Hi. I am seeking some guidance in understanding how the dosing works on typical gas sorption equipment (the attached shot is from a Micromeritics 3 Flex).
I describe what I think is the case below and am asking others to correct me or add to my description as necessary. I then ask my question regarding equilibrium at the end - feel free to skip to the end.
In setting up an analysis, the operator defines a set of target pressures to collect data points at. The operator also defines absolute and/or relative tolerances for the target pressure. I.e. so the instrument does not need to reach exactly the defined pressure value.
During the analysis the instrument injects a quantity of gas into the tube that it expects might get it close to the target pressure. The quantity is determined based on measured or estimated free space, manifold volume etc. (when fixed quantity dosing is not selected). The instrument then waits for "equilibrium".
When equilibrium is reached, if the target pressure has been achieved, within the set tolerances, the instrument will record a data point and move on to the next target pressure the operator has defined in the analysis setup. If the target pressure has not been achieved, the instrument will inject another quantity of gas and wait again for equilibrium. The whole process repeats until all adsorption data is collected. Desorption curves are similar, but gas is being removed via the vacuum system.
My question is, what determines "equilibrium"? The attached image shows a setup where the instrument has been asked to check for equilibrium every 10s. The rate of change between checks is around 0.015%. I have seen the instrument inject more gas when the rates of change between intervals are much higher than this so I am confused about what is the guiding criteria for "equilibrium reached" in the programming of the instrument. Indeed there is no consistent value in any of the numbers displayed that seems to relate to what defines equilibrium. This is very frustrating since it is not easy to understand why some analyses can take hours to equilibrate a single dose, and even then, not necessarily reach the target pressure to record a data point.
Hi Michael,
Have you considered asking directly to the manufacturer? I mean, they built the equipment, programmed the software and their technical team should have quite a bit of experience with this kind of situations. In my experience Micromeritics is quite responsive.
Now, to your question, the "equilibrium" is considered to be achieved when the change of pressure after an injection is lower than a certain value.
Can you go back to the default settings to your apparatus and try again?
Also, are the samples that you measure similar among each other? Usually microporous samples take many hours to achieve equilibrium during adsorption, so a sample with larger pores "will have to wait" for the equilibrium in the other stations.
Hi Fernando, thank you for your reply.
We deal through a distributor, who is the first point of contact. Let me say that this avenue is still being explored before going straight to Micromeritics.
I understand the sample dependence. That is not really the issue. But you are right - it is exactly the reason that port 2 above was waiting for ~63,000s for other ports...
Worded differently, I would ask almost the same question by saying; does anyone know the value/purpose of the information in the analysis window that says; "Rate of Change = XXXX%"? Because it does not seem to give me any indication of how close to equilibrium I am. Is there some not-displayed criteria for equilibrium?
I have watched the instrument closely before and NEVER seen the transducer PERFECTLY match two values between an interval, but I have seen very low "Rate of Change" values between intervals (< 0.015%) without it accepting equilibrium. Yet often much larger "Rate of Change" values immediately before it does do another dose, but without appearing to reach the "Limit" value.
Hi there,
Usually, there are 2 Micromeritics Softwares come together, the first one that you are showing it now in the post and the second one should be "ROA" or Rate of Adsorption. In order to get the equilibrium at each P/P* point, in ROA software they will record the relationship between time and amount of adsorption at that P/P* point (they will record it until equilibrium, so at that equilibrium, no matter how long it takes, the amount of adsorption will not be changed anymore). I think you can use that data to predict the equilibrium time for each individual point as well (not exactly, but it could be your guideline). I hope this might help.
Regards,
Art
There are some equipments that allows you to put the time of equilibrium you want, and the tolerance that you have for deviating the target, probably it wasn't accepting the value for Low tolerance. (lower tolerance usually increases time of experiment, as the equipment will be doing more smaller injections to avoid passing the value)
I had the same problem prior to the lockdown, and talked to Micromeritics about it - they confirmed esssentially what Darren Lapham said.
A small thing I would add is concerning the existence of ultramicropores, as evidenced by long equilibration times in the ultra low pressure region. Jacek Jagiello has done some work with using H2 to probe the ultramicropores (see link at bottom) and I've made some preliminary using dual isotherm measurements (H2/N2) which indicates that those samples which take so long to equilibrate often have pores of widths around 0.4 nm. The porosity data you can calculate from the low pressure region of the N2 isotherm can also be supplied by a (much faster) H2 isotherm, so you can start your isotherm at around 10-4 P/P0, and use both sets of data. SBET and most other classical parameters remain broadly uneffected by the reduced N2 isotherm.
Article Exploiting the adsorption of simple gases O2 and H2 with min...
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