The isotherms looks a bit like in paper Zhai et al, RSC Adv. 2017, 7, 22548. It seems that my sample sorbs linearly. When I tried to evaluate it, I got C value around 2 which means huge error for the result. I would be grateful for any ideas how to increase C value or how to proceed with measurement or evaluation.
The C value is an indication on the equilibrium between adsorption and desorption. A small C value indicates low affinity between nitrogen molecules and the surface. What is the BET surface for the sample, and the pore volume and pore size distribution. Also check the Roquerol plot, are You in the right region of p/p0, Roquerol should be rising as a function of relative pressure. And share some isotherms and transformation plots to the question, it is much easier to see if something is wrong from a graph.
I strongly recommend the IUPAC report of 2015 when picking your points for BET calculations.
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