In contrast, Raman spectroscopy can resolve the formation of crystals in the mid-infrared with much higher chemical specificity even in the presence of water, but it is inherently limited to study the properties of the crystals in solution rather than capturing much information about the liquid phase. Other optical techniques such as second harmonic generation and polarized light microscopy can both detect the onset of the crystallisation, but are not ideally suited to infer detailed mechanistic insight into the nascent molecular structures. The method that is proposed in this study is using THz-TDS to determine the concentration of the solute prior and during crystallisation. In contrast to UV-VIS spectroscopy the method does not require the presence of a chromophore but rather exploits the differences in relaxation dynamics of the dipole moments of the solvent molecules that strongly affect the absorption measured at terahertz frequencies. In analogy to the quantification by UV-Vis a calibration curve over a range of concentrations is required but for the THz-TDS method the sample temperature needs to be taken into account as well.