Terahertz time-domain spectroscopy (THz-TDS) is a useful far-infrared spectroscopy tool that is increasingly used in the field of solid state crystallography to complement x-ray diffraction methods to distinguish between polymorphs and other solid forms, as it probes collaborative motions in molecular systems that are largely influenced by the different crystalline packing. Its use can be extended to study crystallisation itself and it has been shown that even low amounts of crystallinity can be detected in semi-crystalline systems, and we recently developed a setup that allows for the in-situ investigation of crystallisation from the liquid phase over a range of concentrations and temperatures. Ultraviolet-visible (UV-Vis) spectroscopy has been widely applied for determining the concentration of a range of chemical species in a liquid, such as in the field of solution chemistry and water quality monitoring. By measuring the absorption spectrum of the sample of interest, the identity of a known compound or molecule in solution can be confirmed and using a calibration curve, the concentration of the compound can be determined quantitatively. However, despite its excellent sensitivity and ready availability some drawbacks of the method remain: the electronic transitions that give rise to the absorption in the UV-Vis are not very specific and often overlap and it is not always straightforward to utilise UV-Vis spectroscopy for measuring the concentration of chemicals in semi-crystalline systems, as the crystal particles will result to scattering losses and signal distortion.