Ionophore-based Heterogeneous Calcium Optical Titration

Recently, the field of complexometric titration significantly progressed with the advent of ionophore-based titration reagents. We present here for the first time an ionophore-based heterogeneous optical titration method for calcium. The all-in-one reagent was composed of chromoionophore, ion-exchanger and calcium ionophore in dichloromethane (DCM). The endpoint of this reverse titration scheme was determined by the color change of the DCM phase (blue to red). The ion-exchange theory borrowed from ISE membranes was used to simulate the titration curve in a simplified manner. The phase separation speed during titration was found much faster at and beyond the endpoint. A mechanism was proposed for this phenomenon, with zeta-potential measurements on ion-selective nanospheres to support the hypothesis. Calcium titration in a urine sample was also successfully demonstrated and compared with EDTA titration with potentiometric ion-selective electrodes as endpoint indicator. The proposed reagent serves as an alternative to the classical EDTA.