2
$\begingroup$

Why are the gas sample tubes of hydrogen filled at a reduce pressure when observing hydrogen's emission spectrum? In my HS chemistry lab, we looked at gas sample tubes of hydrogen through a spectrometer and I am wondering why they were filled at a reduced pressure?

$\endgroup$
  • 2
    $\begingroup$ Think of the distance that an electron travels before hitting an atom of hydrogen. If the gas were dense then that distance would be short. In other words it would require more voltage to get a discharge across the tube, and more voltage to maintain the discharge. With more voltage, you'd also get more current when the discharge does occur. That would mean more heat. // ;-) I'd guess part of the reason here too is safety. A high voltage with very, very low current isn't deadly. I got a nasty jolt and a small electrical burn from such an apparatus, but lived. $\endgroup$ – MaxW Jan 3 '16 at 20:47
  • $\begingroup$ Googling 'Paschen curve' may prove enlightening (pun intended). $\endgroup$ – Jon Custer Jan 4 '16 at 1:16
2
$\begingroup$

Not only does it take a high starting voltage and a current-limiting device (e.g. inductor) to maintain a continuous discharge at higher pressures, but the quality of the light emitted is changed by atomic (and ionic) interactions, tending to broaden lines into a quasi-continuous spectrum.

A good example is the sodium-vapor lamp, often used for street lighting. In the example cited, compare the line spectrum of a low pressure lamp with that of a high pressure one (though there is also a contribution from other gases in the second image) .

The low-pressure hydrogen glow discharge corresponds well with calculated values and is a useful tool in showing how theoretical calculation of orbitals can be translated into an observable spectrum. Calculation of high-pressure phenomena is much more difficult.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.