Sciencemadness Discussion Board

Hydrofluoric acid

Polverone - 20-5-2002 at 17:30

Seeing as acids are a popular topic right now... I want to share my attempt to make hydrofluoric acid. Now, HF gives me the willies. It's a *really* dangerous substance. But one day this week I felt like trying to make it. So I took ceramic-grade calcium fluoride (fine powder) and put it in an ordinary soda-glass test tube with concentrated sulfuric acid. This I heated outdoors in full safety gear, always dancing around to keep the wind at my back. Various sources indicate that HF is prepared by heating sulfuric acid and calcium fluoride together at ~300 C. I maintained the sulfuric at a boil for a considerable time, with very dense white fumes coming from the mouth of the tube, but I am not sure I really prepared HF. Afterward, I cleaned the tube and it appeared completely unharmed. I was under the impression that hydrofluoric acid would rapidly attack glass.

Perplexed, I placed some "Whink" rust remover (3% HF) in a similar test tube and left it for 24 hours. Not a mark on the tube. I tried heating some Whink in the tube, and there was a similar lack of results. Now I am puzzled, as I am positive that HF is used to etch glass commercially, but I have utterly failed to do so. Perhaps it's time to visit the library.

So, any other home chemists with a bit of a death wish ever tried to make this substance? Perhaps I will try again with larger quantities (but as I said, this makes me very nervous).

madscientist - 20-5-2002 at 17:36

Hrm, strange... all I can think of for explanations are:

1) The glass might not contain enough silicon dioxide (I highly doubt this, as you said it was regular soda glass).
2) The hydrofluoric acid isn't concentrated enough to cause a rapid reaction (highly unlikely that would make that substantial of a difference).
3) The test tube is being eroded, but not in a very noticeable manner; it is becoming thinner but noticeable defining lines between eroded glass and untouched glass have not been created.

I'm thinking that it is most likely the third. I believe the reaction for reaction of hydrogen fluoride with silicon dioxide are as follows.

SiO2 + 4HF --} SiF4 + 2H2O

Coen - 20-5-2002 at 17:42

Yes, this is rather strange. The reaction between CaF2 en H2SO4 is even used commercially sometimes.

Very diluted HF doesn't etch glass really fast. But I have some 40% HF, which etches it practically immediately when brought in contact with glass.

When I get my glovebox I will also try this experiment and posts results.

It's quite important for me because we want to make BF3 for nitration purposes and we need dry HF for that.

Polverone - 20-5-2002 at 17:45

Actually, there was a followup on this experiment that I hadn't posted because nobody seemed interested. It turned out that the glass was quite etched; I just couldn't see it at the time because of all the beads of liquid and clinging powders in the tube. Once thoroughly cleaned I could see that the tube had been considerably etched.

If you have 40% hydrofluoric acid, can you make dry HF gas by dripping it into concentrated sulfuric acid, just as you can get dry HCl gas by dripping hydrochloric acid into concentrated sulfuric? This seems like a simpler and safer (if any production of HF gas can really be considered safe) method than heating sulfuric acid and CaF2 together.

Coen - 20-5-2002 at 18:00

Well, I guess that's possible. But I don't have very much of HF solution. It's maybe a little "cleaner" indeed as far as HF will ever be "clean" to work with. But I need really a lot of HF so I guess I still have to use the CaF2 route.

PHILOU Zrealone - 20-5-2002 at 18:01

solubilities are about equal for CaSO4 and CaF2!
CaF2 + H2SO4 (=equilibrium=) 2HF + CaSO4

Heating is needed to get HF since it is more volatile.
Bp is arround 20°C

Maybe heating was not strong enough or H2SO4 not concentrated enough; typically H2SO4 boils between 160 and 270°C when it is more and more concentrated!

Better use NaF (soluble in water and in H2SO4 + heat)!

CaSO4 and NaHSO4 catches water for a better volatilisation!

40% HF corrodes fast glass, 6% doesn't even after long exposure!

Dry HF corrodes glass readily:
SiO2 + 4HF --) SiF4(g) + H2O

Better make a PTFE reactor with a cold trap to catch the liquid HF below 0°C; the lower the best! to avoid sniffing very toxic HF gas.

