Hi friends,
As we all know, Isotherm is the relationship between quantity adsorbed and pressure at constant temp. But in isotherm plots, quantity adsorbed is usually plotted against P/P0. Why it is so? Instead of pressure, why it is taking the value of P/P0? Can anybody explain?
Regards
Ansari P
Hi Ansari,
as there's a huge diversity in pressure units, a dimensionless plot p/po will save you the hassle of converting pressure units.
Best regards
Niko
The BET calculation for surface area is normally done by sub-critical temperature physisorption (e.g. N2, 77.4K). Physisorption then is sensitive to not simply absolute pressure, but relative pressure, p/p0, i.e. the ratio of absolute pressure to the saturation vapor pressure of the adsorptive (analysis gas) at analysis temperature. If the temperature were changed from 77.4 K to say 75 K, the absolute pressure at which the monolayer of adsorbed gas would form (from which surface area is calculated) would be lower than at 77.4K. Raise the temperature a bit, and that absolute pressure would also be raised! So all physisorption calculations (BET, BJH, DFT) are always done with respect to relative pressure p/p0.