Sciencemadness Discussion Board

Strong acids

cnidocyte - 14-2-2011 at 14:16

I like this one http://en.wikipedia.org/wiki/Trifluoromethanesulfonic_acid
They call it trifilic acid. This stuff reacts with alcohols to form ethers and alkenes.

Heres a cool one
http://en.wikipedia.org/wiki/Fluoroantimonic_acid
even the molecular structure scares me.

Flourosulfonic acid is a nice one too
http://en.wikipedia.org/wiki/Fluorosulfonic_acid

peach - 14-2-2011 at 14:52

Superacid

Superbase

Lewis acid / base theory

garage chemist - 14-2-2011 at 16:02

This reminds me that I've been wanting to prepare fluorosulfonic acid for quite some time.

Brauer gives the method of adding HF gas through a steel capillary to liquid SO3 contained in an aluminium vessel. Simple and quantitative, but I have no means of generating anhydrous HF gas. Same for the reaction of ClSO3H with HF.

A more convenient method is the reaction of KHF2 (potassium bifluoride) with oleum containing 60% SO3. The FSO3H is distilled off in a glass apparatus- very convenient.
The problem is that I don't have any KHF2, and only a very small amount of 40%HF that I don't want to use up.

Now, in the FSO3H wikipedia article, it is said that CaF2 may be used instead of KHF2. This would be perfect if it worked.
I also have quite some NaF, which could perhaps also be used if CaF2 turns out to work.
The following reference is given:

Cotton, F.A.; Wilkinson, G. (1980) Advanced Inorganic Chemistry: A Comprehensive Text, 4th Edition, p.246, Wiley-Interscience Publications.

Can anyone with access to this reference provide an excerpt?

I want FSO3H because it can be used to make perchloryl fluoride, FClO3, by simple heating with KClO4 (Brauer p. 166, "Chlorine Trioxide Fluoride").
I read about this compound in the book "Ignition!" (downloadable in the Library), where it is hyped as a major discovery in the field of storable high-energy liquid oxidisers for liquid-fueled rockets.
It is a remarkably stable and unreactive gas (can be washed with aqueous NaOH, does not decompose in glass at 600°C)
and quite easy to handle compared to other halogen-based rocket oxidisers.

Fleaker - 14-2-2011 at 20:00

I'd trust Cotton and Wilkinson. One's a Nobel laureate, the other certainly deserved it (but was a huge grump!). I'll get you the reference, I have a copy of the red book.

I'd imagine that you could make FSO3H in much the same fashion as HF (i.e. distillation setup), what with the product being removed.

Here's my question to you, why not heat CaF2 with conc. sulfuric acid and distill the HF into the oleum? Or is the idea to do this all in glass? How easily does KHF2 lose its HF?

This is mad science at its best.

garage chemist - 14-2-2011 at 21:23

The idea is to do it in glass, and without generation of gaseous HF as intermediate, and without KHF2.
Looking forward to the reference.

Jor - 14-2-2011 at 22:16

A very crude method not involving any complex apparatus made of materials inert to HF:

Just put NaF in a cheap erlenmeyer and warm it, put a rubber stopper on it with a plastic tube leading into water, and add H2SO4 dropwise. After as much as possible HF has been absorbed, titrate the acid, and add 0,5 eq. of KOH. The crystallise the KHF2. That's how I would do it. Or use this HF generator to bubble it straight into oleum, with a plastic drying tube in between (these are cheap).
Yes you will lose your erlenmeyer or it will be severely damaged, but if you do it on a large scale you can get this valuable reagent at the cost of some cheap glassware.

woelen - 14-2-2011 at 23:34

Are you sure that the HF is of acceptable purity? I expect that quite some SiF4 will be formed as well and this will lead to formation of SiF6(2-) ions in aqueous solution. The FSO3H formed with the resulting impure KHF2 may contain quite some H2SiF6. I'm not sure how difficult it is to remove this impurity.

Making HF can be done at much lower cost and does not require the use of the relatively expensive and hard to get NaF. Just use potteries grade CaF2 and gently heat this with conc. H2SO4. HF will distill out. Unfortunately, this most likely also will contain some H2SiF6, because potteries grade CaF2 almost certainly contains some silica-based material as well.

peach - 15-2-2011 at 05:54

I was wondering if one could build a little still for such purposes using surplus PTFE tubing; or another of the fluoropolymers.

