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hydrofluoric acid

Posted by Albert Russ  
hydrofluoric acid
July 31, 2006 06:28AM
I was wondering if anyone could share some experience in cleaning and etching quartz with me. I have recently obtained some hydrofluoric acid and would like to use it to etch quartz in order to obtain interesting structure. I want the quartz to show clean however and I understand it needs to be completely submerged not to become frosted on surface. If more diluted acid is used I suspect sulfuric acid might speed up the reaction. The acid by itself is extremely slow with glass in relatively safer concentrations. Anyone with long experience in HF?
avatar Re: hydrofluoric acid
July 31, 2006 09:19AM
The general policy on this board is not to give advice on the use of HF (other than strongly to give the advice 'DON'T USE IT') because of the extreme dangers of this substance.

There may be people who can help you with your issue privately, but this is not the place to give out information on the use of HF.

Jolyon
avatar Re: hydrofluoric acid
July 31, 2006 11:41AM
    
There have been many allusions to the dangerous of hydrofluoric acid in the forums, but little detail.

You don't have to swallow hydrofluoric for it to cause injury or death. Because it is a lovely small molecule, it can be absorbed directly through the skin, and the vapours are absorbed directly through the lungs. When absorbed through the skin, it not only causes painful burns but also acts throughout the body to bind with calcium. This causes damage to the bones, but more importantly, makes calcium unavailable for critical chemical reactions in the body. One result can be heart failure.

A lethal dose of hydrofluoric acid can occur when as little as 2.5% (about the size of your hand) of the body's surface is exposed to concentrated hydrofluoric acid. You would be suprised at how little an amount of hydrofluoric acid is necessary to achieve that exposure. A geological lab technician in Australia splashed some hydrofluoric acid on his upper legs, providing exposure to an estimated 9% of his body. They amputated one leg after 7 days, and after 15 days he was dead of "multi-organ failure".

We all suffer from the (sometimes subconscious) "it can't happen to me" syndrome. But it *can* happen and it *has* happened.
Re: hydrofluoric acid
July 31, 2006 02:08PM
Hi Albert

I hope that Daniel's post has made you understand why HF is considered to be a "lab-only" chemical, not to be used outside of a properly equipped lab, and most certainly not by people who are not trained in it's use.

You know, you can sit on a rock and toss a pint bottle of nitroglycerine back and forth between your hands day after day and not get killed by it - might even think the stuff is safe to handle. But just drop the bottle once...

HF is MUCH more dangerous than nitro - because you don't know that it is killing you while it is.

Anyway, if you are going to continue to try to use the stuff, please let us know where to send the flowers and charitable donations in your memory.

Seriously

Alan
Re: hydrofluoric acid
July 31, 2006 02:44PM
That last sentence was original, Alan .

Albert: don't use sulfuric acid either! The only acids I use are hydrochloric 10% with special acid gloves and 5% acetic acid (white vinegar). Dont mess with quartz...

remember to DISPOSE OF THE ACID PROPERLY!!!!!!!!

Phil.
avatar Re: hydrofluoric acid
July 31, 2006 02:45PM
    
Phil,

10% sulphuric acid is considerably less dangerous than 10% HCl !!
Re: hydrofluoric acid
July 31, 2006 03:08PM
Nice! I might start using some. No one sells that here :( though there is some in toilet bowl cleaners.... but i dont want my specimens to smell like lemon.
avatar Re: hydrofluoric acid
July 31, 2006 03:14PM
You can get sulphuric acid from car batteries.

But not much benefit in using it to be honest, I bought some small bottles of Conc. sulphuric a few years ago and the only thing I've ever used it for is cleaning the toilet! Hydrochloric is far more useful.

Jolyon
Re: hydrofluoric acid
July 31, 2006 03:41PM
Sulphuric acid is not very usefull for cleaning. It forms many insoluble precipitates. Sulphuric is for me much safer then hydrochloric cause it has no agressive fumes but better do not spill it on yourself;)

All chemicals are potentialy very dangerous if handled improperly. Its only about knowledge and safety, theres no secret about HF - just must be used properly by trained person and in equiped lab. Nothing to be done at home!

