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
Insert Range
  1. Enter the values:
    • Starting relative pressure. 0.01
    • Ending relative pressure. 0.9
    • Number of points. 50
  1. Click OK.
  2. Click Insert Range again. Enter the values:
    • Starting relative pressure. 0.99
    • Ending relative pressure. 0.45
    • Number of points. 6
  1. In the pressure table, scroll to point number 51.
    1. Click Insert to insert a new row.
    2. Enter 0.95

    The table should now display 57 points.

Preparation
  • Unrestricted evac. from. 10.0 mmHg
  • Vacuum setpoint. 10 mmHg
  • Evacuation time. 0.50 hours
Free Space
  • Evacuation time. 0.50 hours
  • Outgas test duration. Select checkbox and enter 180 seconds.
P0 and T
  • Select Calculate P0 from the analysis temperature.
  • Select Enter the analysis temperature then enter an analysis temperature of 25.0 °C.
Dosing
  • Absolute pressure tolerance. 2.000 mmHg
  • Relative pressure tolerance. 5.0 %
Equilibration
  • Minimum equilibration delay. 600 s
Backfill
  • Disable both backfill options.
 
  • When the Backfill option is disabled at the start of an analysis, the sample must be evacuated before analysis.
  • When Backfill sample is deselected, a message displays indicating that disabling this option may damage the instrument. When dosing water vapor, there is no risk of generating high pressures; therefore, it is safe to deselect this option. Click Yes, then OK to close the window.

Adsorptive Properties

  1. 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.

  1. Non-ideality factor derived as

where

Z = compressibility factor at the saturation pressure of the vapor
T = analysis temperature
  1. Density conversion factor derived as:

where

Vm = molar volume of the liquid at the analysis temperature
  1. 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.
  1. 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.
  1. Click Psat vs T; enter:
Row Saturation Pressure (mmHg) Temperature °C
1 4.584 0.0000
2 9.209 10.000
3 23.776 25.000
 
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