A change in absorbance in the ultraviolet range cannot be detected by eye but can be measured by a suitably equipped colorimeter.
It is also necessary to use fused quartz cuvettes because glass is opaque to ultraviolet.
Many different color reagents have been developed for determining the concentrations of different substances.
For example, Nessler's reagent can be used to determine the concentration of a solution of ammonia.
In thin layer chromatography (TLC) color reactions are frequently used to detect compound spots by dipping the plate into the reagent or by spraying the reagent onto the plates.