Determination of Binary Phase Diagrams of Aqueous Systems That Are of Importance for Processes in the Stratosphere

Introduction 

Depletion of ozone in the earth’s stratosphere, especially over the North and South Poles, has been recognized as a serious problem for the past twenty-five years or more. This phenomenon is linked to the release of chlorofluorocarbons at the earth’s surface that are transferred by convection to the mid or upper stratosphere over time (about five years) [1]. Here the action of short-wavelength UV radiation causes them to undergo photolytic dissociation with the release of free chlorine radicals. For example:

CF₂Cl₂ + hv → CF₂Cl + Cl (1)

The free chlorine radicals then react with methane to form HCl, or with ozone and then NO₂ to form chlorine nitrate:

CH₄ + Cl → CH₃ + HCl (2)
Cl + O₃ → ClO + O₂ (3)
ClO + NO₂ → ClONO₂ (4)

Both HCl and ClONO₂ are considered to be reservoir species because their gas phase reaction is quite slow [2]. However, in the winter months, special conditions occur in the polar stratospheric regions that allow the formation of Polar Stratospheric Clouds (PSCs). The PSCs play a crucial role in the depletion of ozone (O₃). During the polar stratospheric winter they promote the conversion of stable inorganic chlorine compounds (ClONO₂ and HCl) into Cl₂ in the absence of sunlight via reaction on the cloud particle surfaces:

HCl(surf) + ClONO2(surf) → HNO₃(surf) + Cl₂(g) (5)

where (surf) indicates that the molecules are attached to the particle surface and (g) the gas phase. As a result of this surface reaction, gaseous Cl₂ is released into the gas phase during the polar night [3, 4]. When sunlight returns in the springtime (September/October in the Antarctic, March/April in the Arctic), the large amount of Cl₂ gas that has built up over the winter breaks down through photolysis:

Cl₂ + hv → 2 Cl (6)

This large production of Cl radicals then leads to substantial losses of ozone over a short period of time. Many chemical cycles contribute to this loss of ozone, but the cycle first proposed and most well known is the Molina and Rowland cycle for which the two scientists shared the Nobel Prize in chemistry in 1995:
Cl + O₃ → ClO + O₂ (7)
ClO + O₃ → Cl + 2 O₂ (8) 
netto: 2 O₃ → 3 O₂ (9)

Summary

This article describes how binary phase diagrams of relatively complex systems with several stable solid phases can be determined using DSC and complementary IR spectroscopy. DSC experiments supply the important information for mapping the phase diagrams as well as enthalpies of fusion from which the concentration of the phases can be determined. 

Because of its extremely high sensitivity, DSC is able to detect phases with very low transition enthalpies or that are present in very low concentration. The use of complementary analytical methods is however necessary to confirm the DSC results if unknown components are to be identified or different transitions occur at practically the same temperature.

_{Determination of Binary Phase Diagrams of Aqueous Systems That Are of Importance for Processes in the Stratosphere | Thermal Analysis Application No. UC 182 | Application published in METTLER TOLEDO Thermal Analysis UserCom 18}