Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Detecting hydrogen gas via IR spectroscopy

  1. Jul 5, 2010 #1
    Say I want to monitor the interface of an electrochemical cell via IR spectroscopy. If H2 gas is generated, there must be a characteristic IR signature to this event. How can I figure out what this is? Are there calculations for this? I'm sure this is in the literature somewhere, but I haven't been able to find it. Any references or help would be appreciated.
  2. jcsd
  3. Jul 5, 2010 #2


    User Avatar
    Science Advisor

    What's "characteristic" of H2 will depend on what else is around. Take a look at the IR spectra for H2 and whatever other gases are (or may be) present, find some band which is characteristic for H2 in that context, and that's what you want to measure.
  4. Jul 5, 2010 #3
    The hydrogen Paschen series (emission / absorption) lies in the IR band.


    See also;

    Use of spectroscopy to detect H2 gas will not be easy unless you can get precise control and containment of any H2 produced by your reaction. As they reflect relatively small energy level exchanges, detection of the Paschen series require more sensitive measurements.

    Spectroscopy requires that the hydrogen molecules (H2) are first broken up into hydrogen atoms (H) which can then be caused to emit (or absorb) energy. Measurement of the presence (or absence) of IR energy at the known Paschen series wavelengths would be your indication that the reaction has occured.

  5. Jul 6, 2010 #4


    User Avatar
    Science Advisor

    This is wrong.
    1) Hydrogen gas molecules, at pressure, don't have a spectrum that looks like the atomic spectrum at near-vacuum that the Paschen series is about.
    2) Almost no hydrogen gas is in an excited n=3 state at room temperature (or anywhere near it), for which reason you wouldn't see much absorption in those lines anyway.
    3) It does not require precise measurement to see Paschen lines. Do you know what a typical resolution of a spectrometer is? Less than 1 cm-1. The first Paschen lines are separated by thousands of cm-1.
    4) IR spectroscopy, as a rule, looks at vibrational and rotational transitions, not electronic ones.
    5) IR/Vis/UV spectroscopy does NOT require that a molecule be 'broken up' into atoms at all - (or you wouldn't even have vibrational/rotational spectra).
    6) Spectroscopy in general doesn't require any such thing either. You think molecules don't emit and absorb radiation as well?

    The only major issue measuring the IR vibrational spectrum of hydrogen is that it's homonuclear, which means you have to use Raman.
  6. Jul 6, 2010 #5
    I was a bit confused by some of that, thanks for clearing things up.
  7. Jul 7, 2010 #6
    My apologies for any confusion my post created... I allowed myself to follow a one-track, very impractical scheme which proceeded from my knowledge that ATOMIC hydrogen absorbs/emits in the IR (Paschen series) while ignoring completely the fact that MOLECULAR hydrogen will have an IR spectrum due to vibrational and rotational transitions...

    The complex process in my thinking (which lay behind my post) involved something along the lines of the following;

    1. Collect and contain all gases produced by the reaction.
    2. Separate the produced gases with a chromatograph.
    3. Isolate the hydrogen (if any).
    (Note; the step above (and a mass spectrometer) would be sufficient to detect hydrogen... but johng23 asked about the use of IR spectroscopy, so step (3.) is simply preparative in nature.)
    4. Proceed with IR spectroscopy on the isolated sample to detect absorption / emission in the Paschen series for hydrogen.
    (Note; this is the step where the sample would be treated such that any hydrogen is excited to the n=3 state etc.)

    So... once again, my apologies.

    Edit; where I say "IR spectroscopy" I mean emission / absorption spectroscopy in general with detection focused in the IR.

    Last edited: Jul 7, 2010
  8. Jul 7, 2010 #7


    User Avatar
    Science Advisor

    That's fine. But arguably, if you're using GC, that'd probably be enough given that there's a limited number of substances involved. Could be a bit tricky though, given that most GCs use hydrogen as a carrier gas.

    As far as spectroscopy goes I only see two realistic options, which is either IR-Raman or to forget about IR and go to UV. Molecular (or atomic) hydrogen at room temperature simply doesn't have any strong lines in the IR. Being a diatomic, it only has one vibrational mode, and being homonuclear, that vibrational mode has no dipole moment change associated with it, making it invisible to ordinary IR absorption.
    In practice, Raman is what's generally used for H2 detection.

    Well so do I, and I include IR-Raman. But oftentimes people mean absorption only when they say IR (as opposed to Raman in the IR).
  9. Sep 26, 2010 #8
    Before I hijack this thread, in answer to the original posters question, this link may be useful:

    Okay, I am an organic chemist. I am hoping that as in past that my background can gain me some sympathy for a certain degree of wrong-headedness.

    alxm has virtually captured my interest in hydrogen's spectra entirely.
    1) I have searched for atomic absorption spectra for hydrogen. This is difficult to find. However, I believe the emitted and absorbed spectra that the Paschen series is about is measured from hydrogen gas. Is this incorrect?

    2) I have thought about the Bohr atom and those absorptions. This comment captures my thoughts entirely. If hydrogen emits as much energy as it does, then there shouldn't be any gas in the n=3 state. Intuitively, this just seems wrong. If I stand near the coals of a fire, I can feel the emitted heat as it cools. I am not aware of any threshold at which this no longer occurs.

    4) Hydrogen has protons and electrons, what absorbs?
  10. Jul 13, 2011 #9
    At high pressure hydrogen molecule absorbs IR light at wavenumbers around 4500.
    H2 is homo-nuclear molecule,no dipole moment.But it can have quadrapole moment and become IR active.
    You can search for IR spectra of solid para-hydrogen, and will find a bunch of papers on it.
    gas phase IR spectra is comparable to that of solid para hydrogen spectra.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook