tion away from an established chemical equilibrium or probes may react further after the sampling step. Therefore, spectroscopic methods, which allow a feedback free analysis of different components of the mixture, are of special interest for investigations of reacting systems.

It is beyond the scope of the present work to discuss the different options for the analytical methods. We only present one example here, showing what can be achieved with modern instrumental analysis. Fig. 4.15 shows a 13C NMR-spectrum of a formaldehyde + water + methanol mixture taken with an online technique with a 400 MHz NMR spectrometer. Signals from a large number of different species can be resolved. Obviously, the band assignment is non-trivial for such complex mixtures and special techniques, such as two dimensional NMR, have to be applied. One of the most attractive features of NMR spectroscopy compared with other spectroscopic methods is that the quantitative evaluation of spectra such as that shown in Fig. 4.15 can be achieved without calibration, as the area below the peaks is directly proportional to the number of the different nuclei in the solution if the NMR experiment is carried out properly.

In the following discussion, we will focus on reactive vapor—liquid equilibria. Measurements of reactive liquid—liquid equilibria can be done in standard batch cells operated as mixer-settlers using appropriate analytical methods, as long as only data on the fully established equilibrium is needed. Practically no data seems to be available in the literature on liquid-liquid equilibria in mixtures that have not reached chemical equilibrium.

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