[KeithM]

I am trying to seal a gas (0.3% HCl in 97.7% N2) in a small fused silica gas cell with a side-arm.

During the fill process and with a temporary seal on the side arm I can see the infra-red HCl trace on the spectrophotometer, but when I seal it off permanently with the gas torch, there is literally no trace any more. The cell and side arm are all transparent fused silica (Spectrosil 2000) and so the sealing temperature is in excess of 1600C but it is just on a point of the side arm as we use a needle flame from a portable oxy-butane torch.

I am drawn to the conclusion that either the heat is dissociating the HCl or the HCl is reacting with the hot glass but I cannot think of a mechanism for either. I can't find any reliable info on the dissociation temperature of HCl but it seems to sythesise in a hydrogen-chlorine flame at 2600C so that would seem to go against the dissociation idea.

Anybody got any ideas please?

[Borek]

How hot is the cell during sealing? Perhaps it heats up enough so that you are losing substantial part of the inside gas? Unlikely, but worth asking

[KeithM]

The flame is white hot and fused silica melts/softens at around 1600C but that is just at the flame point on the side arm. Of course the cell itself does get hot but thats not easy to measure. I'd guess no more than 200C as the neoprene temporary seal does not melt or burn.

During sealing the side arm is not connected to the gas filling chain, it is sealed off with a neoprene sleeve and clamp and I am happy that its OK like this as I can keep in the spectrophotometer beam for about half an hour to an hour like this. (It slowly leaks over the half hour and we can watch this in the intensity of the trace)

I guess the flame heat could accelerate the leak, and that could be the only explanation but I was wondering if there was any chemical reaction explanation possible.

[cth]

Could it adsorb on the wall surface? I think silica has on its surface silanol groups Si-OH. If you treat them with a base to clean the cell, they may become basic themselves.

How do you clean and treat the silica surface before you put the gas into it?

[KeithM]

Hhmm, could be adsorption, I'd wondered about this so we flushed the cell a few times with 100% HCl gas first.

I'm planning to try a soda glass (ordinary glass) cell soon as this will cut the flame temperature needed for sealing down to about 550C. May take a while.

[Lv2sfo]

could you submerge a portion of the vessel in liquid nitrogen to condense and freeze any present HCl before sealing?
 that would eliminate any issues with reaction with hot glass