2.4.4. Kaiser Test and Ultraviolet-Visible (UV-Vis) Spectrophotometry

For the Kaiser detection, fresh test solutions were prepared and added to a small amount of sample placed in a test tube, as described in reference [65]. The test tube was further put in a sand bath and kept at a temperature of 105–110 °C for 5 min.

The resulting solutions were further analyzed using an Evolution 300 UV-Vis spectrophotometer (ThermoFisher Scientific, Madison, WI, USA). The absorbance values were measured in standard quartz cuvettes between 400 and 800 nm with a bandwidth of 2.0 nm and a scan speed of 240 nm/min. The acquired data were processed using the VISIONpro dedicated software (version 2.0).