Premade, isostatically pressed teflon bottles and thermoformed PFA/ FEP flasks are extremely expensive, hundreds of dollars a go. However, you can sometimes find some of the equally expensive tubing going as off cuts from spools. One method I envisaged, for small amounts, would be to use large ID teflon tubing and fill one end of it with the reactants, and then have the remaining length as the recovery section.

If you have a lathe, or access to someone with one, you could buy a bar of teflon and bore it out to produce the hot flask for larger volumes. You could do this without a lathe, using a drill press and a tap & die set. Teflon tape or fluoroelastomer could be used as possible thread sealants. If the fit is tight enough, you could possibly even do away with the threading.

Temperature for CaF2 method: 265C
PTFE decomposition products become detectable: >200C
Recommend service temperature for PTFE: around 250C (e.g. this PTFE clip by Chemglass and this PTFE sleeve from QuickFit)
Melting point of PTFE = 327C

Which brings me on to the suggestion of using teflon it's self as the source of HF, by thermal decomposition. The question then becomes, how the heck do we contain that and get a pure result if we're melting the teflon in the first place.

After searching around about this idea a month or two ago, I found this handbook guide to resistance, specifically made with HF in mind. <--- Link

--------------------------------------------------------------------------------------------------------------------------------


Here are some rough ideas.


A desktop furnace built in a thermos flask, with a temperature limit around 480C. <--- Link


And a rough picture of what I'm talking about.


I think the first place to start with such a DIY attempt would be to build the mini oven, bore out a teflon bar, fill it with oil and see how it behaves at the CaF2 reaction temperature; prior to attempting the HF it's self. The mini oven looks like it could be lots of fun on it's own, even if not of use here. I decided to ignore the idea of purposefully decomposing PTFE to begin with as, whilst the PTFE it's self is a well known source for the HF, the need for the other materials to handle it above PTFE's melting point and the possible interactions becomes more complex.

PTFE's sister polymers;

PFA, melts at 305C
FEP, at 260C
ETFE, 260C

^All unsuitable for the hot area and questionable for the tubing. Bottles made from these tend to have fairly thin walls as well, compounding the problem. If I were making the bottle from PTFE, I would make the walls a good 2 or 3mm+ thick, probably more, to be more sure of it's stability. I would also be cautious of heating much above the reaction temperature, and opt for more insulation instead.

The reaction temperature is not very far at all over the normal service temperatures. And has the added bonus that the decomposition product is also the product you're trying to make. As sulphuric can be redistilled to give a reasonably clean acid, this leaves the CaF2 as your only source of impurity; and there are likely methods for cleaning it up out there.







Take care! :)

[Edited on 15-2-2011 by peach]

entropy51 - 15-2-2011 at 06:25

HF can be distilled out of the reaction using a retort fashioned from a length of Pb pipe.

peach - 15-2-2011 at 06:32

Nice link.

By a stroke of coincidence, the melting point of both lead and PTFE is 327C.

garage chemist - 15-2-2011 at 06:52

Soldering together a cylindrical retort with lid and side-arm out of roofing lead sheet is probably the best bet, since massive teflon is expensive and hard to work with.
Or even better, cast the whole retort as one piece...
The lead retort will get attacked somewhat, but it won't contaminate the HF gas, and that's important.

I don't have any CaF2 and pottery suppliers in my country do not stock this.
I know a shop for mineral and gem collectors that offers natural fluorite as large, unpolished rocks of various colors at a very low price. These are meant to be polished and resold as semi-precious stones.
Purity is, of course, unknown and probably varies drastically from piece to piece.


[Edited on 15-2-2011 by garage chemist]

ScienceSquirrel - 15-2-2011 at 07:02

Hydrogen flouride is an extremely toxic gas, distilling it at home sounds like an excellent way to die. :(

peach - 15-2-2011 at 07:13

I agree. However I suspect one or two of these boys are also posting on versuchschemie, the origin of the potassium in a flask thread.

Casting it is a nice idea. New roofing lead is also mighty expensive! By casting it, you could start with scrap lead and only buy as much as you need. I have melted pans full of lead on a counter top oven; and there are enough of those going for free at the tip.

The UK is currently going through a lead replacement scheme, where the old lead water mains are being replaced by blue MDPE whenever there's significant ground work going on or water pressure problems. As a result, sticks of the very thick walled lead pipe will show up at the scrap yards. Maybe it's the same in Germany?

I would be concerned about making a large container out of roofing lead for this. As metal researcher demonstrated with his sodium still, metals tend to fail even when below their melting point if they are not supported or they are under load, and roofing lead is not particularly thick. Giving another thumbs up to the thick walled cast retort suggestion.

entropy51 - 15-2-2011 at 07:14

Quote: Originally posted by ScienceSquirrel  
Hydrogen flouride is an extremely toxic gas, distilling it at home sounds like an excellent way to die. :(
Yeah, I know but a couple of friends and I used to do it in high school, before we knew better.

