AMPEROMETRIC TECHNIQUES IN FLOW-INJECTION ANALYSIS - DETERMINATION OF MAGNESIUM IN SERA AND NATURAL-WATERS

Magnesium was determined in a flow-injection system by formation of the magnesium-Eriochrome Black T (EBT) complex at pH 11.5 (5% 1,2-diaminoethane in 0.1 M KCl) and amperometric measurement of the excess of EBT at +0.175 V on a glassy carbon electrode. From modelling calculations, pH and concentrations of EBT (3.75 X 10(-5) M) and masking agent EGTA (1.12 X 10(-4) M) were chosen so as to minimize interference from calcium. Electrode fouling from EBT oxidation products was countered by use of 0.2% surfactant (Hostapur SA 93) in the carrier stream. Iron(III) was masked with triethanolamine. For analysis of natural waters the linear working range was 0.027-1.8 mg kg-1 Mg and the R.S.D. was 0.7% at 1.6 mg kg-1 Mg (n = 11); the detection limit was 5.5 mug kg-1. Analysis of human blood serum samples was effected by two methods: direct injection of serum after a 20-fold dilution (Hostapur SA 93 countered the association of EBT with serum albumin) and after dialysis (2.5 min) to separate Mg from acidified serum in the flow system. The linear working range for direct injection was as above and the R.S.D. was 0.4% (n = 9) at 1.0 mg kg-1 Mg. For each method the results were compared with those from flame atomic absorption spectrometry.