From: Wayne Wolsey <wolsey**At_Symbol_Here**MACALESTER.EDU>
Subject: Re: [DCHAS-L] Hydrofluoric acid
Date: April 1, 2012 8:13:50 PM EDT
Reply-To: DCHAS-L <DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU>
Message-ID: <BAY166-W488D4BE127D9D419D46B54C44F0**At_Symbol_Here**phx.gbl>

On the subject of the relative acid strength of HF, the original statement has some aspects of being correct (whether HF is a strong or weak acid). The acid strength of HF has long been a paradox. Certainly in aqueous solution, HF behaves as a typical weak acid--due to some hydrogen bonding with water. However, in the absence of any water, anhydous HF is one of the strongest protonic acids. Liquid HF boils at 19.5 C, so can be used as a solvent easily in the laboratory. Pure liquid HF would not be encounted routinely unless a researcher were using it for some special project.
There is an acidity scale sometimes used for very strong acids, the Hammett scale. On the Hammett scale, anhydrous sulfuric acid and HF have nearly the same value of H(sub zero) = - 11. Here again, the extreme acidity of 100 % HF is a result of hydrogen bonding. 3 HF = H2F(+) + HF2 (-) (over simplified). The fluoride ion has such a strong attraction for a proton, that it will solvate with another HF molecule forming the bifluoride ion HF2(-), leaving behind a proton--which will link up with still another HF molecule. There is no protonic acid which can be added to pure HF(liq) which will increase the proton concentration. Compounds which function as acids, by increasing the H2F(+) concentration, are strong Lewis acids, such as SbF5. SbF5 + 2HF = H2F(+ ) + SbF6(-) .
A discussion of anhydous HF can be found in: T.C. Waddington, Non-Aqueous Solvent Systems, Academic Press, pp, 47-81, 1965.
Wayne Wolsey
Professor Emeritus
Macalester College
St. Paul, MN

On Sun, Apr 1, 2012 at 12:29 PM, Alan Hall <ahalltoxic**At_Symbol_Here**> wrote:

As a coauthor, I will repond. And, I stand corrected.

I will not argue chemistry with a chemist, as I am a Medical Toxicologist which implies a small knowledge of chemistry but certainly not mastery of the field. In the medical part of the world I deal with on a daily basis, HF is a significant occuptional hazard and if a suitable substitute could be found for those processes in which HF is so far required, I'd be happy to lead the cheering section. I am, however, not turning blue from breath-holding. Obviously HF is a "weak acid" with a pKa of about 3.2 as stated in the article, although fluorine is the most electronegative in the periodic table (hope I got that one right).

Until then, we who are involved in occupational heath and safety must muddle through with what is available to us and promote OEHS as best we can.

That said, the chemistry comment is correct. Let's let chemists be chemists be chemists and Medical Toxicologists be themselves as well

Thank you for your comments.

Executive Committee
Division of Chemical Health and Safety
Alan H. Hall, M.D.
Medical Toxicologist


> Date: Sun, 1 Apr 2012 10:39:27 -0400
> From: secretary**At_Symbol_Here**DCHAS.ORG
> Subject: [DCHAS-L] Hydrofluoric acid
> To: DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU

> From: Ernest Lippert <ernielippert**At_Symbol_Here**>
> Subject: Hydrofluoric acid
> Date: March 31, 2012 10:30:09 PM EDT
> With reference to the Case report on hydrofluoric acid (HF) vapor facial exposure (Journal of Chemical Health & Safety, January/February 2012; Si=E9w=E9, et al., p 7) the first sentence of the Introduction is incorrect. Hydrofluoric acid is not "one of the strongest inorganic acids". It is, in fact, a weak acid. See any good basic chemistry text such as Chemistry, J. McMurry and R.C. Fay, Prentice Hall, ISBN 0-13-737776-2, p. 590-591, 1998.
> The sentence would better be stated as "Hydrofluoric acid (HF), although a weak acid, is the most tissue-destructive of the inorganic acids, and …"
> Regards,
> Ernest L. Lippert

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