 500mL.

I have the heat source and obviously the bisulfite, and I can get a quartz tube made.
The tube you used to lead the SO3 into the cooling vessel, you used quartz, I imagine a borosilicate tube would break or break the quartz tube due to the different thermal expansion coefficients?

I also wonder in terms of minimal quartz glass parts, if a small quartz beaker could be heated to the desired temperature, and a round bottom flask of cold water held a bit above the beaker would work to isolate small amounts of SO3

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No, the tube was ordinary glass. Only the test tube and the extension are quartz.

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woelen

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Price has changed since last time I checked. Now the minimum offtake is 1 liter of 65% oleum for EUR 107 excluding VAT.
Probably in absolute terms this is not expensive, but for me, this is quite a lot of money, which can buy me many other interesting items.

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Congratulations GC for developing this elegant method for making SO3! You continue to prove the high utility of your newly developed tube furnace where you also made pioneering innovations. It is tantalizing to think that such powerful reagents as thionyl chloride and chlorosulfonic acid will soon be within the reach of the home chemist.
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This is very interesting. Essentially its like the persulphate method because after the low-T phase both give you sodium pyrosulphate which actually decomposes to SO3. The difference being that the bisulphate dehydrates in the initial phase, whereas the persulphate liberates O2. The difference in yields between this and what you posted for the persulphate is large - I presume its entirely due to the later not being heated to nearly the temperature of the present method.

The difficultly with the method is its hard to get the quartz tube. There are several alternatives - I was wondering if you considered using an all iron construction, steel is almost impervious to conc. H2SO4 where its used as a lubricant, while it'll be interesting to see what effect it actually does have on the yield. My suggestion - not very much - contact with the walls is far less efiicient than with the Fe2(SO4)3 mix the gases had to pass through last time.

[Edited on 24-3-2008 by len1]
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Yes, you could simply use sodium persulfate instead of bisulfate and probably get more than 80% SO3 yield because the substance is anhydrous to begin with.
When heating sodium persulfate over the bunsen burner without H2SO4 addition I never got any SO3, simply because I did not get it hot enough. Now I know that persulfate would work extremely well without H2SO4 addition, it just needs MORE HEAT.
The H2SO4 addition seems to enable a small amount of the SO3 in pure sodium pyrosulfate to be released below 600°C- but only a small amount, not more.

The essence of this story is that you are going to miss a lot of interesting and useful things if the bunsen burner or a blowtorch is your strongest heat source in the lab.
Everyone who does experimentation on his own should make an electric heater capable of reaching 1000°C or more, it really pays off!

Im am absolutely confident that we will eventually achieve a valid and workable synthesis of Phosphorus if people would just start working on it again, and this time use electric resistance heating.
Just look at what BromicAcid has done in this field in the past, it reminds me a lot of my early experiments on SO3 production with NaHSO4! He -or someone else- needs to do those experiments again with an electric furnace.
And for Phosphorus, metal would be a suitable construction material for the retort.

For SO3, I cannot see iron or even stainless steel working though. Contrary to cold conc. H2SO4, hot concentrated H2SO4 attacks steel very strongly, by oxidising it, with evolution of SO2. SO3 would do the same.

[Edited on 24-3-2008 by garage chemist]

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microcosmicus
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 Quote: he essence of this story is that you are going to miss a lot of interesting and useful things if the bunsen burner or a blowtorch is your strongest heat source in the lab. Everyone who does experimentation on his own should make an electric heater capable of reaching 1000°C or more, it really pays off!

Or enclose the flame ---- a blowtorch can serve quite well as the burner
for a small furnace. Your typical plumber's torch is going to put out
a kilowatt or more of power, likely several when run at full blast, but
open flames are inefficient, with only a few percent of the heat produced
going towards heating the material in the retort. However, once you
enclose the flame in a furnace, the same blowtorch which could not
heat 100g of bisulfate all the sudden can easily melt 100g of brass.

As far as power output, flames are generally going to put out more
than electrical heat. Rather, the advantage of electricity is that it
can be controlled more precisely and that no chimney is required.
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Excellent work!! This is a cheap and effective synthesis for SO3, one I will probably do at home as well. The only limitation this method has (shared in common with your other attempts GC) is that it is a batch process and is not a continuous means of producing SO3. However, this could easily be scaled up and run inside an improvised kiln.

I firmly believe that stainless steel would be suitable for contact with SO3, at least at the lower end.I used it before when working with my vanadium pentoxide method. Borosilicate will not work at all for this, it stresses* from 600C on and is unusable for this synthesis. Vycor would work, and it's cheaper than quartz.

As a correction to garage chemist: PTFE, PFA, and FEP and other fluoromers are not suitable for use with SO3 at temperatures greater than 170 C. In my personal experience, significant charring occurred to both FEP and PFA tubing. PTFE handles it the longest, but will discolour. I have pictures of what it did to FEP tubing in the ''SO3 by catalysis thread'' along with my stainless steel setup which held up flawlessly to ~650 C.

@microcosmicus--it is true that enclosing your heat source makes a massive difference. For us at home, it is easily done with kaowool and kiln brick: I build improvised small 'furnaces' with a kiln brick as a base, and a rolled up bit of kaowool blanket. I highly recommend that anyone interested in high temp. work or catalysis, or distillation (I find more uses for this material every day) buy some kaowool ceramic insulation! It is extremely useful for many things, I can not emphasize that enough!

Edit: correction from S.C. Wack noted--at 600*C, all Pyrex formulations are not yet plastic, but they do begin to strain, and upon cooling may very well break. They do begin to melt from 850 Centigrade on (according to Corning's site), so around orange heat.

