Vapor Pressure Analyser Science Team Agenda Vapor Pressure

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Vapor Pressure Analyser Science Team

Vapor Pressure Analyser Science Team

Agenda • Vapor Pressure • How is it measured? • Knudsen Effusion Method •

Agenda • Vapor Pressure • How is it measured? • Knudsen Effusion Method • VPA from Surface Measurement Systems • Case Studies

Vapor Pressure The vapor pressure is an important physical property that defines the amount

Vapor Pressure The vapor pressure is an important physical property that defines the amount of vapor phase material that exists in equilibrium with the original material. All materials enter the vapor phase by sublimation (solid – gas) or evaporation (liquid – gas). The vapor pressure of a material at thermodynamic equilibrium is a fundamental property of the material and is only a function of temperature.

Methods for Vapor Pressure Measurement The vapor pressure of material is required to be

Methods for Vapor Pressure Measurement The vapor pressure of material is required to be registered with the EPA (US Environmental Protection Agency) or the EC (European Community). The OECD/OCDE Guideline 104 outlines the accepted methods to accurate determine the vapor pressure of different materials. Surface Measurement Systems’ Vapor Pressure Analyser (VPA) was designed using the Knudsen Effusion method described on this guideline.

Vapor pressure measurements Knudsen Effusion Method Gravimetric measurement of the weight loss by evaporation

Vapor pressure measurements Knudsen Effusion Method Gravimetric measurement of the weight loss by evaporation through a small orifice into vacuum. Clausius-Clapeyron equation Incubator (20~70 o. C) Microbalance Vacuum Sample pan Reference pan Knudsen cell Sample Temperature sensor Pre-heater (20~400 o. C) Sample pan 5

Vapor pressure measurements Material held in a cell, with an orifice of known dimensions

Vapor pressure measurements Material held in a cell, with an orifice of known dimensions will effuse through the hole via sublimation, under vacuum which prevents “back scattering” by air molecules. The rate of mass loss at given temperature is recorded as a function of time Through measurement of the rate of mass loss, temperature, molecular weight and orifice dimensions, the Knudsen equation can be applied to provide vapour pressure data. Time Gradient = dm/dt Sample mass • • log (P) Gradient = b • • • 1/T a • dm/dt is the rate of mass loss P is the vapour pressure of the solid R is the universal gas constant (8. 314 m 3. Pa. K-1. mol-1) T is temperature in Kelvin A is the area of the orifice M is the molecular weight of the solid DH is heat of sublimation a and b are constants 6

Vapor Pressure Analyser

Vapor Pressure Analyser

Case Studies • Benzoic Acid • Naphtalene

Case Studies • Benzoic Acid • Naphtalene

Vapor pressure of benzoic acid Benzoic acid mass loss rate under high vacuum and

Vapor pressure of benzoic acid Benzoic acid mass loss rate under high vacuum and different temperature steps. Relationship between the vapour pressure and the temperature plotted to calculate Enthalpy of vaporization 9

Vapor pressure of benzoic acid Temperature [°C] DM/DT [mg/min] Vapour Pressure [Pa] Vapour Pressure*

Vapor pressure of benzoic acid Temperature [°C] DM/DT [mg/min] Vapour Pressure [Pa] Vapour Pressure* [Pa] 30 0. 0008 0. 23 35 0. 0013 0. 36 40 0. 0023 0. 65 45 0. 0040 1. 11 50 0. 0067 1. 91 1. 9 55 0. 0111 3. 17 3. 25 60 0. 0178 5. 11 5. 16 0. 0278 8. 05 65 8. 22 Note: The cell orifice size was 167 micron Experimental values are in very good agreement with literature data * published by: Monte et. al J. Chem. Eng. Data 2006, 51, 757 -766, Kruif et. al. J. Chem. Thermodynamics 1982, 14, 201 -206 Colomina et. al. J. Chem. Thermodynatnic. 1982, 14, 779 -784 . 10 10

Vapor pressure of naphthalene Vapor pressure at 25 o. C: Measured value: 11. 08

Vapor pressure of naphthalene Vapor pressure at 25 o. C: Measured value: 11. 08 Pa Environmental Protection agency value: 11. 60 Pa 11

Conclusions • The VPA represents a simple way to measure vapor pressure of different

Conclusions • The VPA represents a simple way to measure vapor pressure of different materials under the OECD/OCDE Guideline. • Automated instrument with user friendly analysis software for one-click data analysis and report generation