Diffusional Titration of Metal Ions in Microliter Samples with Potentiometric Indication

Abstract

Calcium, magnesium, copper, and zinc ions were determined in 1- and 20-μL samples in the millimolar and micromolar concentration ranges with diffusional microtitration using EDTA as the reagent and a microfabricated gold/mercury amalgam electrode for potentiometric end point indication. Typically, a reproducibility of about 3% was obtained in 20-μL samples with nitrogen jet stirring. A spatially averaging indicator electrode designed to compensate for minor inhomogeneities was tested in 1-μL samples without stirring, yielding 4% reproducibility. Linear calibration graphs in 20-μL copper, zinc, and calcium samples and in 1-μL copper samples were obtained over broad concentration ranges. Based on the pH dependence of stability of EDTA−metal complexes, copper could be determined selectively in low-pH medium in the presence of magnesium, calcium, and other metal ions. With proper pH adjustment, the hardness of tap water could also be obtained in 20-μL samples using a combination of standard additions and dilu...