The truth is that all mineral acids can cause serious burns or injuries when handled improperly. One must know his knowledge and equipment limits:)
Re: hydrofluoric acid
August 01, 2006 06:36AM
Thank you guys for your replies. I should have told you in my first message that I have a very detailed knowledge about HF as well as all hazards associated with it. I am an analytical chemist (I ackgnowledge that I have experience with virtuallay everything including anhydrous HNO3, except HF) and I plan to use it in a well equipped laboratory where I work, full equipment using a digestor with negative pressure-I am not insane to do the work using liter volumes in my basement. Actually there is not much use in sulfuric acid itself except that it draws away some water and creates more oxidizing enviroment, if HF is diluted, addition of sulfuric acid speeds up the reaction. It acts than as more concentrated, if you mix sulfuric and nitric acid you also get the same results as if the nitric acid was far more concentrated by itself. I guess there are very few people that have direct experience with HF because it is not a usual nor practical chemical in a chemical laboratory unless it specializes in specific experiments that require it.
Re: hydrofluoric acid
August 01, 2006 08:33AM
I read all your comments throughly and decided to share some knowledge with you on some of the most common acids. I hope it will be usefull. I try to write it as simple as possible.

HCl- is a strong acid, it is almost completely dissociated as well as H2SO4. It is perfect for dissolving carbonates and thus cleaning their surface, and many other minerals. It could be the most general acid in mineral cleaning.
Safety: As far as splashing, HCl is not as hazardous as other acids because its only power is its full disociation, H+ ions always have a place to bond on live tissue. I do not reccomend on trzing it, even if rinsed immidiately, concentrated HCl may irritate skin and destroy its uppermost level. However, there should be no further damage to tissue if rinsing is not delayed. Unlike sulfuric acid, it liberates HCl gas that is both irritating and toxic. The release of HCl gas is proportional to the acid's concentration. If mixed with some strong oxidiying agents, there is danger of chlorine gas liberation.
HCl itself does not possess oxidizing properties and is sometimes mixed with hydrogen peroxide to achieve this.

H2SO4- again a strong acid just like HCl. It can be used with similar results but its major dissadvantage is that there are fare more insoluble sulfates than chlorides. Its properties are very much dependent on its concentration. Diluted H2SO4 is only a strong acid, just like HCl. As it becomes very concentrated to anhydrous (100%), it becomes a good oxidizing agent and extremely hydroscopic. It is in fact so hydroscopic that it destroys tissue as it draws water from it and reacts with many organic compounds so that these compounds release even more water. As a result, plant and animal tissue can become almost carbon like. The advantage of this is that for instance an inert mineral can be cleaned from excess fungus and organic material with concentrated sulfuric acid- actually the acid does not remove this stuff completely but due to the destruction it makes it is much easier to rinse and scrub off afterwards. A disadvantage of this property is that concentrated H2SO4 is very hazardous because it pulls water from tissue, even from organic molecules and oxidises and releases H+ at the same time. Therefore spills of concentrated H2SO4 on body are much more dangerous than HCl or diluted H2SO4. Sometimes tissue destruction is immidiate and severe.

HNO3- is an acid in low concentration similar to HCl but produces very few insoluble compounds. It has significant oxidising properties which become very apparent at high concentrations. At higher concentrations above azeotrope (over 65%) HNO3 not only becomes highly oxidizing but at over 80% is hydroscopic and reacts with water quite intensely. It is not as hydroscopic as sulfuric acid though. Like HCl, it liberates toxic gases (NO2 mostly) which is not only highly irritating but also poisonous. At concentrations around 80% and over, the acid is fuming vigorously and is very dangerous to deal with. 65% HNO3 small contacts or splashes cause yellow stains on skin that peel off within days if washed immidiately. Fuming 80% acid or more causes severe burns on skin immidiately that are very different from the commercial concentrated 65% and differences are apparent even if washed immidiately. I had one bad experience in such. HNO3 is also very dangerous with many organic compounds at such concentration as vigorous niotration, oxidation or ignition can occur. HNO3 at such concentrations is not practical for mineral cleaning due to its dangers while better results can be obtained when carefully heating deluted acid.

