If I add phosphorus pentoxide to formic acid at atmospheric pressure and 20°C will they react? At what rate will CO be produced?

I have not been able to find a reference discussing this.

  • 6
    $\begingroup$ I have no particular knowledge about this, but P4O10 dehydrates many stable substances. Formic acid is not stable, very slowly decays to H2O and CO, so it is adviced not to keep it very tight closed. So I predict it may be vivid to violent reaction. $\endgroup$
    – Poutnik
    Commented Oct 30, 2020 at 15:34
  • 3
    $\begingroup$ Using sulphuric acid instead is the laboratory method to produce CO. $\endgroup$
    – Poutnik
    Commented Oct 30, 2020 at 15:51
  • 2
    $\begingroup$ Also, P2O5 is a stronger dehydrating agent than H2SO4. $\endgroup$ Commented Oct 30, 2020 at 17:53

1 Answer 1


As noted, the dehydrating action of H2SO4 on HCOOH liberates CO gas. To quote a source:

The sulfuric acid removes the elements of water (i.e., H2O) from the formic or oxalic acid and absorbs the water produced.

As P2O5 is at least as dehydrating as Sulfuric acid, expect the same per the reaction:

$$\ce{P2O5 + 3 HCOOH -> 3 CO (g) + 2 H3PO4}$$

  • $\begingroup$ Note that P4O10 does not get hydrated to H3PO4, but rather to polymerized metaphosphoric acid (HPO3)n. $\endgroup$
    – Poutnik
    Commented Nov 1, 2020 at 8:07

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