Protocol for Obtaining Reference Isotherm Using RM 8850 and Methane.

*

Materials: The adsorbent is NIST reference material 8850 (Zeolite Y). The sample mass to be 

used should be optimized to achieve the best signal-to-noise ratio, according to the experimental 

parameters of the instrument being used. The adsorptive, methane, should have a purity of no

less than 99.999 %.

Sample pretreatment: The zeolite should be outgassed using a turbomolecular pump and slowly 

heated to 350 °C (1 degree per minute, nominally) and held at that temperature for at least 

12 hours under high vacuum (<1 cPa) with continuous pumping. If the outgassing is performed 

in a separate manifold, prevent contact with air when transferring to the analysis instrument. 

Measurement of adsorption isotherms: Methane adsorption isotherms should be measured at 

25 °C up to 7.5 MPa. The same measurement is also to be done without the adsorbent to perform 

a “blank correction.” This blank should be subtracted from the adsorption isotherm.† The

Setzmann and Wagner equation of state‡ is to be used to determine the density/compressibility of 

the gas for the determination of surface excess uptake. RM 8850 has a skeletal density of ≈2.53

g/cm3, which is to be used when performing buoyancy correction for gravimetric technique or 

void volume determination for volumetric technique. 

Comparison to the empirical reference isotherm function: Measured isotherms should fall within 

the 95% uncertainty interval of the reference isotherm function, which in units of mmol/g, is 

characterized by the function,

nneeee,rr rr = aaaa + bb 2 + cc 3

√ee + dd dd ± UUkk=2

where a = 4.2040 (0.0406), b = 0.0818 (0.01000), c = ‒0.0132 (0.0008), d = 1.4400 (0.0146), e = 

1.0700 (0.0115), and Uk=2 = 0.090 mmol/g. No physical meaning should be associated with this 

equation nor with its parameters.

* H. G. T. Nguyen et al., A Reference High-pressure CH4 Adsorption Isotherm for Zeolite Y: Results of an

Interlaboratory Study, Adsorption 26 (2020) 1253. Available On-line. † H. G. T. Nguyen, J. C. Horn, M. Thommes, R. D. van Zee,L. Espinal, Experimental Aspects of Buoyancy

Correction in Measuring Reliable High-pressure Excess Adsorption Isotherms Using the Gravimetric Method, Meas. 

Sci. Technol. 28 (2017) 125802. Available On-line. ‡ U. Setzmann, W. Wagner, A New Equation of State and Tables of Thermodynamic Properties for Methane 

Covering the Range from the Melting Line to 625 K at Pressures up to 100 MPa, J. Phys. Chem. Ref. Data 20 (1991) 

1061. Available On-line.
To reiterate, the following recommendations for measuring of this high-pressure Zeolite Y/CH4

adsorption isotherm are offered:

• Sample Activation: Sufficiently complete sample activation is crucial. RM 8850

requires special handling to ensure the sample is activated completely. As noted above, 

the zeolite must be activated at 350 °C for at least 12 h under high vacuum (≤1 cPa) to 

realize the reported reference isotherm. If the sample is activated ex-situ, exposure to air 

and moisture must be avoided to obtain the correct sample mass.

• Sample Volume Determination: Proper determination of sample volume is needed both 

for buoyancy correction in a gravimetric system, as well as the void volume 

determination in a volumetric system. When required in data analysis, a skeletal density 

of 2.53 g/cm3 should be used for RM 8850.

• Buoyancy Correction/Void Volume Correction: A buoyancy correction must be applied 

when using a gravimetric system. Although less important at low-pressures, buoyancy 

effects are significant for high-pressure measurements and cannot be overlooked. This is 

analogous to the use of void volume in a volumetric instrument to determine surface 

excess uptake, and the effect of using the wrong volume also becomes more significant 

with increasing pressure.

• Equation-of-State: In general, identify the equation-of-state used to calculate fluid 

density, and use critically evaluated equations, such as those contained in the NIST 

Reference Fluid Thermodynamic and Transport Properties Database (REFPROP). The 

Setzmann and Wagner equation of state should be used for CH4 at 25 °C to replicate the 

reference data.

• T, P, and m: Ensure good control and measurement of temperature, pressure, and 

sample mass, as these are important in accurate determination of the uptake.

• Blank Correction: A blank run subtraction should be performed whenever possible, as it 

corrects for small uncompensated transducer nonlinearities, effects of temperature 

heterogeneities coupled with the compressibility of the adsorptive, and other 

experimental effects.