I'm using Quantachrome iQ for the surface properties. If I use the classical range of multipoint BET ( P/Po from 0.05 to 0.3 ) some times the result of the C- constant is negative sign ( which is not logical ) , but when applying the BET assistant software (selects the P/Po) in the very linear range ... the C- constant will be positive
According to the BET theory in IUPAC Report (http://www.ims.uconn.edu/wp-content/uploads/sites/190/2014/01/05079-NOVAWin11.0-RevN.pdf), the parameter C is exponentially related to the energy of monolayer adsorption. It is now generally agreed that the value of C rather gives a useful indication of the shape of the isotherm in the BET range. The range of linearity of the BET plot is always restricted to a limited part of the isotherm, often within the p/p° range of ∼ 0.05–0.30. The parameter C should be positive (i.e., a negative intercept on the ordinate of the BET plot is the first indication that one is outside the appropriate range).
The process for data selection and optimization has been fully automated and is based on the rules proposed by Prof. Jean Rouquerol ( http://micro-report.com/automatic-bet-surface-area/ ).
(a) the quantity C should be positive;
(b) application of the BET equation should be restricted to the range where the term n(1 – p/p0) continuously increases with p/p0;
(c) the p/p0 value corresponding to nm should be within the selected BET range.
The work of Rouquerol is good for indicating the máximum relative pressure that you can use for your BET plot. Put simply for non-porous materials the linear range will be located between about 0.05 to 0.35. If the simple has mesopores then this range is shortened depending on how narrow the mesopores are since their filling will cause an upswing on the amount adsorbed so that if you include those points then the intercept could go negative. I have had some samples that are not microporous but due to the very narrow mesopores the linear range for BET was only from 0.05 to 0.2. If the simple is microporous then the linear range is shifted to lower relative pressures and becomes narrower, typically 0.02 to 0.15 or 0.02 to 0.12.
Best regards,
Malcolm
According to Malcon Yates, the Rouquerol method allows you to determine the maximum pressure you have to use in the BET method. To determine the maximum and minimum pressures, I recommend using a different linearization of the BET equation, reflected in the publication:
Characterization of activated carbons by BET equation-an alternative approach. J. Ads Sci Tech (1995) 12 (1) pp 51-66
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