Surface area characterization — regarding the BET adsorption apparatus, which statement is correct?

Introduction / Context:
The Brunauer–Emmett–Teller (BET) method estimates specific surface area of porous and nonporous solids by nitrogen or other gas adsorption at cryogenic temperatures. The apparatus and the principle are often misunderstood in entry level labs.

Given Data / Assumptions:

• Adsorptive gas is usually nitrogen at 77 K or krypton at lower coverages.
• Pressures employed are subambient, typically in the relative pressure P/P0 range 0.05 to 0.3.
• Apparatus includes glass or metal manifolds, vacuum system, calibrated volumes, and detectors.

Concept / Approach:
BET does not measure area directly. It infers monolayer capacity from measured adsorption isotherms and converts that to area using known molecular cross sections. The method uses low absolute pressures and high vacuum, not high pressure. Construction materials vary; many systems use glass dewar and sample tubes with metallic manifolds, not necessarily all stainless steel.

Step-by-Step Solution:
Collect adsorption data at several P/P0 points.Fit the BET equation to obtain monolayer capacity n_m.Compute area: A = n_m * N_A * molecular cross section.

Verification / Alternative check:
Cross validate area with mercury porosimetry or microscopy for appropriate materials; results differ when micro porosity dominates and t plot or NLDFT is needed.

Why Other Options Are Wrong:
Area is inferred, not directly measured; pressures are low; and there is no strict requirement that every wetted part is stainless steel only.

Common Pitfalls:
Using an incorrect P/P0 fitting range or ignoring degassing quality yields significant errors in calculated surface area.

Final Answer:
None of these.