Determination of Vapor Pressure and the Enthalpy of Vaporization by TGA - METTLER TOLEDO

Determination of Vapor Pressure and the Enthalpy of Vaporization by TGA

The vapor pressure of a chemical compound is a measure of its volatility: the higher the vapor pressure at a given temperature, the greater the probability that the substance is in the gaseous state rather than the condensed phase (liquid or solid). The vapor pressure of a substance increases with increasing temperature. The boiling point is reached when the vapor pressure is equal to the total pressure of the surroundings.

effusion cell 

Figure 1: Schematic view of an effusion cell. The gas phase and condensed phase (liquid or solid) of a substance are in equilibrium.

 

Introduction

Knowledge of this thermophysical property and the enthalpy of the corresponding phase transition are of fundamental importance for areas such as process control, storage of materials and stability.

The data is used for establishing environmental guidelines and to define the maximum allowable limiting values and is also needed for preparing safety data sheets.

There is a need for a simple method to determine the vapor pressure of substances for industrial applications as well as for basic research. The method should be routinely applicable, reliable, rapid and straightforward to perform. In this article, we will show that thermogravimetric analysis (TGA) using a reference substance is a suitable technique.

 

Theoretical Background

Several methods are available for determining the vapor pressure of substances. Some are based on techniques such as ebulliometry or gas saturation methods, others on static methods using a manometer (e.g. HPDSC). Another group of methods makes use of the so-called effusion technique.

The effusion technique is based on the determination of mass loss. Devices such as the Knudsen cell and techniques derived from this have become of special interest. This is because their theoretical description takes into account effects that depend on a particular experimental setup. These can be due to different geometrical factors or result from the necessity of not having to work in high vacuum in contrast to the original Knudsen method. An effusion cell is shown schematically in Figure 1.

The design is an open system in which molecules leave the cell by passing through an effusion hole. In thermal equilibrium, mass loss occurs at a constant rate.

An experimental setup like this can be realized with practically any conventional TGA. Thermogravimetric analysis (TGA) is based on the continuous measurement of the mass differences of a sample as a function of temperature and time. The method is therefore ideal for investigating processes such as vaporization or sublimation [1, 2].

In contrast to the method based on a Knudsen cell in high vacuum [3], the experimental setup can be parameterized at ambient conditions. This is done using a known reference substance and is based on the generalization of the Langmuir equation...



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Conclusions

The vapor pressure of a compound was determined by performing TGA measurements at normal pressure conditions. It was shown that vapor pressure data can be obtained using a rapid reliable and sensitive method.

The deviation of the experimental values from literature data was within 5 %. The substances studied in this work cover a vapor pressure range of several decades above 10 Pa. Beside this, quite accurate values of the sublimation or vaporization enthalpy were calculated based on the Clausius-Clapeyron equation. This makes TGA a convenient and simple method to determine both thermodynamic quantities.



Determination of Vapor Pressure and the Enthalpy of Vaporization by TGA | Thermal Analysis Application No. UC383 | Application published in METTLER TOLEDO Thermal Analysis UserCom 38