Pb reactor could also work, since PbSO4 is much less soluble than PbF2 that is also unsoluble; those precipitate will prevent further attack of the H2SO4 or of the formed HF!
Avoid valuable glass reactor wasting!



daryl - 21-9-2002 at 04:44

I vaguly remember that anhydrous HF does not corrode glass. I think it has to be dissolved in water

Xenos - 21-9-2002 at 12:08

I dont think it does. I read in Asimov on Chem. that a scientist in 1800s etched his glasses with HF gas. But, awhile ago, i aquired some NaF and attempted to make HF. Here was my setup: I made, what looked like a still out of 2" copper fittings, because i read, again in Asimov, that elemental fluorine attacks copper, and then wont attack the CuF layer. Then, not wanting HF gas floating around, i submersed the copper tube in some water, in a PVC tube, becuase i read that HF does not attack PVC. In the copper 'still' i added some NaF and really crappy H2SO4 and heated the 'still'. Nothing happened. When I looked inside, there was a hard, pourous residue on the bottom of the 'still', and no acid. I have a feeling the conc. of the sulfuric acid was my biggest problem. I recently aquired some H2SO4 drain cleaner, 93%, and i would like to try again. As you can see, i attemped to make this experiment as safe as possible, becuase HF is definatly one of the worst chemicals. So, my questions are, how is my setup and was my acid conc. the problem?

daryl - 21-9-2002 at 14:07

Conc H2SO4 is one of the most useful reagents you can have. It is vastly different to dilute acid and is not used by domestic cleaners etc. You have to buy it at a chemical supplier.

I vaguely remember that to make your own conc H2SO4 is too hard and is a PITA. It also detracts from your original purpose

It's not hard....

vulture - 22-9-2002 at 03:32

Just takes some time and a shitload of energy :D

You can heat any % H2SO4 to evaporate off water, since it forms an azeotrope with water at 98% which is almost impossible to distinguish from anhydrous H2SO4 (which you're never going to get).
You'll notice when you reach 98% by the evolution of white SO3 fumes which are highly irritating.


PrimoPyro - 22-9-2002 at 04:41

Excuse me, but maybe this will explain something:

Hydrogen Fluoride: A colorless liquid, HF, r.d. 0.99 m.p.-83*C b.p.19.5*C. It can be made by the action of sulphuric acid on calcium fluoride . The compound is an extremely corrosive fluorinating agent which attacks glass. It is unlike other hydrogen halides in being a liquid (a result of hydrogen bond formation). It is also a weaker acid than the others because the small size of the fluorine atom means that the H-F bond is shorter and stronger. Solutions of hydrogen fluoride in water are known as hydrofluoric acid.

So yes, it is corrosive, but since the aqueous acid does not fully dissociate, is weaker in strength than the other hydrohalogen acids.


vulture - 23-9-2002 at 02:14

The definition of corrosive is usually that if a chemical destroys living tissue, it is labeled as corrosive. In this way HF would be double corrosive, since both the H+ and the very reactive F- damage tissue.

Hydrogen Fluoride

ElliB - 20-10-2002 at 09:26

Are somebody know the correct synthesis-way of CMNT (C7H3ClF3NO2) from HF and a chloronitrobenzene derivate ?

Hoffmann-LaRoche - 20-10-2002 at 10:42

For god´s sake!!!

Is it REALLY necessary to experiment with HF??
HF is really one of the nastiest substances (hydrazine too..) known to man.

But if u really can´t live without it, notice:

IF u get HF on your skin, it is not possible to wash it away as usual, HF causes big, very painful wounds that do not heal.
If u get HF on your skin they can only cut the skin away to prevent the wound from growing.
They have to transplant some skin from your ass or your legs.


It's worse

vulture - 20-10-2002 at 12:42

It will eat right through your skin straight to the bones which will be rapidly attacked followed by evolution of poisonous fluorides which, once they are in the blood stream, cause heart failure.

The only thing that can help you is lot's of water and calciumgluconate gel.

IrC - 8-3-2005 at 22:50

"The only thing that can help you is lot's of water and calciumgluconate gel."

If you get a bad enough spill (% conc. vs body area) on your skin you need to inject liquid IV type calcium gluconate directly into the area. The gel alone will not save your life. Fluoridation of the bones results in the need for amputation, and the loss of blood calcium combined with Mg binding will likely stop your heart, this is especially true with inhalation of HF.

