I am trying to verify the results of EPA method 26A (http://www.epa.gov/ttnemc01/promgate/m-26a.pdf). The Method claims to be able to separate and quantify a mixed halogen/halide gas stream. Although I believe the chemistry holds true for Cl₂/HCl and Br₂/HBr, I'm not so sure about F₂/HF. Despite the Method not mentioning F₂, it commonly produces data for both HF and F₂. I am dubious that the F₂ results are real and am trying to develop a chemistry argument to support that position, if possible.

In summary, the Method takes a mixed gas stream (F₂/HF) and passes it through a 0.1 N H₂SO₄ solution, then a 0.1 NaOH solution; sodium thiosulfate is later added to the alkaline potion. The theory is that the HF is captured in the acid solution, while the F₂ passes through unreacted. In the alkaline solution, the F₂ reacts to form HF and the hypohalous acid (HOF, I assume). The thiosulfate converts the HOF to another HF. When the acid and alkaline solutions are analyzed for fluoride ion, the quantity is assumed to be the HF and F₂ in the original sample, respectively.

Considering that F₂ is so reactive, I would think that any F₂ in the gas stream would react with the water in the acid solution and that theoretically no F₂ would make it to the alkaline solution. Is this a reasonable assumption?

If any F₂ did get to the alkaline solution, I am not sure that the conditions are favorable for the creation of HOF; F₂ reacting with water to produce HF seems more probable. Unless the acid solution is ignored by the F₂, I don't think that a true separation is taking place and any fluoride detected in the alkaline solution would represent blow through or some other error. Does this seem like a reasonable conclusion?

Thanks in advance for any input!

  • $\begingroup$ I have no idea whether this is feasible or not, but I imagine you could figure it out with some theoretically simple controls. First pass a mixture of the gasses (preferably in known amounts) through the dilute acid solution and measure fluoride. Then take the same mixture and directly add it to the base solution without thiosulphate and measure fluoride. Finally, take the mixture and pass it though a basic solution with thiosulphate. It should be possible to figure out if the fluorine gas makes it through the acid solution, and also if thiosulphate actually does anything. $\endgroup$ Commented Jan 21, 2014 at 23:46
  • $\begingroup$ I believe the better option to catch $HF$ is to freeze it from the gas stream, as it has relatively high (~19 celsius) boiling point. $CO_2$ cooled trap will do it for sure and ice-cooled trap may do it as well. Fluorine is really active and does react with water. AFAIK, water burns in fluorine. $\endgroup$
    – permeakra
    Commented Mar 23, 2014 at 19:12
  • $\begingroup$ Reading the standard closely- its says only identify hydrogen halides in Chlorine and Bromine only. $\endgroup$ Commented Apr 15, 2014 at 8:11

1 Answer 1


The answer is no, because as you remark $\ce{F2}$ will react with water and produce $\ce{HF}$. So it is not the case that $\ce{F2}$ passes through unreacted.

I'm puzzled by this sentence:

Despite the Method not mentioning F₂, it commonly produces data for both HF and F₂

Can you clarify what you mean by "it produces data"?

  • $\begingroup$ When the Method is performed on a mixed halide/halide acid stream, halide ions can potentially be in both the acid and base solutions; ultimately you get two halide ion concentrations. Test report data I've reviewed show F ion in the base solution and conclude the presence of F2; from this discussion, I do not believe that is the case. However, the Method is often used by EPA and industry as if it can quantify and separate a mixed F2/HF exhaust stream. $\endgroup$
    – John
    Commented Aug 4, 2014 at 20:26

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