Factors to Consider When Estimating Water Vapor Pressure

Under equilibrium conditions, the partial vapor pressure of water above an aqueous sample is defined by thermodynamic parameters. The measurement of vapor pressure, therefore, is linked to thermodynamic principles, though in a real system, which may or may not be at equilibrium, the vapor pressure is not necessarily that which is to be expected at equilibrium. In order to select a methodology for determining vapor pressure or related properties, the limitations imposed by any requirement for equilibrium and the consequences of a departure from equilibrium must be understood. The purpose of this commentary is to clarify the generic constraints that exist when one attempts to determine the water vapor pressure of a sample. 

As indicated above, the equilibrium partial vapor pressure of water defines its thermodynamic state. The thermodynamic state is usually described by use of the water activity concept, which defines water activity as aw — [pjpw Ir 

It is important to note the constraints imposed by the definition of activity. These are (1) that the system be at equilibrium and (2) that the comparison be an isothermal compari- 

The requirement for equilibrium should be considered first. It is necessary that region C be truly at equilibrium. Unless this region is at equilibrium, the relationship between the sensor and its immediate environment is not uniquely defined. Hence, for sensor measurements to be definitive, equilibrium must exist in region C. For region A, in areal measurement situation, equilibrium is not required to be established. It is sufficient that the pw of the immediate environment (pA) be in a steady state relationship with the sample, unchanging on a time scale defined by the investigator. 

The temperature requirement should also be considered. Region C must be at a uniform, known temperature, Tc. This follows from the requirement for equilibrium. Region A should 

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A2.1 Factors to Consider When Estimating Water Vapor Pressure A2.2 Dew-Point Method for the Determination of Water Activity A2.3 Measurement of Water Activity Using Isopiestic Method A2.4 Direct Manometric Determination of Vapor Pressure A2.5 Measurement of Water Activity by Electronic Sensors... 

A2.1 Factors to Consider When Estimating Water Vapor Pressure A2.1.1... 

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