An optical absorbance method was developed to measure the concentration of alkali metal ions in aqueous solution at neutral pH. The method was based on the formation of a ternary complex between an ionophore/metal ion complex and the anionic form of 7-(n-decyl)-2-methyl-4-(3′,5′-dichlorophen-4′-one)-indonapthl-1-ol (MEDPIN). Detection of the ternary complex was made in the organic phase after ion-pair extraction. Concentrations of sodium and potassium in the aqueous phase were detectable with high sensitivity and selectivity. The absorbance spectrum of MEDPIN in octanol was strongly dependent upon the potassium concentration in the aqueous phase when the octanol contained the potassium ionophore valinomycin; the absorbance at 610 nm increased by 159% upon addition of 4 mM potassium with an isosbestic point at 501 nm. Potassium was detected with a sensitivity of 0.1 mM and a standard error of 3.4% at concentrations below 25 mM. In chloroform, 80 mM sodium in the aqueous phase caused the absorbance of MEDPIN at 620 nm to increase by 830% in the presence of the sodium ionophore hemisodium. From studies of the spectroscopic properties of MEDPIN in various solvents, at varying concentrations of MEDPIN, and at different pH values in the aqueous solvent, it was concluded that the mechanism of MEDPIN sensitivity to a cation/ionophore complex involves an ionexchange transfer reaction in the organic phase, resulting in deprotonation of MEDPIN.
