| Abstract
| - The present work reports a selective and simple flow injection method for the direct and simultaneousdetermination of calcium and magnesium ions in red, rose, and white wines. Both ions react withmethylthymol blue (MTB) at a strongly basic medium to form colored complexes that are monitoredspectrophotometrically (λmax = 610 nm). The simultaneous determination is achieved by onlinemasking of magnesium by 8-hydroxyquinoline (8-HQ). Incorporating an online dilution mode basedon the “zone sampling” technique in the FI system, the determination of both analytes was achievedwithout any pretreatment of the samples, in the range 0−350 mg L-1 and 0−200 mg L-1 for Ca(II)and Mg(II), respectively. The 3σ detection limits were quite satisfactory (2.1 and 1.8 mg L-1 forCa(II) and Mg(II) respectively), and the precision was 1.2% (at a mixture of 100.0 mg L-1 Ca(II) +100.0 mg L-1 Mg(II), n = 12). A detailed study of interferences proved that the proposed method ishighly selective. The application of the method to the direct analysis of red, rose, and white winesyielded excellent results compared with those obtained by using FAAS as a reference method (er<2.8%). Keywords: Calcium; magnesium; flow-injection; spectrophotometry; methylthymol blue; simultaneous determination; wine; zone sampling dilution
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