There is a difference between fluoride and fluorine.  Fluoride is an anion found in many salts.  Fluorine is an atom found covalently bonded in many compounds.  This distinction is particularly important when considering the toxic effects of fluoride.

 

Compounds recently mentioned, such as trifluoroacetic acid (TFA) and sodium tetrafluoroborate, do not contain "free" fluoride.  The F atoms are covalently bonded to C in TFA and to B in sodium tetrafluoroborate.  The later salt is sneaky since it is a salt, but the anion is the complex ion, BF₄^-, not fluoride, F^-.  The covalently-bonded F atoms do not ionize in water.  (Trichloroacetic acid, of course, is not of concern with regard to fluoride exposure.)

 

Hydrofluoric acid, HF, is also sneaky - but in a deleterious fashion.  It is weak acid (pK_a = 3.14) which means that most of the molecule is unionized in water, and as an acid, not so dangerous.  However, when HF penetrates the skin the small amount of ionized fluoride present reacts with calcium ions in the blood to precipitate calcium fluoride (K_sp = 0.0002).   As this precipitation occurs the acid equilibrium will continue to shift (via the common ion effect) to generate more fluoride, and eventually "all" of the fluorine in HF will become free fluoride that reacts with Ca²⁺.  This leads to hypocalcemia in the blood; not good.

 

Calcium gluconate has the effect of restoring the Ca²⁺ levels in the blood. Wikipedia has a good webpage about HF burns, with gruesome pictures if you like that sort of thing.

 

Note:  TFA is indeed harmful but largely as a strong corrosive acid; (pK_a = 0.5).  Under the effect of three very electronegative fluorine atoms the less-dangerous (weak) acetic acid (pK_a = 4.7) becomes a strong acid as the anion (CF₃COO^-) becomes greatly stabilized by the F atoms.  But there are still no free fluoride anions.

 

It is also instructive to look at SDSs for these compounds:

 

(Sigma) for HF:  use calcium gluconate

(Sigma) for NaF: use calcium gluconate

(Sigma) for TFA:  no mention of calcium gluconate

(Fisher) for ammonium tetrafluoroborate;  no mention of calcium gluconate        

 

It is interesting, and puzzling, that the Sigma SDSs for ammonium tetrafluoroborate and sodium  tetrafluoroborate, have long sections about HF - as if HF is generated by ammonium tetrafluoroborate (and sodium or silver tetrafluoroborate) via hydrolysis or some other pathway.  This is not discussed.  If anyone else has an insight into this, please comment.

 

So, should one apply calcium gluconate to TFA skin exposure?  Some ER physicians have tried this since they apparently forgot their general chemistry lessons.  Google Scholar found the papers below; I have italicized the relevant conclusions.

1)  Full-Thickness Chemical Burn From Trifluoroacetic Acid: A Ca... : Annals of Plastic Surgery (lww.com)

Full-Thickness Chemical Burn From Trifluoroacetic Acid

A Case Report and Review of the Literature

Annals of Plastic Surgery: November 2018 - Volume 81 - Issue 5 - p 528-530

doi: 10.1097/SAP.0000000000001579

 

Trifluoroacetic acid does not seem to involve the systemic toxicities that result from hydrofluoric acid burns, and there is no role for calcium gluconate in acute management based on this case. 

 

2)  Chemical burns caused by trifluoroacetic acid

16 August 2013; https://doi.org/10.1111/cod.12059

 

Background

Trifluoroacetic acid is a very strong carboxylic acid. The acid has been suspected to have similar toxic effects as hydrofluoric acid on skin contact. Hydrofluoric acid is highly toxic, owing to skin penetration by fluoride ions. A spill of hydrofluoric acid on the skin may be fatal. As trifluoroacetic acid contains fluorine, patients with chemical burns caused by trifluoroacetic acid have been given particular attention when treated in the hospital.

Methods

Five patients with chemical skin burns caused by trifluoroacetic acid were reviewed with regard to the extent of the burn, treatment, blood samples taken, and systemic effects.

Results

The chemical burns reported were limited (< 1% of the total body surface). The burns healed as expected for chemical burns caused by acids. None of the patients showed any symptoms or signs that are typical for hydrofluoric acid burns.

Conclusion

Localized chemical burns caused by trifluoroacetic acid should be regarded as being similar to burns from other acids, with the exception of hydrofluoric acid. To our knowledge, there are no indications that trifluoroacetic acid causes the same toxic effects as hydrofluoric acid.

 

Dave