We also distilled ether over a bunsen flame, without any problems.

God watches over fools and teenage boys. Apparently amateur chemistry was a lot safer in the 1950's than it is today.

[Edited on 15-2-2011 by entropy51]

ScienceSquirrel - 15-2-2011 at 07:50

I have distilled ether with a bunsen flame and used a lot of dangerous things in a professional context where you have the kit and the precautions.
God was sleeping on his watch when Fone had a go at APAN in kilo quantities and Myfanwy tried his hand at phosgene. :(

Regolith - 15-2-2011 at 09:00

Just for accuracy sake his name was Phone. We warned him too. "It was cold where he was" though, so it was safe... God helps those who help themselves like Kevlar and a blast suit. For the love of god have protection for the above. Damn ( checks texts ) forgot what a scary friken chemical that is converts to acid on contact with lungs. Eats more substances than Rosie Odonell... Thats something I'd make at somebody else's house.

Magpie - 15-2-2011 at 09:49

I like the idea of casting metal retorts using the lost wax process (or similar). I still want to cast an RBF out of silver. This is a craft that I hope we can develop on this forum someday.

I think I have mentioned before that I have a 1923 high school lab manual that has an experiment for etching glass with HF. Two cc of CaF2 are placed in a 5cm lead dish. This is moistened with con sulfuric acid then warmed with a small flame. Then a paraffin covered glass plate with the writing "fluorine, Moissan" exposed on the glass, was placed over the dish.

There was no mention of using a hood, or the toxicity of HF! Apparently God was the unseen partner in high school chemistry labs in those days.

ScienceSquirrel - 15-2-2011 at 09:56

There was the friendly Kipps apparatus sitting in the corner leaking hydrogen sulphide and experiments with troughs filled with mercury as well ...
Happy days :)

entropy51 - 15-2-2011 at 10:35

Quote: Originally posted by ScienceSquirrel  
There was the friendly Kipps apparatus sitting in the corner leaking hydrogen sulphide and experiments with troughs filled with mercury as well ...
Happy days :)
Small potatoes indeed compared to a kilo of APAN. And we all survived, apparently none the worse for it.

In high school we etched glass as Magpie described, in the hood. There were two wax bottles of HF in the stockroom as well. They were slowly emptied by those of us who needed it for home use. IIRC we carried it home in little plastic bottles, most likely in our pocket.

Regolith - 15-2-2011 at 11:02

Entropy I assume you meant the aqueous acid of HF. Or you live in Siberia and the classroom never went above 19C.

peach - 15-2-2011 at 12:02

Quote: Originally posted by Magpie  
I like the idea of casting metal retorts using the lost wax process (or similar). I still want to cast an RBF out of silver. This is a craft that I hope we can develop on this forum someday.


There's a group on yahoo called Hobbicast (or hobbycast) which I joined when I was about 13 or something like that. There are a lot of guys on there building furnaces and casting things at home. You will enjoy the discussion. I have seen people lost casting with styrofoam, with the styrofoam in place as the metal goes in! :D The results are not winning any beauty competitions, but it works.

To make the internal hollow for an RBF, you'd need to use a core (as they do when casting engine blocks); which will leave a surface finish that is a serious bitch to clean. I can't think of any easy way to spin or roto cast it either. The core finish is machined down and then honed if it's going through something like a chamber, but doing that with an RBF would be mighty difficult given the lack of access to the insides.

It may be easier with a sphere to form it from sheet and silver solder it together. Another possibility would be plating the silver (or gold) onto a form or base metal instead; if you're getting ambitious, platinum plating.

Casting a small retort for things like this HF idea from lead I would imagine to be far simpler. Particularly if the internal surface finish doesn't much matter. I mean, if it doesn't have to be polished to a glass finish.

[Edited on 15-2-2011 by peach]

Magpie - 15-2-2011 at 16:29

Thanks for the info peach. Someday when I get serious about metal casting I look those boys up.
Quote: Originally posted by Fleaker  
... Cotton and Wilkinson. One's a Nobel laureate, the other ... a huge grump!).


