Experiments EXPERIMENTAL SYSTEMS 1 SOLUTION-PHASE REACTIONS

While experiment and theory have made tremendous advances over the past few decades in elucidating the molecular processes and transformations that occur over ideal single-crystal surfaces, the application to aqueous phase catalytic systems has been quite limited owing to the challenges associated with following the stmcture and dynamics of the solution phase over metal substrates. Even in the case of a submersed ideal single-crystal surface, there are a number of important issues that have obscured our ability to elucidate the important surface intermediates and follow the elementary physicochemical surface processes. The ability to spectroscopically isolate and resolve reaction intermediates at the aqueous/metal interface has made it difficult to experimentally estabhsh the surface chemistry. In addition, theoretical advances and CPU limitations have restricted ab initio efforts to very small and idealized model systems. 

The tube-in-tube experiment is a very powerful method to determine the sequence of precipitation of secondary minerals as function of temperature for a chosen chemical system. Chemical reactions occur quickly (within 40 days) and the transport by diffusion of chemical elements is efficient. Similar crystallization sequences are observed in both experiments, suggesting that the transitions between the different mineral phases are not only controlled by the composition of the solution but also by temperature. The experimental design does not strictly correspond to the geometry encountered at the Soultz-sous-Forets site and therefore needs to... 

This brief survey shows that there are many options for measuring phase equilibria in reacting systems, which allow to carry out such studies for a wide range of systems and conditions. The main limitation for experimental investigations of reactive vapor-liquid equilibria is related to the velocity of the reaction itself if phase equilibrium measurements of solutions are needed, which are not in chemical equilibrium, the reaction must be considerably slower than the characteristic time constant of the phase equilibrium experiment. Apparatus are available, where that time constant is distinctly below one minute. For systems with reactions too fast to be studied in such apparatuses, it should in many cases be possible to treat the reaction as an equilibrium reaction, so that the information on the phase equilibrium in mixtures, which are not chemically equilibrated is not needed. 

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