[Edited on 24-3-2008 by Fleaker]

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Seems to me the needed piece of kit is a small ( say 1 cu.foot?) electric ceramic kiln. With a top temp of around 1250C it would let you make a nice slip-cast porcelain retort, and with slight modification would become the heat source for the SO3 reaction as well.
Little kilns like this go for resonable money on ebay - usually being sold off by rich, middle aged women who have dabbled in ceramics, got bored, and are selling off all the expensive gear they've bought

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gc, can I ask where you got that quartz tube from? I've been searching google for the last 20 minutes for "quartz furnace tube", "quartz test tube" and the like, yet the closest i could find was a tube with a glass joint on one end and the other end was open. Not to mention it was only half of the size of yours
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I got this quartz test tube from german ebay. You have to search for the terms quarzglas reagenzglas on ebay to find the specific model I use- with joint 14/23, the one for EUR 18.50.

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Most universities also have glassblowing shops, and for money may make you things. I have seen some that sell glass flowers and glass swans and the like on the side.

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 Quote: Originally posted by DJF90 I've been searching google for the last 20 minutes for "quartz furnace tube", "quartz test tube"

First hit on google (20 seconds of searching) - 2.5x20 cm. test tube for $30. Maybe it's a location thing...Not so sure about 600C for Pyrex softening...another google challenge http://www.corning.com/Lifesciences/technical_information/te... ...maybe it weakens some to weight before that. As said before in other threads, the metalcasters on the net are quite familiar with builing gas-fired furnaces. The refractory on my older Lindberg is 2 layers of different kinds of material, held together with some grey mortar. The inner bricks look for all the world like ground-up perlite held together with some sort of binder, like typical firebrick from any local hobby pottery or welding shop supplier. But the outer layer planks look exactly like ground-up vermiculite. So I wonder if there are different ways to skin that cat, in making your own refractory brick instead of cement (maybe I should google?). Seem to be drifting off topic here. microcosmicus National Hazard Posts: 287 Registered: 31-12-2007 Member Is Offline Mood: spin up Yes, perlite or vermiculite held together with furnace cement makes for a good high-temperature insulating material. For a formula and further information see Wasser's page: http://web.archive.org/web/20010822084118/people.ne.mediaone... http://www.schundler.com/ccfoundry.htm http://www.schundler.com/ccfoundry.htm A coffee-can furnace made of this stuff powered by a blowtorch should be useful for heating retorts to generate SO3, P, etc. Personally, I prefer to use this stuff as insulation around an inner shell made of a harder refractory material. IIRC, vermiculite, it is not so good for high temperature applications as perlite because of steam issues. The annealing point of Borosilicate glass is 600C, and its strain point is 510C. Already at the lower temperature. the glass has become soft enough that it will sag and bend noticeably under pressure. By contrast, the strain point of Vycor is at 890C and that of pure silica is 950C. As for metalcasters, both 12AX7 and I happen to work with molten metal. Maybe I should make a publication here about furnace building one of these months. Fleaker International Hazard Posts: 1226 Registered: 19-6-2005 Member Is Offline Mood: nucleophilic @microcosmicus--I also do casting and metalwork. If you've not already joined, please join us at backyardmetalcasting.com/forums and abymc.com. There is ample material on that site on constructing many furnaces in far grander design than we've attempted here. I think it is a good idea to put up a post on furnace construction and proper materials. I am thinking more and more that this reaction could be conducted in an improvised setup--a reaction vessel buried in charcoal, with a louver to control the amount of oxygen (thus preventing overheating). Charcoal without a ready supply of oxygen will barely melt aluminum, which melts about 650 C, so it should work well Neither flask nor beaker. "Kid, you don't even know just what you don't know. " --The Dark Lord Sauron MagicJigPipe International Hazard Posts: 1553 Registered: 19-9-2007 Location: USA Member Is Offline Mood: Suspicious "Little kilns like this go for resonable money on ebay - usually being sold off by rich, middle aged women who have dabbled in ceramics, got bored, and are selling off all the expensive gear they've bought" That's funny you should say that as my Mom is a semi-rich, middle aged woman that dabbles in glass-melting (to make jewelry). She has a small kiln. One of these days, she will quit using it and I shall harvest the nichrome out of it. Anyway, what I wanted to ask is I have a piece of equipment that I bought for$25 from Ebay a while back. It is some sort of glass tube/probe (annealing?) oven. It works sort of like a toaster. The tube goes in and when the timer is done it pops it out. The max temp appears to be 750C. I wonder if I could modify this to make some sort of tube furnace? I know you need more information... I shall post a picture soon.

PS... Is this question in the wrong place? If so, I apologize and I will not object if it is moved.

"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think, free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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It has occured to me that H2SO4 mist is a known carcinogen on a par with benzene. The SO3 which is hard to condense must be the worst fom of this H2SO4 mist as the particles are tiny. Can anyone confirm.

 Quote: Strong inorganic acid mists containing sulfuric acid are known to be human carcinogens based on sufficient evidence of carcinogenicity from studies in humans that indicate a causal relationship between exposure to strong inorganic acid mists containing sulfuric acid and human cancer. Occupational exposures to strong inorganic acid mists containing sulfuric acid are specifically associated with laryngeal and lung cancer in humans.

Im beginning to lose some of my interest in this hobby, its seems I cant step left right or centre without fear of a carcinogen, a precursor, or a poison. In physics the scares were much fewer and far between.

PS I hope people dont think health questions are unrelated to the relevant chemistry. On a hobby site I think its paramount (and in professional literature its concetrated on far too little in my opinion).

[Edited on 4-4-2008 by len1]
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