A kinetic method for the simultaneous determination of magnesium and calcium ions in solution is described. The method is based on the dissociation reactions of cryptand (2.2.2) complexes; a stopped-flow injection technique is used with spectrophotometric monitoring of the phthalein complexone complexes of the released metal ions. Analyses were done at a rate of 80 h-, with injected sample volumes of 80 ~1. A microcomputer system for data acquisition and control of the system is described. Flow injection systems, particularly when operated in the stopped-flow mode, have proven very suitable for kinetic types of chemical analysis [l] . Alkaline earth metal ion complexes of polyether diamine-type ligands, cryptands [2], show welldefined dissociation patterns when exposed to suitable scavenger ions [3]. The pseudo first-order dissociation reaction has a rate constant characteristic of the metal ion in question, thus allowing differential kinetic analysis to be applied [ 4, 51. The unusual kinetic and thermodynamic Cryptowl (2.2.2)
properties of the cryptand (2.2.2) complexes of magnesium and calcium form the basis of the simultaneous determination of the two metal ions by stopped-flow injection analysis. The magnesium complex dissociates very quickly, whereas the calcium complex dissociates relatively slowly (Fz,, = 0.15 s-l) when potassium ions are used as scavenger.