DIFFUSIONAL TITRATION OF METAL-IONS IN MICROLITER SAMPLES WITH POTENTIOMETRIC INDICATION

Calcium, magnesium, copper, and zinc ions were determined in 1- and 20 -mu L samples in the millimolar and micromolar concentration ranges wi th diffusional microtitration using EDTA as the reagent and a microfab ricated gold/mercury amalgam electrode for potentiometric end point in dication, Typically, a reproducibility of about 3% was obtained in 20- mu L samples with nitrogen jet stirring, A spatially averaging indicat or electrode designed to compensate for minor inhomogeneities was test ed in 1-mu L samples without stirring, yielding 4% reproducibility. Li near calibration graphs in 20-mu L copper, zinc, and calcium samples a nd in 1-mu L copper samples were obtained over broad concentration ran ges, Based on the pH dependence of stability of EDTA-metal complexes, copper could be determined selectively in low-pH medium in the presenc e of magnesium, calcium, and other metal ions, With proper pH adjustme nt, the hardness of tap water could also be obtained in 20-mu L sample s using a combination of standard additions and dilutions, The setup d eveloped is extremely simple yet practical for metal ion determination s in microliter samples.