As with other acids, the onset and severity of the burn depends on the amount of hydrogen fluoride, its concentration, and the duration of contact. Hydrogen fluoride penetrates tissues including the skin and nails and an apparently minor or mild exposure can cause a serious burn if not promptly and properly treated. Although hydrogen fluoride solutions weaker than 50% may cause first degree burns (redness) which may not become apparent for up to 24 hours, injuries from solutions over 20% generally induce unrelenting pain within 30-60 minutes. Hydrogen fluoride solutions stronger than 50% cause pain within 5 - 10 minutes and tissue injury becomes apparent rapidly. This is usually manifest as redness followed within 30-60 minutes by a paleness of the tissue proceeding to frank skin blanching and finally a subjacent bluish-black necrotic appearance. At this time, the pain is usually intense. The skin becomes hard and leathery. As the acid is one thousand times less dissociated than hydrochloric acid, and therefore is a weak acid, substantial quantities of the uncharged moiety can penetrate deeply into the tissues.
Tepperman (1980) reported a fatality due to severe hypocalcaemia resulted from exposure to a concentrated solution affecting only 2.5% of the total body surface.

Most importantly, absorbed fluoride binds magnesium and calcium irreversibly resulting in hypomagnesaemia and severe hypocalcaemia, the latter of which is the usual cause of death from cardiac disturbances. Hyperkalaemia may accompany widespread cellular injury and this can complicate hypocalcaemia.

OK people, keep your calcium antacid tablets ready and eat them like candy just before playing with your HF, and stock up on the gluconate of course, both liquid and gel (It helps to scavenge an epi pen to fill with the liquid).

Immediate removal from exposure and decontamination is essential. Affected areas should be flooded with water for at least 15 minutes. In cases of serious exposure ( > 2.5% body surface area to anhydrous hydrogen fluoride; 10% body surface area to 70% hydrogen fluoride, 25% body surface area to more dilute solutions) early calcium supplementation should be instituted. Patients with renal or cardiac failure may need close observation to prevent hypercalcemia. Local treatment of the exposed areas of the skin are aimed at decontamination and inactivation of the fluoride ion by complexing with calcium, magnesium, or quaternary ammonium compounds. In severe cases, aggressive support may be needed with treatment of pulmonary oedema (inhalation), arrhythmias and hypotension (all routes of exposure). Skin burns may need local or intra-arterial injection with calcium gluconate if water decontamination and other topical calcium (or magnesium) salts have not relieved the pain.

Monitoring of blood calcium, electrolytes, and magnesium is essential in serious exposures (Trevino et al, 1983). Arterial blood gases, and electrocardiographic monitoring is also necessary. Blood calcium and electrolytes should be measured serially every 6 h for at least the first 24h in severe cases. In the absence of full laboratory support, semi-quantitative analysis for calcium may be performed in urine (Sulkowitch Test). A chest X-ray should be obtained for any suspected inhalation.

Monitoring of developing hypocalcaemia is essential. Removal or inactivation of the fluoride from the site of contact is important and absorbed hydrogen fluoride must also be inactivated. The standard assessment should include immediate electrocardiographic monitoring of all serious exposures and immediate assessment of serum calcium and electrolytes. This should be repeated every 6 hours for 24 hours, or as indicated by the clinical course.

In known or suspected serious exposures, early administration of intravenous (if unavailable, oral) calcium supplementation
should be effected. In patients with normal renal function, excess calcium will be excreted; however, reports of hypercalcaemia during the course of hydrogen fluoride treatment have been noted. Calcium gluconate, 10 ml of a 10% solution may be given intravenously over several minutes with 1000 mg in 1L of 5% dextrose solution over several hours. With known or suspected ingestions, serious skin exposures or inhalations, it is essential to treat the patient conservatively DESPITE THE POSSIBLY BENIGN INITIAL APPEARANCE.

The critical effects of hypocalcaemia may be delayed for 2-24h.

A skin burn of more than 1% body surface area, especially when there has been a delay in decontamination, should require admission of the victim to a burns unit for close monitoring (hypocalcaemia and arrhythmias) and treatment (principally administration of calcium). All persons with extensive skin burns secondary to hydrogen fluoride gas, aerosol, or concentrated solutions should be evaluated for inhalation injury unless they were wearing respiratory protection at the time of the exposure.

Just FYI

Mailinmypocket - 26-11-2012 at 07:22

Interesting article from the old days...

image.jpg - 332kB image.jpg - 362kB image.jpg - 428kB image.jpg - 421kB

daragh8008 - 5-12-2012 at 15:14

Quote: Originally posted by Mailinmypocket  
Interesting article from the old days...

