Colorimetric chelating reagents form colored complexes with metal ions in pH-controlled solutions. Their selectivity depends on the dissociation constants of metal ions and their sensitivity depends on the molar absorptivity of the complex. However, few colorimetric chelating reagents are highly selective. To increase selectivity, the choice of masking reagents or solvents for the extraction procedures is important. The maximum wavelength of the complex is also an important factor for selectivity and sensitivity. For example, Nitroso-PSAP forms complexes with several heavy metal ions, but the maximum wavelength of the Nitroso-PSAPFe complex is considerably longer than that of the other metal complexes. Thus, iron can be determined without interference from other metal ions.
Real-time monitoring of electrolytes is increasingly important for clinical use. For example, monitoring sodium and potassium ion concentrations in the blood flow is indispensable during cardiac surgery. Although lithium ion is used in the treatment of manic symptoms, its serum level must be strictly monitored due to its toxicity. For simple and quick determination of these alkaline and alkaline earth metal ions, polyvinyl chloride (PVC) membrane electrodes have been widely used. The concentration of neutral carriers, plasticizers, and counteranions used to prepare the PVC electrode determines its ion selectivity. A large number of crown ether compounds have been developed. Some of them are superior to naturally existing neutral carriers such as valinomycin, which is highly selective for potassium ions.