It's entropy which "cares for" the perfect mixture of gases at any time, so they won't separate.

Thank you sir

The gases are filled inside the cylinder may be in liquid form (not all case), when it is open attain an inverse effect at room temperature. All real gases (H2 and N2 ) are filled inside the cylinder by Pv=nRTrule. Under this supper critical conditions I feel these gas may not involved any reaction. Do you find any weight changes in cylinder that will possible case for undergone reaction.

In an old (~5 year) gas cylinder we have found it best to give it a quick purge before using it, creating turbulence in the bottle. Also the gas mixture (containing NO) had changed, we found oxidised NO2.

Thank you Andrew. But why is the composition changing with time.. since gas molecules don't react at room temperature.

The bottle we had, was old, but we used it little by little each 3 months (calibration gas). If nothing changed in the bottle, there must have been an outside influence... perhaps O2 from the ambient air migrating (diffusion?) inwards each time the bottle was used? How else could NO be oxidised to NO2 inside a gas bottle? If all things remain equal, the simplest answer must be correct. What do you think?

Yes it might be possible dee to gas composition changes in the case of NO cylinder. There are two different (liner and bend) structures available for NO and NO2 molecules respectively. The linear NO is involve two electron system being tend to accept one oxygen from other molecule to form stable NO2 gas. This effect i considering the charge de localisation between two electro negative atoms is favor free energy shifts by little pressure changes inside the container due to its surrounding atmosphere. But i guess it does not form NH3 gas or H2O (example) due to high free energy factors.

Dear Nikhil,

I am not in accordance with Peters response (entropy takes care of everything). In principle it is possible that you get a gravitational segregation in your standard gas cylinder. However, diffusion in the bottle will partly counteract. this process. It might occur if you have two gases in the standard which largely differ in molecular weight (e.g. butane and Hydrogen in N2 or so).

I doubt, that you will see this effect in standard GC analysis. But you might check by rolling the gas bottle (properly sealed with protection cover!!!!) along a floor for several times.

As mentioned by different people here standard gases might react in the bottle. But no sense to speculate, if this is possible because you didn´t mention what kind of gas you are dealing with. However, all standard gases have a specified lifetime, so you should check this and replace the standard.

Otherwise (if standard is not outside the specified time and regurlarly used) I would check your instrument for potential drift. Could be anything from slowly degrading/increasing split ratios, detector degrading and so on.

Regards

Stefan

Addendum to my own post:

Does the drift effect ALL components in the same degree? If yes, its possibly not an issue of the standard. If not, does it effect lighter components more than heavier ones? Then replace the standard. No valve is leaktight (don´t believe any manufacturer :-)) and you might have an expired standard with preferential loss of light(er) components through the main valve,

Regards

Stefan

Hi all, glad to see your discussion here.

I fully agree with Stefan on gas separation in the cylinder, esp when the molecular weight differs a lot, eg. He in N2+CO2. One of our customer met this issue after the cylinder was put in their factory for months, then their laser machine keeps warning on wrong gas.

That's why in our factory when filling multi gases in one cylinder containing H2 or He in Ar or CO2, rolling it is a must before its delivery to the customer.

BRs,

Yuanbiao

So what about the effect of Pressure and Temperature?

as you know, if we suppose the gases to treat as real gas, according to PV=nRT, the density belongs to the pressure and temperature ...

so my question is:

Is there any order of magnitude for the density difference to claim on gas/gas gravity separation?

Are the P&T important?

For instance, I'm facing a huge drum filled with N2 & O2, fixed in a isolation condition without any motion. Will the N2 and O2 be separated in a long time? if yes, do you have any estimation on that?

Nikhil Gn There have been experiments in which gaseous mixtures in pressure vessels undergo separation due to difference in their densities.