HF- hydrofluoric acid is not a strong acid (it is much less dissociated as an aqueous solution because the hydrogen bond is too strong for free ions to form readily). Acid strength is defined in its dissociation in water, not in its other potentially hazardous chemical or physical properties. In fact, if HF was a strong acid, it would not penetrate tissue so well because H+ would attack tissue right from the skin sufrace, slowing the molecule's propagation. Obviously the undissociated molecule is very small which only benefits its fast penetration through tissue. Eventually it reacts with almost anything within the tissue, creating severe burns and tissue necrosis. At the same time, F- ion precipitates Ca2+ and forms insoluble CaF2. Drop of Ca2+ and Mg2+ in blood levels, called hypocalcemia and hypomagnesia in severe cases proves fatal because of their importances in many physiological processes including heart muscle functioning. Hydrofluoric acid is the only known chemical that effectively dissolves quartz, glass and other silicates. It is unfortunate because the only chemical possible is so dangerous. Glass can be attacked by hot concentrated phosphoric acid or molten NaOH or KOH, but the rate of attack is very slow and the results are very bad. I am not sure about quartz. HF, when concentrated enough, does not attack many metals and can be transported in steel tanks.

HOOC-COOH oxalic acid is best in removal of rust stains and selenite. It is a weak organic acid that bonds meny metals in very stable complexes.
CaSO4 for example is almost insoluble. Sulfates in general are extremely stable because sulfuric acid is a very strong acid which binds very well, and no acid is able to release sulfate ion into sulfuric acid and take its place. Usually weak acids that create salts can be released with a stronger acid. That's why carbonates are so easy to dissolve- H2CO3 is a very weak acid that can be substituted by almost any other acid and because it is not very stable, it decomposes and creates CO2 bubbles. HCl is a very strong acid but both HCl and HNO3 can be released if H2SO4 is applied on chlorides or nitrates, creating more stable sulfates. If we want to dissolve CaSO4, in a sense no acid can help us. Oxalic acid is one solution but not as an acid but as a complex-forming agent. Oxalic acid binds Ca2+ into a very stable complex, dissolving CaSO4, otherwise very difficult to deal with. At the same time, oxalic acid is poisonous because it binds Ca2+ in live organisms into complexes. Calcium is still present in an organism, but not a necessary free ion but a very stable complex that is useless. Hypocalcemia is induced just like in case of HF poisoning, only due to a different chemical process. Fe oxides in form of rusty crust can be removed by binding Fe into a soluble complex. It is strange that HCl and HNO3 are unable to dissolve a simple oxide of such a metal as iron. The problem is that Fe forms complicated oxides and peroxides that are difficult to attack with regular acids and are thus surprising insoluble. Oxalic acid and oxalates are unstable at higher temperatures and decompose into CO, CO2 and H2O. If there is too much iron oxalate on a specimen that does not go of, it is possible to heat up the mineral (if heat does not destroy the mineral itself), oxalate decomposes and the mineral, with reformed Fe oxides, can be repeatedly treated with oxalic acid. Because of thermodynamic instability of oxalic acid and its salts, one other danger arises. Termal decomposition releases carbon monoxide and this starts occuring once the acid bath boils. Therefore oxalate solutions should be heated carefully and never boiled.

Conclusion.
In general, it is important to note that each acid is a solution for a specific problem. There is no universal acid that dissolves everything and each acid is characterized by its specific set of properties. It is also important to note that concentration and temperature may change the acid's properties and abilities to achieve something exrtremely- a few percent off can seem like a totally different substance. Sometimes, acids must be even combined for example to dissolve gold and so on. It is important to know that each acid is usefull but in a different way- and each acid is dangerous but a different way. I ackgnowledge that HF may be the most dangerous but that's because it posesses unique properties that coincidently are not compatible with human body.
Re: hydrofluoric acid
August 01, 2006 02:27PM
Sorry about that, Albert...

We are so used to novices showing up here asking about the use of HF that we are pretty much programed to say: "Are you NUTS!" :~}

I'm sure you can appreciate the need for people to be cautious about giving certain types of info to people they don't know - especially when it comes to extremely dangerous things like HF. At one point, Jolyon had one or two people who were using HF, even though they were not trained and didn't have access to a proper lab, telling others here that it was just fine to use it for mineral cleaning, no problem... It was at that point that Jolyon instituted the present policy regarding the discussion of HF here.

Regards

Alan
Re: hydrofluoric acid
August 01, 2006 02:48PM
    
Daniel,

'' it not only causes painful burns '' This is untrue, the reason HF is so deadly, it doesn't burn, you don't even feel it until it is busy attacking bones-too deep to clean off with water!

Barry
avatar Re: hydrofluoric acid
August 02, 2006 01:24AM
    
There have been excellent overviews given here. Just two more points to mention (in my opinion).