Do you have a reference for this? :D

madscientist - 15-2-2011 at 17:06

garage_chemist, here's an excerpt from the Sixth Edition of Cotton's Advanced Inorganic Chemistry (p. 74):
Quote:
Fluorosulfuric Acid

Fluorosulfuric acid is made by the reaction:

SO<sub>3</sub> + HF = FSO<sub>3</sub>H

or by treating KHF<sub>2</sub> or CaF<sub>2</sub> with oleum at ~250C. When freed from HF by sweeping with an inert gas, it can be distilled in glass apparatus. Unlike ClSO<sub>3</sub>H, which is explosively hydrolyzed by water, FSO<sub>3</sub>H is relatively slowly hydrolyzed. The acid is one of the strongest pure liquid acids and, as noted in Sect. 2-11, is used in superacid systems. An advantage over other acids is its ease of removal by distillation in vacuum. The extent of self-ionization

2FSO<sub>3</sub>H <--/--> FSO<sub>3</sub>H<sub>2</sub><sup>+</sup> + FSO<sub>3</sub><sup>-</sup>

is much lower than for H<sub>2</sub>SO<sub>4</sub> and consequently interpretation of cryoscopic and conductometric measurements is fairly straightforward.

In addition to its solvent properties, FSO<sub>3</sub>H is a convenient laboratory fluorinating agent. It reacts readily with oxides and salts of oxo acids at room temperature. For example, K<sub>2</sub>CrO<sub>4</sub> and KClO<sub>4</sub> give CrO<sub>2</sub>F<sub>2</sub> and ClO<sub>3</sub>F, respectively.

Fleaker - 15-2-2011 at 20:50

Quote: Originally posted by Magpie  
Thanks for the info peach. Someday when I get serious about metal casting I look those boys up.

Quote: Originally posted by Fleaker  
... Cotton and Wilkinson. One's a Nobel laureate, the other ... a huge grump!).


Do you have a reference for this? :D


@Magpie--I should have said was in both cases as regrettably they are both now dead. Wilkinson is far before my time, but my mentor knew Cotton quite well being an inorganic chemist--very smart guy, but very obstinate and intractable. I heard many a story about Professor Cotton and how helpful he was to his group members (I mean that in earnest). I'm sure most here know about the incident with the ACS chairmanship...

Also, check out backyardmetalcasting.com. Casting a silver RBF would be VERY difficult if one is to do it economically, even by lost wax. It can be done (the journeyman's test of skill was to cast a teacup, saucer, and spoon through clever fileting and core use). Regardless, you'd have to pour very hot and the silver would be very gassy. I've melted many kilograms of silver--the hotter you get it, the more porosity. In a thin wall flask (say 3 mm thick) you may get away with it if the sand or investment is fairly coarse. You'd be better of to use silver sheet and braze it or somehow weld it (not easy given its thermal conductivity).
Even though the layer is thin, you might just try silvering the flask interior.

@Madscientist--this [6th] is the edition I have so I couldn't have helped him with the exact reference he wanted. Thank you.

@everyone--given the experiments garage chemist conducts...I fear little for his safety. He's arguably one of the most competent people doing amateur science as far as I can tell.



[Edited on 16-2-2011 by Fleaker]

watson.fawkes - 16-2-2011 at 05:46

Quote: Originally posted by Fleaker  
Casting a silver RBF would be VERY difficult if one is to do it economically, even by lost wax. It can be done (the journeyman's test of skill was to cast a teacup, saucer, and spoon through clever fileting and core use). Regardless, you'd have to pour very hot and the silver would be very gassy. I've melted many kilograms of silver--the hotter you get it, the more porosity.
Is the dissolved gas only hydrogen, or are there other constituents? For example, hydrogen is the predominant gas in gassy aluminum. It's the only one with appreciable solubility. It's picked up from moisture adhering to the charge and from atmospheric water. I've seen the solubility data for aluminum, but not for silver.

peach - 16-2-2011 at 10:14

Have you looked into using degassing tablets for the silver?



Buget Casting Supplies

garage chemist - 18-2-2011 at 13:14

Thanks for the info, madscienctist. That's just what I wanted to know.
For distilling straight HF from CaF2 + H2SO4 you'd need a lead or silver retort, but if you use oleum, you can do it in glass. Isn't that fascinating?
The FSO3H could be used as a source of free HF by hydrolysis.
Conversion of CaF2 into a highly reactive HF carrier in an all-glass setup!

In order to avoid etching of the glass flask, one would have to make sure that there is enough free SO3 in the oleum to react with all the HF. So one would have to add an excess of SO3 with the oleum to the CaF2, and collect excess SO3 as the forerun before the FSO3H product.

