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?