1) As mentioned above, hydochloric acid will release HCl into the air. If you are messing with concentrated acid, even a well ventalated area may not be enough to prevent you from getting a good whiff of the stuff. It burns, baby (I was using 12 Normal HCl). You really need air flow taking fumes away from you, or be sure it is reasonably dilute.


2) An old chemistry lab warning! When diluting acids, add the acid to water. NEVER add water to acid. Mixing water and acid releases heat. If you add acid to water, the heat is absorbed reasonably well by the water. However, if you add water to acid, the acid (being concentrated) will be on the bottom below the water, and the heat released can be enough to launch the water (and acid) right back out of the flask or cylinder. I saw it happen with sulfuric acid with water dumped on top, and we had bad acid rain.
Re: hydrofluoric acid
August 02, 2006 06:39AM
You are right, Paul. Actually the results of mixing acid with water vary differently but since sometimes the reaction is so vigorous and sometimes it is not, it's better to follow the same step and pour acid into water for safety reasons. H2SO4 releases lot's of heat when mixed with water but the heat released is verz much dependent on its initial concentration. If close to 100%, pouring water on it would be suicidal because enough heat would be released for the acid to be splashed all over. The same accounts for 80% HNO3 and over. Besides violent reaction with water, lot's of NO2 becomes released. 65% commercial HNO3 usually only heats up if water is added to it, so does HCl but for safety reasons it's better to always add acid to water and even that slowly to avoid any accidents unless you really know what you are doing. Sounds like a chemistry class but good knowledge about acids won't hurt before proceeding to mineral cleaning. I guess I am too freaked out to work with HF after reading all the safety instructions on the internet...
DebS
Re: hydrofluoric acid
August 03, 2006 02:51PM
I would never use hydrofluoric or sulphuric acid. Ever. It's extremely dangerous.

I have used oxalic acid in dilution to get rid of iron both on minerals and in toilets!

I have also used hydrochloric, a weakened variety sold at hardware stores for calcite, but I HATE even using it. I have always used it outside, never inside. And with a full face shield and in a locked but airy shed. I hope not to use it ever again.
avatar Re: hydrofluoric acid
August 03, 2006 05:21PM
    
Barry:

If you will google "hydrofluoric painful burns" you will see numerous citations on the painful burns that can be caused by hydrofluoric acid, including some case studies.
avatar Re: hydrofluoric acid
August 04, 2006 09:57PM
    
Unfortunately too many are too scared of chemicals - 'it will blow up', 'you will from just looking at it' - but I must admit hydrofluoric is one of the relatively few chemicals that make me very cautios (I have letters after my name too, have dealt with large volumes of exotic substances, been in charge of lab security).

I have seen people in developing countries seriously hurt because they have used HF and not known what they work with and I know of numerous mineral dealers who are idiots ... careless idiots.

Fine with me that we do not dispense advise on working with HF as such, but allow my to put forward one piece of advise to those who fail to appreciate HF should not be used except under exceptional circumstances:

If you work with HF always use the gear mentioned in the MSDS, etc. and ALWAYS have a jar of calcium gluconate gel next to where you work. Before opening the HF, you check where that jar is and that the cap is not stuck. If you get HF, however dilute, on your body put on calcium glucionate gel immediately - only rinse if it is a major spill, calcium gluconate gel goes first. The calcium binds the HF and may (MAY!) prevent tissue damage.
When you've done that, then you call the ambulance.

Calcium gluconate gel is readily available in shops dealing in chemicals, safety gear and most pharmacies can get or make it. I have even seen mineral dealers selling it (mail me if you need an address).

Claus

____________________________________________________________________________
Claus Hedegaard
Google me to find me!
Re: hydrofluoric acid
August 12, 2006 10:07AM
    
Deb,

Hydrochloric Acid isn't that bad (at least at 10%) as I regularly use it when ever I can get my hands on it...
The only danger is if you spill some when it hasn't been diluted or if you accidently get a whiff of it while pouring but apart from that I find it quite safe but always have a nearby tap and plenty of basic substences ready.

Barry
Richard Bielek
Re: hydrofluoric acid
September 03, 2006 10:19PM
Dear Albert,

our common friend, Mgr. Kusik, has once used HF and since that time his finger is twice bigger. It is up on you to continue playing with hazards but it may happen that your mother will again remove the glaze from your WC. Anyway, I support your scientific approach and I am on your side while cleaning the minerals, including mines, with selected chemicals.
Your friend
Richard
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