Wow! That article is wild. At work they make me wear latex gloves under heavy butyl gloves, lab coat, rubber apron, full face shield, working in a fume hood in a spill basin, with full acid spill kit and two tubes of calcium gluconate beside me and carrying a radio to some one else familiar with the dangers, just to work with HF. I wonder how many people did themselves harm after reading that back in the day?

Mailinmypocket - 5-12-2012 at 15:26

Quote: Originally posted by daragh8008  
Quote: Originally posted by Mailinmypocket  
Interesting article from the old days...

Wow! That article is wild. At work they make me wear latex gloves under heavy butyl gloves, lab coat, rubber apron, full face shield, working in a fume hood in a spill basin, with full acid spill kit and two tubes of calcium gluconate beside me and carrying a radio to some one else familiar with the dangers, just to work with HF. I wonder how many people did themselves harm after reading that back in the day?

Most likely a couple. Although people can have the same types of accidents doing chemistry today too, albeit fluorides are especially dangerous. If done carefully the SiF4 synthesis was fun and fairly safe- I never had the idea to try making it before reading that article either!

daragh8008 - 5-12-2012 at 15:33

Something about the guy in the picture leaning over his setup having a good look at the process. Eeeck! No thanks.

Mailinmypocket - 5-12-2012 at 15:51

Quote: Originally posted by daragh8008  
Something about the guy in the picture leaning over his setup having a good look at the process. Eeeck! No thanks.

Oh that part! Hahahaha! I have to agree with you! I never thought twice of it. He makes it look like something you would do on the livingroom coffee table to entertain company with "stunts!"

"Who wants a custom home etched brandy sniffer!? Let me just generate some hydrogen fluoride,and melt some wax! It's a glass eating chemical!!" :)

[Edited on 5-12-2012 by Mailinmypocket]

Manifest - 7-12-2012 at 18:04

Could you try getting a bit of glass and weighing it with a precision scale and put it in acid for a bit and then weigh it again.

siegfried - 17-12-2012 at 15:25

Sounds like HF is as bad as 2,2-dichlorodiethyl sulfide aka mustard gas.

Mailinmypocket - 17-12-2012 at 15:28

Quote: Originally posted by Manifest  
Could you try getting a bit of glass and weighing it with a precision scale and put it in acid for a bit and then weigh it again.

This would of course reduce the mass of the sample of glass in question!

When I get a chance I will place a pre-weighed microscope slide into some 50% HF solution and we will see the results. The reaction is quite violent by the way.

Zyklon-A - 19-12-2013 at 18:15

I heard that HF (aq) causes (very bad), but rather un-painful burns.
''Aqueous hydrofluoric acid is a contact-poison with the potential for deep, initially painless burns and ensuing tissue death.'' Wikipedia.

Mailinmypocket - 19-12-2013 at 20:09

Indeed. The fluoride easily penetrates the skin and disturbs electrical functions in the heart and other parts of the nervous system. The exact mechanism by which, I'm not sure.

Polyolefin - 20-12-2013 at 03:58

Another thing I have in mind is thermal decomposition of HFC refrigerant. It isn't hard to do in presence of catalyst and water. When the coil in refrigeration compressor burns out under refrigerant vapor due to a short or an overload, HF level can reach such a level that it can make the tainted refrigeration oil a chemical burn hazard.

If you run it through glowing hot copper mesh in a glass tube and run the output through cold water, you might get a fairly good deal of HF.

blogfast25 - 20-12-2013 at 06:02

Quote: Originally posted by Mailinmypocket  
The exact mechanism by which, I'm not sure.

Hypocalcemia is a large part of it: HF precipitates Ca<sup>2+</sup> as CaF<sub>2</sub> in cells, causing these cells to die very quickly. Hence very painful burns that heal only slowly. Considerable exposure leads to slow and agonising death.

This is why having calcium gluconate at hand as an antidote is recommended: large amounts of it can bind the HF/F<sup>-</sup>, reducing the damage.

Zyklon-A - 20-12-2013 at 11:25

How bad would a 3% solution be.

blogfast25 - 20-12-2013 at 15:18

Still bad enough to what is necessary to prevent any getting on your hands or eyes but of course much less dangerous than 40 % or anhydrous HF.

Zyklon-A - 20-12-2013 at 15:43

Might buy some 3% HF at Home Depot.