By George Gamow
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For all other C values the single point curve lies above the BET curve and their difference vanishes as the relative pressure approaches unity. Thus, as the value of C increases the knee of the isotherm becomes sharper and moves toward lower relative pressures. For lower C values, curves X and Y diverge and higher relative pressures must be used to make single-point surface areas conform to those obtained by the multipoint method. 13). It is this term which contains the C value. 1 indicates that, regardless of the C value, the relative error is reduced by using higher relative pressures.
1, and linear BET plots are found at even lower relative pressures. 5 The single point BET method * The BET theory requires that a plot of [W(P0/P -1)] " l versus P/Po be linear with a finite intercept [cf. 38) and Fig. 1]. By reducing the experimental requirement to only one data point, the single-point method offers the advantages of simplicity and speed often with little loss in accuracy. 1 Derivation of the single-point method The slope s and intercept i of a BET plot are s= ~ (cf. 1) For reasonably high values of C the intercept is small compared to the slope and in many instances may be taken as zero.
34) When /J equals unity, N/Nm becomes infinite. This can physically occur when adsorbate condenses on the surface or when P/Po = 1. 35) m but k AP (cf. 11) with n = 1. 38). A plot of l/[W(P0/P) -1] versus P/Po, as shown in Fig. 35. 1 Typical BET plot. 13), namely St = W™N_^ M (Cf. 13) and the specific surface area can be determined by dividing St by the sample weight. 44) that when sufficient adsorption has occurred to form a monolayer there is still always some fraction of surface unoccupied.