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I've read that hydrofluoric acid (HF) is extremely dangerous to touch, but what exactly makes it so toxic? It's weak acid ($K_a = 7.2 \times 10^{–4}$) and dissociates approximately 1/1000 as much as Hydrochloric acid does in water, so why is it so much more dangerous? Wouldn't a stronger acid cause more damage? What exactly happens when human skin comes into contact with Hydrofluoric acid that causes it to be so lethal?

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The confusion arises from the term weak, which has only to be interpreted in chemical terms.

Weak acid, as you say, just means that the acid does not readily dissociate, not that its effects are weak! Just to say one, HF corrodes glass, something that not even smoking HCl does.

Wikipedia has a nice summary of HF toxicity (see also the references in the article itself):

Hydrofluoric acid is a highly corrosive liquid and is a contact poison. It should be handled with extreme care, beyond that accorded to other mineral acids. Owing to its low dissociation constant, HF as a neutral lipid-soluble molecule penetrates tissue more rapidly than typical mineral acids. Because of the ability of hydrofluoric acid to penetrate tissue, poisoning can occur readily through exposure of skin or eyes, or when inhaled or swallowed. Symptoms of exposure to hydrofluoric acid may not be immediately evident. HF interferes with nerve function, meaning that burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury.
Once absorbed into blood through the skin, it reacts with blood calcium and may cause cardiac arrest. Burns with areas larger than 25 square inches (160 cm2) have the potential to cause serious systemic toxicity from interference with blood and tissue calcium levels. In the body, hydrofluoric acid reacts with the ubiquitous biologically important ions Ca2+ and Mg2+. Formation of insoluble calcium fluoride is proposed as the etiology for both precipitous fall in serum calcium and the severe pain associated with tissue toxicity.

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This question appears to have a revived interest? I would like to clarify this Q&A you don't mind.

  1. HF(aq), like other hydrogen halides, does completely give up its proton to $\ce{H2O}$ but since F is the most eletronegative element on earth, it will pair up with hydronium ion in the aqueous environment, making the proton "less available", at least to a pH electrode. This is one of strongest hydrogen bond known.

$\ce{HF(aq) -> F^-.... H3O+}$

Thus HF(aq) has a depressed pKa in water whilst its "real acidity" can be much higher.

  1. Homo-association of HF only occurs in very high concentration (close to unity). You can estimate the extent of homo-association of anhydrous HF by measuring its conductivity. Anhydrous HF has lower conductivity (<1.6 $\mu$S/cm) than deionized water (~5 $\mu$S/cm) [1] so it's probably not terribly important in driving disassociation of HF, esp in water.

  2. HF causes deep burn, often with a delayed onset because while the skin prevents charged raw hydronium ion from penetrating too deeply, HF or the hydronium-flouride ion pair can be absorbed and releases the hydronium ion (by exchanging with water abundant in tissue) at deeper level. (Thus it's often reported as more painful than other acid burn)

  3. Although fluoride in ppm level is anti-microbial and prevents cavity (by reacting with calcium mineral in tooth), high concentration of fluoride is toxic because, as cited in previous answer, fluoride interferes with calcium metabolism in forming insoluble calcium fluoride. It causes skeletal fluorosis and suppresses some essential metabolic enzymes.

  4. HF is corrosive because of its proton; it's corrosive to bones because of its fluoride. HF corrodes glass and HCl doesn't because silicon tetrafluoride is a more stable complex than the highly reactive silicon tetrachloride. It has little to do with whether the acid is weak or strong.

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Nice answer and thanks for sharing +1 – Christiaan Mar 19 '15 at 4:56

I am not a chemist and unfortunately I will have to refer to wikipedia, but the ionization constant for HF (10−3.15, according to wikipedia) does not take homo-association into account.

Homo-association is an association between a base and its conjugate acid through a hydrogen bond. This process stabilizes the conjugate base, thereby increasing ionization. Often this behavior increases acidity, especially at high concentrations. In case of HF (double arrows indicate equilibria):

2HF <- -> H2F+ + F (autoionization of HF)
HF + F <- -> HF2 (homoassociation)

The formation of the bifluoride anion (HF2) encourages the ionization of HF by stabilizing and removing F from solution hence pulling the dissociation equilibrium of HF (HF + H2O <- -> H3O+ + F-) to the right. Thus, the usual ionization constant for hydrofluoric acid (10−3.15) understates the acidity of concentrated solutions of HF.

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Although the importance of that would depend on the reaction constants of those three reactions... do you by chance know what they are? – nico Mar 17 '15 at 13:29
@nico - great comment - but no. I'm just a humble biologist and stumbled upon this info after digging into it. HF is absolutely a strong acid from my perspective and this proves it, albeit just qualitatively. A quantitative answer has not even appeared on chemistry.SE, where a few related questions were posted. – Christiaan Mar 17 '15 at 13:31

protected by Chris Mar 16 '15 at 13:03

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