Vapor Analysis
When running vapor analyses, it is important to:
- Remove the cold trap dewar to prevent vapor from freezing in the cold trap.
- Thoroughly degas the vapor source. See Degas the Vapor Source Tube.
- Use the metal jacket on the sample tube when performing analyses at temperatures above ambient; see Use a Metal Jacket on the Sample Tube.
- Close the door on the vapor enclosure.
Specify Analysis Parameters
Create standalone parameter files for analysis conditions and adsorptive properties for the materials typically analyzed.
The values used in the following example are for silica-alumina reference material having a relatively large pore volume and analyzed at 25 ºC with water vapor derived from a 35 °C source.
Analysis Conditions
Create an analysis conditions parameter file using the parameters in the following table:
Button | Enter or select | ||
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Insert Range |
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Preparation |
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Free Space |
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P0 and T |
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Dosing |
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Equilibration |
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Backfill |
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Adsorptive Properties
- Create an adsorptive properties file.
If the adsorption test is solely for the purpose of discovering how water is taken up by the sample, the Density conversion factor, Therm. tran. hard-sphere diameter, and Molecular cross-sectional area values are of little consequence. All that is needed is the adsorption isotherm and, perhaps, the desorption isotherm.
- Non-ideality factor derived as
where
Z | = | compressibility factor at the saturation pressure of the vapor |
T | = | analysis temperature |
- Density conversion factor derived as:
where
Vm | = | molar volume of the liquid at the analysis temperature |
- Thermal Transpiration hard-sphere diameter. enters into calculations when high precision requires that low-pressure measurements take into account small differences in temperature along gas passageways. The subject is thoroughly treated in Ross Sidney and Olivier, James P., On Physical Adsorption, Interscience Publishers, NY, 1964.
- Molecular cross-sectional area. Required when, from the isotherm, the surface area of the sample material is to be calculated. The technical literature, beginning with the extensive tabulation of McClellan and Harmsberger [“Cross-sectional Areas of Molecules Adsorbed on Solid Surfaces,” J. Coll. and Interface Sci. 23, 577-99 (1967)], is replete with adsorbed cross-sectional areas on various substrates at different temperatures. In the case of silica-alumina and water, these values range from 0.108 to 0.198 nm2. A value of 0.125 nm2 gives BET surface area results that agree well with nitrogen analyses.
- Click Psat vs T; enter:
Row | Saturation Pressure (mmHg) | Temperature °C |
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1 | 4.584 | 0.0000 |
2 | 9.209 | 10.000 |
3 | 23.776 | 25.000 |
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