Working with HF

Buffered HF is available from semiconductor specialty chemical houses. Concentrated HF(aq) is readily available and "diluted" HF is just concentrated HF diluted 1:10 with distilled (or better, Nanopure) water.

EXTREMELY IMPORTANT SAFETY COMMENTS ON THE HAZARDS OF HYDROFLUORIC ACID

This is just what I have to say. HF is *dangerous*, and you should read up on it yourself, not just go on what some fool graduate student has to say. Your safety is your responsibility. HF is a weak acid. It is not its acidity per se that presents a risk to those exposed to it. Rather, the fluoride ion is the hazard. You may scoff, saying "Ha, drinking water has Fluoride ion in it!" Right you are, but most water also usually has arsenic in it...in both cases the issue is one of concentration. Fluoride ion makes for stronger bones and teeth because it reacts with calcium to form strong bonds; but it also binds to free calcium ions to form insoluble CaF2. In fact, too much fluoride in one's drinking water will make teeth and bones crumble. The full story, according to Greenwood and Earnshaw's Chemistry of the Elements, is as follows (p.946):

"The highly corrosive nature of HF and aqueous hydrofluoric acid solutions have already been alluded to and great caution must be exercised in their handling. The salient feature of HF burns is the delayed onset of discomfit and the development of a characteristic white lesion that is excruciatingly painful. The progressive action of HF on skin is due to dehydration, low pH, and the specific toxic effect of high concentrations of fluoride ions: these remove Ca2+ from tissues as insoluble CaF2 and thereby delay healing; in addition the immobilization of Ca2+ results in a relative excess of K+ within the tissue, so that nerve stimulation ensues. Treatment of HF burns involves copious sluicing with water for at least 15 min followed by (a) immersion in (or application of wet packs of) cold MgSO4, or (b) subcutaneous injection of a 10% solution of calcium gluconate (which gives rapid relief from pain), or (c) surgical excision of the burn lesion. Medical attention is essential, even if the initial effects appear slight, because of the slow onset of the more serious medical symptoms."

Pragmatically, the single most important thing about working with HF is to take it very seriously and to not take a chance at not realizing if you are exposed to it. As mentioned above, HF is very slow-acting, and you will not get an instant burning sensation, as with hycrochloric or sulfuric acids, if you come in contact with it. By the time you feel an HF burn, it is far too late. Almost all minor superficial exposure to HF is minor IF it is *immediately* noted and acted upon. If you get *any* amount of HF on your skin, rinse with copious amounts of water. This is usually enough if you do so immediately. An important exception: places you can not rinse well. For example, the cuticles of your fingers and under your fingernails. Any exposure of such areas should get professional, and informed, medical attention. *DO NOT ASSUME MOST DOCTORS KNOW HOW TO TREAT HF BURNS* *EXPLAIN THE MEDICAL EFFECT OF HF ON TISSUES* *HAVE A COPY OF ONE OF THE REFERENCES LISTED BELOW, AND BRING THEM WITH YOU TO THE HOSPITAL* Or don't, hey, it's your life.

Two quotes from reference (1), below, illustrate the point:

Case 1: A drop of concentrated HF splashed on to the finger nail of a patient. The finger was insufficiently washed. The exposed point turned gradually to a white-yellowish colour, but no further visible changes were observed. Pain occurred 7h later which continued for about 30 h. Examination of the tissue under the nail then showed that a pea-sized area had already been destroyed by necrosis requiring surgical treatment.

Case 2: HF had dropped on to the finger of a patient. Insufficient treatment resulted in the amputation of the exposed finger. Chemical analysis of this finger gave a fluoride content of as little as 0.16 mg. If you wear gloves, don't become aloof and think you are safe. The perspiration on your skin makes it difficult to know if a hole has formed in the glove and that a droplet of liquid on your skin is HF rather than sweat. Wash your hands thoroughly after you remove your gloves, and change gloves at minimum every ten minutes when working with HF, washing and drying your hands thoroughly inbetween. Also change gloves every time you finish working with HF solutions and are ready to move on to something else.

Let me close by reprinting some sage commentary with some excellent references from a previous SPM discussion group posting:

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Date: Wed, 14 Apr 1999 13:49:55 -0600

From: Ken Westra <ken@amc.ab.ca>

Subject: HF Hazards with references

Message-ID: <#1>

To continue the HF discussion:

HF is a very dangerous reagent. It is only a weak acid, but it acts like a toxin. So, you can get dangerous amounts on you, but you won't know it. The dangers are maiming, amputation, and death. A 25 square inch spill can caused death, if not treated properly. If you know what you are doing, you can treat it properly.(Also, a single drop, if not treated has caused fingers to be amputated) Microfabrcation facilities (both academic and industry) use lots of HF, but there are few problems, because they understand what they are doing and have invested in protective gear and the necessary medical supplies. The problems occur in labs who use HF only once or once in awhile. This is were the horror stories typically come from. And I think making mica samples falls in to this catagory.

My advice is: If you are already use to using HF and have all the needed facilites, go ahead with the mica work. Its nothing that you haven't done before. If don't have experience or the equipment for HF work, find a lab on campus who does (a microfabrication facility, for example)and do it there. If there isn't a lab with experience, either don't do the experiment or spend the time and money to do it right.

Nobody should die for their research (and with HF, this has happened). Sci.chem has periodic discussions on HF. From those discussions I have a few references that anyone using or planning to use HF should have:

1)"Symptons and treatment of HF injuries" D. Peters and R. Miethchen J of Fluorine Chemistry Vol 79 pp 161-165 (1996)

2) "Recommended medical treatment of HF acid exposure" Allied Signal HF Products Division. Morristown NJ 800-622-5002

Ken Westra, MicroFab at the University of Alberta

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Please be careful out there. Good luck cleaning your tips. I'd be interested to hear success stories. I guess you can complain if this doesn't work out to, but remember I'm a grad student and thus already think I have lots to be unhappy about, even if it's not really true.