When addressing the question, "What is Relative Humidity?" we find several definitions. The most relevant one in our context is:
"The amount of water vapor a solution can hold above a salt solution compared to a pure water system at saturation pressure or temperature."
This is fundamentally a function of the partial pressure of water (PH2O) above a salt solution relative to the pure component vapor pressure of water at the same temperature (T) and pressure (P).
Relative Humidity Calculation
Relative Humidity (RH) can be defined using the following formula:
where:
- PH2O(SALT) is the partial pressure of water above the salt solution.
- Vₚ(H₂O) is the vapor pressure of water.
Activity of Water
Notably, this ratio also represents the activity of water:
Since OLI already calculates and reports the activity of water in the calculation outputs, we can use this value to determine relative humidity. For example, if the water activity is 0.95, the relative humidity is 95%.
Thermodynamic Considerations
The concept of relative humidity is grounded in thermodynamics, specifically the expression for the fugacity of water at vapor-liquid equilibrium:
- fH2O(solution, P, T) = fH2O(pure water, P, T) x αH2O
Given that fH2O(solution, P, T) = PH2O×ϕH2O, the equation simplifies to PH2O when gas phase non-ideality is negligible. Similarly, fH2O(pure water, P, T) can be approximated as Psat,H2O, assuming the Poynting correction and fugacity coefficient are insignificant. Thus, we obtain:
αH2O = PH2O / Psat,H2O (as a function of T)
This equation also defines relative humidity.
Important Note
This definition of relative humidity is applicable only in systems with an aqueous liquid phase. In systems dominated by hydrocarbons, this calculation might overestimate the relative humidity. Understanding these nuances ensures accurate modeling and analysis in various water chemistry applications.