One thing is worrying me, and that is the compound pyrosulfuryl fluoride S2O5F2. According to Brauer, this substance is formed from CaF2 and pure SO3 at 200°C, and is not hydrolyzed by 92% H2SO4.
It is colorless liquid boiling at 51°C, fumes only very slightly in air, and is extremely toxic in a similar way as phosgene.
Potential formation of this compound should be avoided as far as possible.
Though the desired product FSO3H is probably not much less toxic than this substance, its formation would mean an uncontrollable loss of fluoride and SO3. It would be impossible to separate SO3 and S2O5F2 by distillation.

Fleaker - 18-2-2011 at 17:05

Peach,

I'm well familiar with degassing tablets, degassing alloys (think lithium phosphide), and the like.

Watson.fawkes,

Actually, the dissolved gas is oxygen. Silver has a propensity to absorb it. It's called spitting. The purer the silver, the more likely it is to happen. When I cast a silver bar I play a slightly rich flame over the surface to slow the cooling (this precludes concentric contraction related rings from occurring) and to ensure oxygen keeps away from the silver as it cools. If I didn't do that, the silver would have a jagged, spikey surface. Many metals dissolve gases. Actually, I should say all metals dissolve gases (including argon) but it's the degree that the resulting porosity can be tolerated that governs the conditions by which the metal is melted.

plante1999 - 26-2-2011 at 13:55

des someone have thinked of a standart glass setup with 10-12 coat of silver mirror in?

blogfast25 - 27-2-2011 at 08:26

Have we collectively forgotten the (in)famous anhydrous HF thread by kilowatt?

http://www.sciencemadness.org/talk/viewthread.php?tid=9842

… which somewhat ominously went ‘dead’ at about 17-02-08? :(

peach - 27-2-2011 at 10:35

I like the PTFE still.

I am concerned by the squeezy bottle being used for dropping the sulphuric. Which will seem amusing, as I have posted photos of myself trying to use them to generate gases. I am not concerned by the plastic, but by the way in which it's connected together; and by an addition funnel being used in the first place. Mine featured an equalising arm and some attempt at controlling the flow. And the worst that'd come from mine going wrong (which happened) was a mess, as opposed to HF. There would be essentially zero control over the flow with that single, tapless, tube into the bottle. It's only going to make things harder and more likely to go wrong if a big squirt of it goes in whilst the still is hot.

I dread to think of the possibilities if it was squeezed in by hand whilst the thing was in an oil bath. Wooooosh! Cough cough!
Quote:
For one thing, this isn't an uncontrolled reaction


Where's the control on an unequalised, tapless dropping funnel?
Quote:
do you think I am just dumping a bunch of sulfuric acid and fluorspar together in a bucket and cooking it in my back yard?


That'd be safer, dumping it all in at the start.

There are also some odd things going on with the temperatures and such. E.g. acid is being added whilst it's hot, rather than cold, and the temperature it's being run at is quite a lot below the point where it would need to be to produce much in a short time.

I think it's doable. Well, I know it is looking at that lead pipe method and hearing about people making it. But that dropping of acid whilst hot should have been ruled out from the start. Never mind doing it sans equalising arm or tap.

Formatik - 27-2-2011 at 12:45

There is some information on HF handling materials and preparation, and some info on super acids (fluorosulfonic acid) here that may be of some interest, including a declassified document on the corrosive action of some super acid mixtures.

Note that the one declassified has some of the same data from the article below. Magic acid (HSO3F with SbF5) also does not attack common glass at any significant rate even at higher temperatures. Some magic acid that is. It will dissolve candles, but not glass.

Attachment: magicacidmetalcorrosion.pdf (51kB)
This file has been downloaded 1136 times

[Edited on 28-2-2011 by Formatik]

Formatik - 28-2-2011 at 19:38

Some great information here on fluorosulfuric acid concerning preparation and properties from Gmelin. In there is also some information on disulfuryl fluoride, and also from Inorganic Synthesis. Disulfuryl fluoride is made from heating CaF2 and an excess of SO3 at 100 to 300 C as stated in Gmelin about a JACS article. With toxicity like phosgene, this is serious stuff now.

Some properties on fluorosulfuric acid from Gmelin, I'm sure some of you were itching to know: at normal temperatures it is a thin, colorless liquid that fumes in air. It has a weakly pungent odor. On dry skin, it only attacks a little; it feels like a fat on contact and then does not have the strongly vesicant properties of HF. Rubber, wood, cork, sealing wax and similar materials are easily, to some extent immediately decomposed.

[Edited on 1-3-2011 by Formatik]

Formatik - 5-4-2012 at 12:43

New link for the files above.